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Using excess natural gas for reverse osmosis-based flowback water treatment in US shale fields
Energy 196 (2020) 117145
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Using excess natural gas for reverse osmosis-based flowback water treatment in US shale fields Aritra Kar , Vaibhav Bahadur * Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2019 Received in revised form 8 February 2020 Accepted 11 February 2020 Available online 12 February 2020
This work addresses three significant issues associated with hydraulic fracturing in US shale fields: flaring/venting of excess natural gas, disposal of flowback water and freshwater procurement. Flaring/ venting of excess gas is a significant contributor to global emissions. This work presents a novel utilization concept, wherein excess gas is used onsite to power reverse osmosis (RO)-based treatment of flowback water to supply freshwater for oilfield operations. This study details technical and technoeconomic analyses of the above concept. An analytical model is extended and improved to quantify RO-based freshwater production for flowback water of different salinities. The technical performance of RO systems is analyzed and compared with two competing gas utilization technologies (thermal desalination, atmospheric water harvesting). The use of these technologies in the top eight US shale fields is analyzed, and a techno-economic analysis of RO-based water treatment is conducted. Results indicate that this concept will significantly benefit the Eagle Ford and Niobrara shales. It can meet 200% of water requirements and reduce wastewater disposal by 60% in the Eagle Ford. Furthermore, such RO-based projects can have favorable payback periods of as low as one year. Importantly, this waste-to-value concept has worldwide relevance since the underlying issues are present globally. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Reverse osmosis Natural gas Flowback water Shale oil Flaring Thermal desalination
1. Introduction Hydraulic fracturing has enabled large scale exploitation of shale reserves and positioned the United States (US) as the top oil producing nation worldwide [1]. While shale oil has been a global game changer in the energy landscape, there are significant negative outcomes associated with hydraulic fracturing. Issues like the risk of earthquakes and groundwater contamination are wellpublicized. This work addresses three other issues associated with hydraulic fracturing: flaring/venting of excess natural gas, sourcing water for hydraulic fracturing, and disposal of flowback water. Flaring of associated natural gas (co-produced with oil) is commonly employed worldwide in regions lacking gas collection, processing and transportation infrastructure. Estimates show that 140 billion cubic meters of natural gas was flared worldwide in 2015 [2], which is equivalent to 4% of the global production and 20% of domestic gas consumption in the US. Flaring in the US has increased by 4 times since 2000 [1], and the US is presently the 4th
* Corresponding author. E-mail address: [email protected] (V. Bahadur). https://doi.org/10.1016/j.energy.2020.117145 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
largest flaring country. The surge in flaring in the US can be attributed to the widespread use of hydraulic fracturing to produce oil from shale formations. Amongst US shale fields, the Bakken in North Dakota (primarily) and the Eagle Ford in Texas are responsible for 40% and 15% respectively of total flaring. Locally, the flaring percentages are significantly higher [3]. Another important consideration is the venting of natural gas during completion of gas wells. Vented gas emissions from shale wells are estimated to be two orders of magnitude higher than the emissions associated with conventional wells [4]. This is significant since methane is a much more potent greenhouse gas than CO2 (product of flaring). The second issue addressed by this work is the water requirement associated with hydraulic fracturing. The fresh water requirement per well ranges from 7.5 to 34 million liters [5], with an average of 9.5 million liters [7]; this is enough to fill four Olympic sized swimming pools [6]. Sourcing water is challenging since many shale fields are located in acute water stress regions [8]. Fifty percent of US Shale wells lie in extreme stress regions, where freshwater procurement and transportation costs can reach 3.2 cents/liter [9]. Fresh water is a critical bottleneck for shale oil production in many regions [2]. The use of brackish groundwater is a possible alternative to freshwater use; however, groundwater
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A. Kar, V. Bahadur / Energy 196 (2020) 117145
Nomenclature
Symbols M A S Dp
Dp b
MR w C
r k ED
h I
Water flow rate (kg/s) Permeability coefficient (kg/m2-s-kPa) Membrane area (m2) Applied pressure across the membrane (MPa) Osmotic pressure across the membrane (MPa) Concentration polarization factor (no units) Membrane rejection (no units) Water salinity (g of salt/kg of water) Modified Van’t Hoff coefficient (kPa-kg/g) Density (kg/m3) Mass transfer coefficient across membrane (m/s) Energy density of natural gas (MJ/m3) Overall efficiency (no units) Іncome (million $)
extraction is stringently regulated. Furthermore, saline water reduces the efficiency of chemical proppants mixed with water [4]. The third issue addressed by this work is the flowback water from the well after hydraulic fracturing (within a few days). The amount of flowback water can range from 10% to over 100% of the injected water [6], and varies from well-to-well [10]. Interestingly, most of the flowback water and excess gas production (which is flared) occurs in the days to weeks following well completion. The total dissolved solids (TDS) concentrations in flowback water range from 10,000 parts per million (ppm) (brackish water levels) to over 200,000 ppm (hypersaline). Water with TDS levels at the higher end of this range is beyond the operating regimes of conventional water treatment techniques [4] like thermal desalination (eg. mechanical vapor recompression), thermal evaporation, crystallization etc [11]. Presently, large volumes of flowback water are trucked or piped from shale fields to disposal sites. Many shale fields lack suitable injection sites in close proximity, which significantly increases the cost of wastewater disposal [12]. From an environmental perspective, improper disposal can contaminate surface and groundwater aquifers [13]. There is recent and growing focus [14] on the need for treatment and reuse of flowback water [15], to prevent irremediable long-term environmental problems [16]. One possible solution to all the three problems is to use the natural gas (currently being flared or vented) to treat flowback water onsite, thereby producing freshwater for hydraulic fracturing. Glazer et al. [6]. analyzed flared gas-powered thermal desalination techniques for freshwater production. The present group has conducted extensive studies on the use of excess natural gas and landfill gas-powered atmospheric water harvesting for producing water for oilfield operations [2]. These technology concepts involve the use of natural gas-powered refrigeration systems to produce cooling capacity to achieve large scale dehumidification of ambient air [17]. Thermal desalination and atmospheric water harvesting each have their inherent advantages and drawbacks, and their viability depends on the local conditions and the quality of flowback water streams [18]. Tavakkoli et al. [19]. conducted a technoeconomic analysis of membrane distillation-based reuse of flowback water and reported favorable numbers of $ 0.74/m3 of freshwater. Thiel et al. [20]. compared the energy consumption of various desalination methods for flowback water generated in shale fields. In general, utilization of waste gas streams for desalination or electricity generation has been widely studied [21]. Eldean & Soliman [22] studied the use of waste gases from oil refinery
T M D r Cc Q e MTU ε
Payback Period (years) Мaintenance costs (million $) Depreciation (million $) Discount rate of the project (no units) Capital investment costs (million $) Freshwater output (m3 water=m3 gas) Specific energy input (MJ=m3 ) Mass Transfer Units (no units) Effectiveness (no units)
Subscripts p f f,in GT pump t¼j s
Permeate Concentrating Inlet feed Gas turbine Pump For the specific jth year Sea water
plants for desalination and electricity generation. Osipi et al. [23]. conducted a techno-economic analysis of different desalination methods for produced water treatment. The use of excess natural gas-powered reverse osmosis (RO) for treatment and reuse of flowback water has not been considered in any available literature, and is the focus of this work. RO is a well understood membrane-based water treatment technique and has been widely commercialized since the 1960s. In this work, we present technical and techno-economic analysis of excess natural gas-powered RO systems for top shale fields in the US. The first section of the manuscript details an ε-MTU (effectiveness-mass transfer units) model. Our work extends a previously developed model and uses it to estimate the water production via RO powered by excess gas. Water production from RO systems is compared with two other methods of producing water for eight shale plays in the US. A detailed techno-economic analysis is conducted to quantify the economic viability of RO technology in shale plays, where it is technically feasible. It is noted that the term ‘water production’ in this study refers to any technique which can produce freshwater (thermal desalination, reverse osmosis, atmospheric water harvesting). The novelty of this work lies in the conceptualization, and in the technical and techno-economic analysis of excess natural gaspowered RO for flowback water treatment for shale fields. We note that there is no existing study on the use of excess natural gas for reverse-osmosis-based treatment of flowback water. Modeling frameworks for quantifying the technical and economic impact of this technology are established and are used to assess the benefits for US shale plays. It is stressed that the methodology developed in this work can be applied globally keeping in mind that shale extraction is being explored in many other regions worldwide. 2. Description of excess natural gas powered reverse osmosis system Reverse osmosis relies on the application of external pressure to overcome the osmotic pressure gradient across a membrane. The membrane is semi-permeable which allows water molecules to pass through but traps the relatively larger salt molecules, thereby purifying water as it passes through the membrane [24]. Desalination produces two separate water streams, where one of the streams is made more dilute and the other more concentrated. In commercial RO-based desalination applications, spiral wound
A. Kar, V. Bahadur / Energy 196 (2020) 117145
membranes are commonly used, due to their high packing efficiency [25]. Water moves across the membrane into the permeate stream, which is desalinated water. The remainder of the flow stream is concentrated due to the accumulation of salts and is collected separately for disposal. Fig. 1 shows a detailed schematic of an excess natural gaspowered RO system for flowback water treatment. Excess natural gas from the wellhead is fed to a gas turbine after cleanup in a gas conditioning module which could include knockout drums and scrubbers. The gas turbine powers the pump driving the RO system. Alternatively, a gas engine could also power the pump and the analysis will not differ significantly. Flowback water is pre-treated before it is pressurized and fed through the membrane. The pretreatment methods depend on the feed water quality. Coagulation, flocculation and filtration are the most common forms of primary pre-treatment [29]. It is important to note that modern pretreatment methods like ultra-filtration (UF) or nano-filtration (NF) membranes can significantly mitigate membrane fouling associated with inadequate pre-treatment of feed water [30]. The quantity and quality of desalinated water depends on flowback water volume and quality, performance parameters associated with the RO membrane, availability of excess gas and the efficiency of the gas turbine.
3. Extended ε-MTU model for reverse osmosis In this work, we extend a previously developed model by Banchik et al. [26]. which used the ε-MTU approach to obtain the performance curves associated with RO-based desalination. The ε-MTU model obtains a relation between the dimensionless parameters ε and MTU. ε is the effectiveness of the mass exchanger and is the ratio of the operating recovery ratio of the system to the maximum achievable recovery ratio. Recovery ratio is defined as the fraction of the feed water recovered as desalinated water. The effectiveness can be considered analogous to the second law (exergy) efficiency in thermodynamics, which is defined as the ratio of the operating efficiency to the maximum Carnot efficiency. Effectiveness is a key measure of system performance and is bounded by 0 and 1. The parameter MTU (Mass transfer Units) is a measure of the size of the RO system. Higher values of MTU suggest larger membrane areas. It is noted that this modeling approach is analogous to the ε-NTU approach [27] which is widely used in the design of heat exchangers. Fig. 2 schematically shows the mass flow streams associated with membrane-based RO. Mass transfer occurs through the membrane from the concentrated water stream to the permeate stream. Banchik et al. [26]. developed an analytical model for this system under certain assumptions, which are detailed below.
3
Membrane rejection is 100%, i.e. no salt passes through the membrane. Permeability coefficient (A) is constant across the membrane and is independent of feed water quality. Concentration polarization factor is constant. Pressure drop due to frictional losses along the channel is negligible. Osmotic pressure follows the van’t Hoff’s equation independent of the feed water quality. The present work extends the previous work by relaxing the 100% membrane rejection assumption, thereby allowing the possibility of salt passage through the membrane. Also, this work considers the membrane rejection to be independent of the recovery ratio of the flow. Membrane rejection is an important parameter in the consideration of flowback water desalination. While it is possible to have membrane rejections of 97e99% [28] for seawater, flowback water can include a wider variety of dissolved salts which may be permeable through the RO membrane. Relaxing the 100% membrane rejection assumption in the model is thus a significant improvement. Also notable is the fact that the model can still be analytically solved. In this study, we use a membrane rejection of 90%, however the model can easily incorporate any other membrane rejection numbers. Next, the mathematical formulation of the presently developed model is discussed. The differential permeate flow rate is due to the difference between applied pressure and osmotic pressure which can be estimated as:
dm_ p ¼ AðDp bDpÞdS
(1)
where m_ p is the permeate mass flow rate, A is the permeability coefficient, Dp is the applied pressure across the membrane, b is the concentration polarization factor, Dp is the osmotic pressure due to the feed water salinity and S is the area of the membrane. The balance of solute in the concentrated feed stream and the permeate stream with the inlet feed water stream, respectively, can be captured as:
ðMRÞm_ f ;in wf ;in ¼ m_ f wf
(2)
ð1 MRÞm_ f ;in wf ;in ¼ m_ p wp
(3)
where MR is the membrane rejection which is defined as the fraction of total dissolved solids that is rejected into the concentrated stream. m_ f ;in is the mass flow rate of the input feed stream, wf ;in is the input feed water salinity, wf is the salinity of the feed water stream as it flows through the spiral-wound membrane and wp is the permeate feed water salinity. The equation corresponding to the overall mass balance of the
Fig. 1. Schematic of a natural gas-powered reverse osmosis (RO) system for treatment of flowback water. A natural gas-fired turbine powers the pump driving the RO system. Flowback water is separated by the membrane into a permeate stream and a concentrated stream.
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A. Kar, V. Bahadur / Energy 196 (2020) 117145
Fig. 2. Schematic depicting various flow streams and mass transfer across the RO membrane. The governing differential equation is formulated for the control volume indicated by the dotted lines.
SR0f can be defined as [26]:
solute can be obtained by adding equations (2) and (3) as:
m_ f ;in wf ;in ¼ m_ p wp þ m_ f wf
(4)
In the above equation, the left-hand side represents the total solute quantity in feed water and the right-hand side is the sum of total solutes in the permeate and concentrated feed streams. Similarly, a mass balance for water yields:
m_ f ;in ¼ m_ p þ m_ f
(5)
The osmotic pressure can be estimated from the modified van’t Hoff equation [26]:
Dp ¼ C wf wp
(6)
where C is the modified van’t Hoff coefficient. The differential equation for the permeate flow rate (equation (7), below) can be obtained by combining the above equations as:
dm_ p ¼ A Dp bC m_ f ;in wf ;in
!! MR 1 MR dS m_ f ;in m_ p m_ p
(7)
In the above equation, all other parameters except m_ p and S can be treated as constants. As per Fig. 2, this differential equation is valid over the length of the channel. The permeate flow rate increases with the increasing area of contact of the flow with the membrane. Next, certain dimensionless parameters are introduced as per the model of Banchik et al. [26]. Firstly, the recovery ratio RR is defined as [26]:
RR ¼
m_ p m_ f ;in
ASDp m_ f ;in
(9)
MTU represents the size (area) of the RO membrane. Next, the osmotic pressure ratio is defined as [26]:
pf ;in Cwf ;in ¼ SRf ¼ Dp Dp
(11)
In the above equation, the factor b captures the influence of concentration polarization, which is a non-linear effect observed in membrane-based purification systems [8]. It occurs due to excess accumulation of salts on the high-pressure side of the membrane [24], resulting in an increase in the effective osmotic pressure [12]. b can be estimated as [25]:
! RRoperation ADp m_ p ¼ exp b ¼ exp krs S MTUoperation krs
(10)
It is noted that the membrane performance increases for smaller osmotic pressure ratios. Finally, the modified osmotic pressure ratio
(12)
RRoperation and MTUoperation are the operating recovery ratio and mass transfer units respectively, which is mathematically equivalent to the recovery ratio and the mass transfer units at the outlet of the spiral wound membrane. The value of b can be obtained by nonlinearly solving equations (12) and (14). A value of b greater than 1 increases the effective osmotic pressure ratio SR’f and reduces membrane performance. Introduction of the above four dimensionless parameters transforms equation (7) into a differential equation in RR and MTU as:
dðMTUÞ ¼
SR0f ð1
RRð1 RRÞ dðRRÞ MRÞ þ 1 SR0f RR RR2
(13)
The right-hand side of the above equation can be analytically integrated, which yields:
(8)
The recovery ratio is a measure of the fraction of water recovered from the inlet feed stream. Secondly, the Mass Transfer Units (MTU) is defined as [26]:
MTU ¼
SR0f ¼ bSRf
MTU ¼ RR SR0f
R1 RR R2 RR þ ln 1 ln 1 R1 R2 R1 R2 R1 R2 (14)
whereR1 and R2 are the roots of the quadratic equation with R1 being the positive root:
x2 ð1 aÞx ab ¼ 0 a ¼ SR0f
(15)
b ¼ 1 MR The physical significance ofR1 is that it represents the maximum achievable recovery ratio for a given membrane and inlet feed. The effectiveness of the mass exchanger can be defined as:
A. Kar, V. Bahadur / Energy 196 (2020) 117145
ε¼
RR R1
(16)
Putting everything together, the modified model is obtained as:
MTU ¼ εR1 a
R1 R2 R lnð1 εÞ þ ln 1 ε 1 R1 R2 R1 R2 R2 (17)
Fig. 3 illustrates the results of the model for multiple modified osmotic pressure ratios SR’f and membrane rejection (MR) values. The plot shows the operation curves at different modified osmotic pressure ratios with MRs of 100% and 90%. With a MR of 90%, the MTU is lesser than the one with a MR of 100%. This is expected as we are over-estimating the osmotic pressure with a MR of 100%; the required membrane area (quantified by MTU) will be lower for the case of 90% MR. In general, the influence of MR on MTU is larger (for any SR’f ) as the effectiveness increases. However, for low SR’f (eg. SR’f ¼ 0:25), the MTU is seen to be fairly insensitive to the MR. It is noted that all the results reduce to those derived by Banchik et al. [26]. in the limit MR/ 1. The above model was presently used to estimate the freshwater output of RO-based desalination of flowback water. The following section details the modeling framework for estimating freshwater output of the RO system, which is powered by natural gas. 4. Estimating freshwater output from excess gas-powered reverse osmosis of flowback water Presently, the energy requirements for RO are met by combustion of excess natural gas (from the wellhead) in a gas turbine, which then powers the pump in the RO system. The operating parameters for this system are listed in Table 1. Implementation of the presently developed ε-MTU model requires estimation of the effectiveness and the mass transfer units. Commercially, spiralwound membranes are widely used because of their high packing density of 500e1200 m2/m3 [28]. A typical industrial spiral wound module 20 cm diameter, and 1 m long with a membrane area of 30 m2 can handle feed water flow rates on the order of 1 kg/s [28]. Using a permeability constant of 3.61 g/m2-s-MPa (Table 1) and an applied membrane pressure of 10 MPa, the MTU is estimated to be
5
1.1. The effectiveness can be estimated from the non-dimensional curves in Fig. 3. For flowback water salinity in the seawater range, and a membrane rejection of 90%, the effectiveness is 0.85. In this work, we conservatively choose an operating effectiveness of 0.8, since operating at higher effectiveness will require high membrane areas which might not be feasible for flowback water applications. The efficiencies of the gas turbine and the pump are key parameters in determining the energy available for water treatment; this study utilizes a pump with an efficiency of 85% (Table 1). The efficiency of the gas turbine depends on the rated output power of the turbine. In the following section, we develop a framework for techno-economic analysis which utilizes micro to small-scale turbines with rated output capacities less than 600 kW and efficiencies ranging between 15e20%. Presently, we use a turbine efficiency of 18% (Table 1). The energy input to the pump per unit of natural gas is estimated as:
epump ¼ ED hGT
(18)
The feed water volume output of the pump per unit of gas utilized can be obtained from the energy input estimated by equation (19). The freshwater output per unit of utilized natural gas is estimated using Q which is the input to the RO module per unit of excess gas.
Q ¼
epump hpump Dp
(19)
The feedwater salinity (as measured by TDS) imposes limitations on the ability to implement RO. From a practical perspective, it is accepted [24] that RO can only treat water with a maximum TDS of 50,000 ppm. Hence, it was assumed that RO will only be implemented for flowback water streams with TDS levels less than 50,000 ppm. For higher TDS levels, the external pressure required to overcome the osmotic pressure exceeds the maximum tolerable membrane pressure [25]. Membrane fouling is also a significant problem at high TDS levels and membrane replacement can be expensive [24]. It is also important to note that this analysis assumes the flowback water to be of similar composition to that of sea water. While this is true to an extent considering that the composition of flowback water is dominated by chloride ions, the overall composition could vary significantly across different shale regions [31]. 4.1. Results- freshwater output
Fig. 3. ε-MTU curves for various modified osmotic pressure ratios. The solid lines represent the performance for a membrane rejection of 100%. Dotted lines represent a membrane rejection of 90%.
Fig. 4 shows the amount of desalinated water obtained per cubic meter of natural gas and the corresponding operating recovery ratio, as a function of the TDS level of flowback water. Feed water salinity is a key parameter in determining the overall performance of a RO membrane. Fig. 5 shows results till TDS levels of 50,000 ppm, beyond which RO will become infeasible. The amount of water recovered is plotted for applied membrane pressures of 6, 8 and 10 MPa. These are typical of the membrane pressures utilized in existing RO installations [28]. A higher membrane pressure reduces membrane fouling over time; however, it increases energy consumption and requires better membrane quality. For flowback water with TDS between 20,000e50,000 ppm, it is possible to obtain 0.3e0.6 cubic meter of freshwater per cubic meter of gas. Note, that the operating recovery ratio decreases linearly with higher salinities of the feed water, which implies a reduction in the volume of permeate water. The above analysis predicts the water production resulting from the use of excess gas for RO-based treatment of flowback water. To quantify the advantages of this concept it is essential to compare the water production with other technologies which also use
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A. Kar, V. Bahadur / Energy 196 (2020) 117145 Table 1 Performance-related parameters for the RO system, pump and gas turbine. Parameter
Value
Reference
A ε
3.61 106 kg/m2-s-kPa 0.8
MR C
0.90 73.45 kPa-kg/g 1060 kg/m3 39 MJ/m3 18% 85%
[26] Estimated value for spiral-wound membranes (using the present model) Selected value [26]
rs ED
hGT hpump k
3 104 m/s
[32] [33] [26]
increases due to higher energy requirements. The key takeaway from Table 2 is that RO is the most effective technology for freshwater production, if the TDS levels allow its use. 5. Applications of the three water production technologies in US shale fields
Fig. 4. Water production from RO-based flowback water treatment versus feed water salinity, for various applied pressures.
natural gas for water production. Two such competing technologies are considered; thermal desalination (TD) and atmospheric water harvesting (AWH). Glazer et al. [6]. studied excess natural gaspowered TD of flowback water. Thermal desalination includes a broad class of methods such as Multi-Stage Flash (MSF), MultiEffect Distillation (MED), Mechanical Vapor Compression (MVC) and Membrane Distillation (MD). Glazer et al. [11]. studied the use of excess natural gas powered-MVR, as it can withstand higher TDS levels than other techniques. The concept of excess natural gas-powered AWH was developed by the present group and involves gas-powered refrigeration cycles to condense water from humid air [34]. The system uses a gas turbine to power the compressor of the refrigeration cycle, and the cooling produced is used to condense atmospheric moisture [2]. This technology does not require a water source, which is advantageous in some situations [17]. However, the water harvest depends strongly on ambient weather; condensation rates are higher in hot-humid conditions as compared to cold-dry conditions [18]. Importantly, as this work shows, AWH is much more energy intensive than RO and TD and can be attractive only in hot-humid climates where there is no flowback water. The amount of water produced per cubic meter of excess gas using RO, TD and AWH is listed in Table 2. RO will maximize water production, while AWH has the lowest water production. Clearly, the low specific energy consumption of RO results in significantly higher water production than the other two technologies. The range of water produced by TD and RO in Table 2 is due to varying recovery rates (salinity and process dependent). The amount of water recovered via RO reduces as the salinity of flowback water
The use of excess natural gas-powered RO, TD and AWH was analyzed for eight of the biggest US shale fields. All these fields face challenges associated with flaring/venting gas, flowback water disposal and freshwater availability. These fields are shown in Fig. 5 and are Bakken, Niobrara, Marcellus, Utica, Permian Basin, Barnett, Eagle Ford, and Haynesville-Bossier. Notably, four of these fields are in Texas, which is responsible for significant US flaring and has significant water-related challenges. In this study, the Marcellus and Utica are analyzed together due to their geographic continuity, which is a common practice. Data on flared gas volume, flowback water volume and flowback water quality for the first seven shale fields was obtained from Glazer et al. [11]. Data relevant to the Barnett shale (Texas) was acquired from multiple sources. Flared gas volumes in Texas are reported by the Railroad Commission of Texas (RRC) [35] and the Energy Information Administration (EIA) [36]. The quantity and quality of flowback water was obtained from Environmental Protection Agency (EPA)’s compilation [37] of FracFocus data [38]. Next, the criteria for deciding the use of specific water production techniques (between RO, TD and AWH) for these shale fields is outlined. As per Table 2, RO should be the preferred technique owing to its highest freshwater yields. However, RO can be used only with flowback water streams with TDS levels less than 50,000 ppm. TD will work for TDS levels between 50,000 and 200,000 ppm. Water with TDS levels higher than 200,000 ppm is too saline for treatment using conventional techniques [11]. AWH can be attractive in such scenarios. While the use of AWH does not solve issues related to flowback water, it can still utilize the excess gas to produce onsite freshwater. 5.1. Results of analysis of water production technologies in US shale fields Table 3 shows the total annual volume of flowback water, the average TDS level and excess natural gas volumes for the eight shale fields of interest. It is noted that the quality and flowrate of flowback water and excess gas production vary within these fields. Table 3 shows that average TDS concentrations exceed 80,000 ppm in the Marcellus, Utica, Haynesville-Bossier, Permian Basin and Barnett shale fields [11]. The use of RO is ruled out and TD processes like MVC can provide desalination solutions. Average TDS concentrations of the flowback water from the Eagle Ford and Niobrara shales are below 50,000 ppm [11]. RO can be used in these fields and should be the preferred option compared to TD. Flowback
A. Kar, V. Bahadur / Energy 196 (2020) 117145
7
Fig. 5. Location of the eight shale fields considered in the present study.
Table 2 Comparison of water production using three technologies which use excess natural gas. Method
Water produced per m3 gas
Reverse osmosis Thermal desalination Atmospheric water harvesting
340e760 L 115e380 L 2e10 L
water from the Bakken shale is hypersaline with TDS levels exceeding 250,000 ppm [11]; this precludes the use of conventional desalination techniques. Importantly, the Bakken accounts for 40% of the total US gas flaring [3]. The significant volume of excess gas can make it economically viable to implement AWH. It is noted AWH is very energy intensive and can be used only in the summer
for the climate in the Bakken. However, it can still generate significant quantities of freshwater in relation to the amount of water used for hydraulic fracturing operations [17]. Table 3 shows the freshwater production achievable in each of the eight shale fields, based on the quantity/quality of flowback water and availability of excess natural gas. The amount of freshwater generated using RO is estimated using the model developed in this work. The freshwater obtained using TD and AWH was obtained from Glazer et al. [11]. and Ozkan et al. [42], respectively. Importantly, our analysis shows that the use of excess gas for flowback water treatment will not lead to 100% gas utilization or treatment of 100% of flowback water. Instead, these numbers depend on the relative volumes of excess gas and flowback water. Table 3 shows the unused gas, and untreated flowback water in each field. As an illustration, RO-based treatment in the Eagle Ford
Table 3 Summary of the performance of various water production techniques for eight shale fields in the US. The volumes reported are cumulative annual volumes. RO, TD and AWH represent reverse osmosis, thermal desalination and atmospheric water harvesting, respectively. Shale field
Average TDS of flowback water (ppm)
Flowback water Excess gas Technology Freshwater production (million m3), Remaining wastewater (million (million m3) (% of total water requirement) m3), (% of initial flowback water) (million m3) used
Unutilized gas (million m3)
Bakken Marcellus/ Utica Eagle Ford Niobrara HaynesvilleBossier Permian Basin Barnett
250000 130,000
51.6 5.5
2730 449
AWH TD
(6.7), (26%) (2.75), (5%)
(51.6), (100%) (2.75), (50%)
None 417.3
40,000 25,000 120,000
252.6 3.3 48
1060 160 17
RO RO TD
(145.2), (200%) (2.2), (23%) (1.7), (34%)
(107.4), (42%) (1.1), (33%) (46.3), (96%)
645.2 155.4 None
140,000
686.4
621
TD
(58.8), (149%)
(627.6), (98%)
None
85000
24.3
310
TD
(12.15), (137%)
(12.15), (50%)
188.5
8
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shale will generate 145.2 million m3 of freshwater from an initial 252.6 million m3 of flowback water. The concentrated waste stream from the RO process, which is 107.4 million m3, still needs to be disposed. Note that the volume of water that needs disposal is only 42% of the initial volume. 223.4 million m3 of gas will be utilized and 836.6 million m3 of gas will be left unutilized. Overall, gas availability exceeds (in a relative sense) the flowback water quantity in the Eagle Ford shale. The situation is reverse in the Bakken, Haynesville-Bossier and Permian Basin, where the flowback water is in excess (relatively), and 100% of excess gas is consumed to run TD processes. In such places, wastewater will be left behind, which consists of untreated flowback water and the concentrated waste stream from the TD process. Table 3 also quantifies the amount of freshwater generated as a percentage of the total freshwater requirement in the shale play. The total freshwater requirement was obtained from a recent report by Freyman [7]. Data for the Haynesville/Bossier and Barnett shale fields was not available in this report. The freshwater requirement was therefore estimated from the total number of drilling permits issued annually multiplied by the average water consumption per well. Data on the total number of drilling permits was obtained from the Railroad Commission of Texas [35]. Table 3 shows that the use of waste gas for flowback water treatment can produce significantly more freshwater than current requirements in the Eagle Ford (via RO), Permian Basin (via TD) and the Barnett (via TD) shales. This concept is particularly attractive for the Eagle Ford shale and can produce twice the freshwater requirements. Water production is also significant in the Niobrara (via RO) and Haynesville-Bossier (via TD) shales. Table 3 also reports the remaining wastewater as a percentage of the initial wastewater volume. It is seen that wastewater volume is reduced by at least half in Marcellus/Utica (via TD), Eagle Ford (via RO), Niobrara (via RO) and Barnett (via TD) shales, which will significantly reduce disposal-related costs. As an extension of the current concept, it is noted that the large volumes of unutilized gas in the Eagle Ford, Marcellus/Utica, Barnett and Niobrara shales can be used to power AWH cycles to further produce freshwater. While this can be an attractive proposition, the implementation of two separate water production projects at one location might be challenging from an economics and logistics standpoint. Another possible application of the unutilized gas is its use in RO-based treatment of brackish groundwater to produce freshwater for oilfield operations. While there are other such applications and technologies, final decisions depend on techno-economic analysis of the suite of technologies discussed in this work. The next section includes a detailed techno-economic assessment of excess gas-powered RO-based treatment of flowback water for the Eagle Ford and Niobrara shales. 6. Techno-economic analysis of reverse osmosis-based desalination of flowback water The economic viability of any RO-based flowback water treatment project will be contingent on a favorable techno-economic assessment. This section details a techno-economic study of excess natural gas-powered RO-based flowback water treatment for the Eagle Ford and Niobrara shales. It is noted that the present group conducted a similar study [2] on landfill gas-powered AWH for water production for oilfield operations. While several techniques can be employed to estimate the economic benefits of a project, this work uses the payback period (PBP) as the criteria of interest. The PBP is the time required to recover investment and is widely used in decision making. A shorter PBP indicates a faster return on investment and reduced risk, which is attractive to investors.
Presently, analysis was conducted from the standpoint of an enterprise developing and using the infrastructure to treat flowback water via excess gas powered RO, and supplying water to nearby oilfields. Separate analysis was conducted for the Eagle Ford and Niobrara shales. Presently, shale operators source water (via pipelines or trucking) and truck the flowback water to disposal sites. Large transportation distances imply significant costs [39] associated with water management, given the large volumes of freshwater and flowback water being handled. A new enterprise which provides complete water management services (water supply, flowback water handling and disposal) and excess natural gas handling service would be of interest to present-day shale operators if the costs associated with water and excess gas management were lowered. The concept introduced in this study can be technologically deployed via the development of mobile, small-scale desalination (RO-based) units. Having mobile infrastructure is key due to the short-term nature of both excess gas and flowback water production. Centralized water treatment facilities in the shale plays, while technically attractive will not be economically viable owing to the costs of trucking water from individual wellsites (or using pipelines to transport water). Mobile units offer the advantages of onsite water treatment, and onsite water production, which is attractive from a logistics standpoint. Importantly, small-scale mobile desalination units are commercially available from several water management companies. Next, we discuss the capacity and costs associated with mobile RO-based desalination. Current mobile RO trailer units are primarily designed [40] to treat brackish water, with TDS levels less than 10,000 ppm. However, it is possible to custom-make trailerbased units [40] for treating water with salinity close to that of seawater (TDS level >30,000 ppm), and to integrate a small-scale gas turbine. Such trailer-based desalination trailer units can have capacities upto 1000 gallons per minute (gpm). Based on the volume of flowback water, the trailer capacity for use in the Eagle Ford and the Niobrara shales was selected as 400 gpm and 50 gpm, respectively. For the economic analysis we obtained [40] an estimate of $ 2.5 million for a custom-made 400 gpm capacity trailer (targeted at Eagle Ford), excluding the costs of the gas turbine and its integration. For the 50 gpm trailer (targeted at Niobrara), the price was estimated to be $ 1 million [40]. It is noted that these costs are conservative estimates. Also, multiple such mobile units will be needed to treat flowback water in the entire shale play. Based on flowback water volumes, it is estimated that 365 and 45 units will be needed in the Eagle Ford and Niobrara, respectively. Also, a utilization efficiency of 50% is assumed in this study (unit is used for 50% of time). All these numbers are detailed in Table 4. It is important to note that the waste stream from the desalination process is included in the total wastewater disposal volume, and is accounted for in the analysis. The cost of the gas turbine unit was estimated using a handbook [32]. Desalination units require turbines with rated output capacity ranging from 50 to 600 kW, depending on the salinity of flowback water and the capacity of the desalination unit. The rated output capacity falls in the class of micro to small-scale turbines which are compact-enough to be mounted on a trailer and integrated into a desalination unit. The running and maintenance costs of the desalination unit and the gas turbine were assumed to be 20% of the capital cost. This is a conservative estimate as other studies [41] on maintenance costs of RO plants and turbines showed maximum costs of 15% and 12% of the capital cost, respectively. All these parameters are listed in Table 4. The revenue for the water treatment enterprise depends on the price charged for desalinating flowback water and providing freshwater to the shale operator. Since prices fluctuate, this study
A. Kar, V. Bahadur / Energy 196 (2020) 117145
9
Table 4 Key parameters and cost inputs for the techno-economic assessment of RO-based flowback water treatment in the Eagle Ford and Niobrara shales. These numbers correspond to a single mobile RO unit powered by excess gas. Parameter
Eagle Ford
Niobrara
Details
Value
Details
Capacity e 400 gpm
Cost of mobile desalination unit
$ 2.5 Capacity- 50 gpm million [40] Calculated using TDS levels of 40,000 ppm and 530 kW Calculated using TDS levels of 25,000 ppm and capacity of RO unit. capacity of RO unit. $1000/kW [32] $ 0.53 $ 1400/kW [32] million 10% of total cost of turbine & desalination unit $ 0.3 10% of total cost of turbine & desalination unit million Onsite equipment $ 0.1 Onsite equipment million $ 3.43 million
Capacity of gas turbine Cost of turbine System integration cost Infrastructure for handling excess gas Total capital cost per unit
Operating and maintenance costs
20% of total capital cost
$ 0.69 million Depreciation 10% of capital cost $ 0.34 million Operator-related expenses per unit 5 operators per unit with mean compensation $ 0.5 of 100,000 $ per year million Total operating, maintenance and depreciation $ 1.33 costs per year per unit million Unit utilization efficiency Assumption 50% Total number of units across shale region 365 Maximum freshwater that can be produced Capacity of RO trailers Utilization Efficiency Discount rate of project 10%
Value $ 1 million [40] 47 kW $ 0.07 million $ 0.11 million $ 0.1 million $ 1.28 million
20% of total capital cost
$ 0.26 million 10% of capital cost $ 0.13 million 5 operators per unit with mean compensation $ 0.5 of 100,000 $ per year million $ 0.69 million Assumption 50% 45 Maximum freshwater that can be produced Capacity of RO trailers Utilization Efficiency 10%
quantifies the benefits for various price levels. The freshwater production rates were estimated using the model described in Section 4. The payback period T for this RO-based project can then be estimated by solving the below equation:
0
T X
It¼j
j¼0
ð1 þ rÞj
@Cc þ
T X Mt¼j þ Dt¼j j¼0
ð1 þ rÞj
1 A¼0
(20)
where It¼j, Mt¼j, and Dt¼j represent the income in a specific year, the maintenance costs and the depreciation, respectively. Cc is the capital (initial) investment in the project. A discount rate r of 10% was presently used and is typical of desalination projects [43]. 6.1. Results of techno-economic analysis Fig. 6 shows the PBP for RO-based flowback water treatment in the Eagle Ford and Niobrara shales as a function of the price charged for treatment (and consequent freshwater production) of flowback water. Firstly, the results indicate that the economic viability of this technology is more favorable in the Eagle Ford shale than the Niobrara shale. The Eagle Ford has much higher volumes of flowback water and excess gas as compared to the Niobrara, which translates to better economic benefits. The payback period is just over a year for prices of 1.1 cents/liters in the Eagle Ford. This price is much lower than current costs associated with flowback water disposal and freshwater sourcing in the Eagle Ford. Wastewater disposal averages 1.5 cents/liter [9] and freshwater costs average 0.9 cents/liter in the Eagle Ford [39]. Shale operators are therefore currently paying an average of 2.4 cents/liter to obtain a liter of freshwater and dispose a liter of flowback water. With this background, payback period of a year or less is a very attractive proposition from an investment standpoint. Overall, this analysis suggests that this technology has significant economic potential in the Eagle Ford which is responsible for a significant fraction of US
Fig. 6. Payback period versus the price charged for RO-based flowback water treatment projects in the Eagle Ford and Niobrara shales.
flaring and where freshwater sourcing can be a deal breaker. The PBPs for the Niobrara shale are noticeably higher than those in the Eagle Ford; a 3-year PBP corresponds to a price of 1.8 cents/ liter. While there is no publicly available reliable data on the flowback water disposal and freshwater procurement costs in the Niobrara, it is expected to be higher than 1.8 cents/liter. In most shale fields like the Marcellus, Bakken and Permian Basin, flowback water disposal costs are much higher than those in the Eagle Ford and are at least 1.6 cents/liter [39]. Freshwater procurements costs typically exceed 1.3 cents/liter [9]. Based on these discussions, a price of 1.8 cents/liter (with a 3 year PBP) represents an attractive
10
A. Kar, V. Bahadur / Energy 196 (2020) 117145
economic proposition for the Niobrara shale. It is noted that this analysis does not consider other factors which will increase the value proposition of this technology. While flowback water disposal is carefully regulated, there are usually no financial repercussions on venting or flaring gas. Any future regulatory push towards taxing emissions will enhance the economic proposition of this emissions reducing technology. This technology also offers several intangible benefits such as reduced water disposal requirements, reduced pressure on surface and groundwater resources, reduction in pollution (from flares) and reduced truck traffic. The carbon footprint associated with oil-gas production can be reduced via large scale adaptation of this technology. All such benefits are not easily monetized, but they contribute towards decision making of projects. 7. Challenges associated with RO-based treatment of flowback water From a technology standpoint, the proposed concept can be implemented using existing technology, with all components of the system in Fig. 1 being commercially available. Treatment and reuse of flowback water is increasingly becoming a common practice, and the use of excess natural gas to power RO systems will be of interest to the vast and growing water management industry that caters to shale operators. It is noted that while the analysis focused on the Eagle Ford and the Niobrara shales (owing to lower salinities of flowback water in these fields), this technology can be used in other shale plays where the local TDS levels permit the use of RO. While the low specific energy requirement of RO gives it a competitive advantage over other desalination methods [24], the current technological state of RO makes desalination of water with TDS levels higher than 50,000 ppm unfeasible [25]. This is a major limitation to the adoption of RO in many shale fields, which have much higher TDS levels. There do exist methods which allow the RO process to tolerate higher TDS levels like Vibration Shear Enhanced Processing (VSEP), RO integrated with nanofiltration and microfiltration (RO-NF, RO-MF) and others [44]. There is significant ongoing research on novel membrane-based technologies and improvements in the conventional methods to tolerate higher TDS levels [45]. Another challenge for flowback water treatment using RO is the handling of organic contaminants, which are difficult to remove using conventional pre-treatment methods like coagulation or flocculation, and lead to membrane fouling [24]. Also, the composition of flowback water varies across different wells which increases the challenges associated with membrane design. 8. Conclusions Water management-related issues can be a deal-breaker for future development of shale resources, and this study proposes a novel solution which can impact the energy, water and environment sectors. The first part of the study develops an analytical model to quantify the performance of RO-based flowback water treatment systems, which are powered by onsite excess natural gas. Comparison with other freshwater production techniques clearly highlights the low specific energy requirements of RO (upto 760 L of freshwater per m3 of gas). The TDS level requirements (<50,000 ppm) of flowback water to be compatible with RO impose significant restrictions on its geographic applicability. However, in shale plays where RO is technically feasible, freshwater production can be very significant (eg. 200% of water requirements of the Eagle Ford). Importantly, the techno-economic analysis indicates that RObased projects would be economically attractive (payback period: 1 year), owing to the high current costs of wastewater disposal and freshwater procurement. Overall, this waste-to-value conversion
technology can solve critical energy-water-environment-related issues associated with shale production. Funding We would like to acknowledge a research grant from the University of Texas for support of this work. CRediT authorship contribution statement Aritra Kar: Methodology, Data curation, Formal analysis, Visualization, Writing - original draft. Vaibhav Bahadur: Conceptualization, Methodology, Writing - review & editing, Supervision. References [1] https://www.eia.gov/tools/faqs/faq.php?id¼709&t¼6 (accessed on 6/16/ 2019). [2] Wikramanayake ED, Ozkan O, Bahadur V. Landfill gas-powered atmospheric water harvesting for oilfield operations in the United States. Energy 2017;138: 647e58. [3] Dalrymple A, Tribune B. North Dakota oil production, natural gas flaring reach new highs. The Jamestown Sun 2018. [4] [a] Howarth RW, Santoro R, Ingraffea A. Methane and the greenhouse-gas footprint of natural gas from shale formations. Climate Change 2011;106: 679.[b] Estrada JM, Bhamidimarri R. A review of the issues and treatment options for wastewater from shale gas extraction by hydraulic fracturing. Fuel 2016;182:292e303. [5] Nicot J, Scanlon BR. Water use for shale-gas production in Texas, US. Environ Sci Technol 2012;46:3580e6. [6] Glazer YR, Kjellsson JB, Sanders KT, Webber ME. Potential for using energy from flared gas for on-site hydraulic fracturing wastewater treatment in Texas. Environ Sci Technol Lett 2014;1(7):300e4. [7] Freyman M. Hydraulic fracturing & water stress: water demand by the numbers. Ceres Report 2014:1e85. [8] Vengosh A, Jackson RB, Warner N, Darrah TH, Kondash A. A critical review of risks to water resources from unconventional shale gas development and hydraulic fracturing in United States,. Environ Sci Technol 2014;48(15): 8334e48. [9] EIA Report. Trends in US oil and natural gas upstream costs. 2016. [10] Nicot JP, Reedy RC, Costley RA, Huang Y. Oil & gas water use in Texas: update to the 2011 mining water use report. Texas: Texas Water Development Board: Austin; 2012. [11] Glazer YR, Davidson FT, Lee JJ, Webber ME. An inventory and engineering assessment of flared gas and liquid waste streams from hydraulic fracturing in the USA. Curr Sustainable Renewable Energy Rep 2017;4:219e31. [12] https://www.nytimes.com/2015/05/05/science/earth/fracking-chemicalsdetected-in-pennsylvania-drinking-water.html (accessed on 6/16/2019). [13] https://www.nytimes.com/2016/12/13/us/reversing-course-epa-saysfracking-can-contaminate-drinking-water.html (accessed on 6/16/2019). [14] Rassenfoss S. Permian operators squeezed by growing water pressure. Project systems technologies oil and gas facilities 2019;71(4). [15] Whitfield S. “Permian: treating produced water for discharge reduces injection,” Project systems technologies oil and gas facilities. 2019. [16] Howarth RW, Ingraffea A, Engelder T. Natural gas: should fracking stop? Nature 2011;477:271e5. [17] Wikramanayake ED, Bahadur V. Flared natural gas-based onsite atmospheric water harvesting (AWH) for oilfield operations. Environ Res Lett 2016;11(3): 034024. [18] Chaitanya B, Bahadur V, Thakur AD, Raj R. Biomass gasification-based atmospheric water harvesting in India. Energy 2018;165:610e21. [19] Tavakkoli S, Lokare OR, Vidic RD, Khanna V. A techno-economic assessment of membrane distillation for treatment of Marcellus shale produced water. Desalination 2017;416:24e34. [20] Thiel GP, Tow EW, Banchik LD, Chung HW, Lienhard V, J H. Energy consumption in desalinating produced water from shale oil and gas extraction. Desalination 2015;366:94e112. [21] Rattner, A.S., Garimella, S., “Energy harvesting, reuse and upgrade to reduce primary energy usage in the USA,” Energy, 36 (10), 6172-6183. [22] Eldean MAS, Soliman AM. A novel study of using oil refinery plants waste gases for thermal desalination and electric power generation: energy, exergy & cost evaluations. Appl Energy 2017;195:453e77. [23] Osipi SR, Secchi AR, Borges CR. Cost assessment and retro-techno-economic analysis of desalination technologies in onshore produced water treatment. Desalination 2018;430:107e19. [24] Greenlee LF, Lawler DF, Freeman BD, Marrot B, Moulin P. “Reverse osmosis desalination: water sources, technology and today’s challenges. Water Res 2009;43:2317e48. [25] Fritzmann C, Lowenberg J, Wintgens T, Melin T. State-of-the-art of reverse osmosis desalination. Desalination 2007;216:1e76.
A. Kar, V. Bahadur / Energy 196 (2020) 117145 [26] Banchik LD, Sharqawy MH, Lienhard V, J H. Effectiveness-mass transfer units (ε e MTU) model of a reverse osmosis membrane mass exchanger. J Membr Sci 2014;458:189e98. [27] Incropera FP, DeWitt DP, Bergman TL, Lavine AS. Fundamentals of heat and mass transfer. sixth ed. Book; 2006. [28] Kucera J. Reverse osmosis: industrial processes and applications. second ed. Book; 2015. [29] Jamaly S, Darwish NN, Ahmed I, Hasan SW. A short review on reverse osmosis pre-treatment technologies. Desalination 2014;354:30e8. [30] Ahmadun F, Pendashteh A, Abdullah LC, Biak DRA, Madaeni SS, Abidin ZZ. Review of technologies for oil and gas produced water treatment. J Hazard Mater 2009;170:530e51. [31] Gregory KB, Vidic RD, Dzombak DA. Water management challenges associated with production of shale gas by hydraulic fracturing. Elements 2011;7:181e6. [32] Boyce MP. Gas turbine engineering handbook. fourth ed. Butterworth-Heinemann Publications; 2012. [33] Gulich JF. Centrifugal pumps. third ed. Spring Verlag Publications; 2014. [34] Bahadur V. From Fire to Water: instead of flaring stranded gas, use it to harvest water for new wells. ASME Mech Eng Mag 2016;138:31e5 [cover article)].
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[35] Texas RailRoad Comission (https://www.rrc.state.tx.us). [36] US Energy Information Administration (https://www.eia.gov). [37] EPA Compilation of FracFocus Data (https://www.epa.gov/hfstudy/epaanalysis-fracfocus-1-data). [38] FracFocus (https://fracfocusdata.org). [39] McCurdy R., “Underground injection wells for produced water disposal,” Chesapeake Energy Corporation, EPA Compilation. [40] Conversations with companies which provide trailer-based RO units. [41] McGivney WT, Kawamura S. Cost estimating manual for water treatment facilities. In: Hoboken NJ, editor. John Wiley & Sons; 2008. [42] Ozkan O, Wikramanayake ED, Bahadur V. Modelling humid air condensation in waste natural gas-powered atmospheric water harvesting systems. Appl Therm Eng 2017;118:224e32. [43] Zhou Y, Tol Richard SJ. Evaluating the costs of desalination and water transport. Water Resour Res 2005;41:W03003. [44] Afrasiabi N, Shahbazali E. RO brine treatment and disposal methods. Desalination and Water Treatment 2012;35(1e3):39e53. [45] Chang H, et al. Potential and implemented membrane-based technologies for the treatment and reuse of flowback and produced water from shale gas and oil plays: a review 2019;455:34e57.
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Using excess natural gas for reverse osmosis-based flowback water treatment in US shale fields Aritra Kar , Vaibhav Bahadur * Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2019 Received in revised form 8 February 2020 Accepted 11 February 2020 Available online 12 February 2020
This work addresses three significant issues associated with hydraulic fracturing in US shale fields: flaring/venting of excess natural gas, disposal of flowback water and freshwater procurement. Flaring/ venting of excess gas is a significant contributor to global emissions. This work presents a novel utilization concept, wherein excess gas is used onsite to power reverse osmosis (RO)-based treatment of flowback water to supply freshwater for oilfield operations. This study details technical and technoeconomic analyses of the above concept. An analytical model is extended and improved to quantify RO-based freshwater production for flowback water of different salinities. The technical performance of RO systems is analyzed and compared with two competing gas utilization technologies (thermal desalination, atmospheric water harvesting). The use of these technologies in the top eight US shale fields is analyzed, and a techno-economic analysis of RO-based water treatment is conducted. Results indicate that this concept will significantly benefit the Eagle Ford and Niobrara shales. It can meet 200% of water requirements and reduce wastewater disposal by 60% in the Eagle Ford. Furthermore, such RO-based projects can have favorable payback periods of as low as one year. Importantly, this waste-to-value concept has worldwide relevance since the underlying issues are present globally. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Reverse osmosis Natural gas Flowback water Shale oil Flaring Thermal desalination
1. Introduction Hydraulic fracturing has enabled large scale exploitation of shale reserves and positioned the United States (US) as the top oil producing nation worldwide [1]. While shale oil has been a global game changer in the energy landscape, there are significant negative outcomes associated with hydraulic fracturing. Issues like the risk of earthquakes and groundwater contamination are wellpublicized. This work addresses three other issues associated with hydraulic fracturing: flaring/venting of excess natural gas, sourcing water for hydraulic fracturing, and disposal of flowback water. Flaring of associated natural gas (co-produced with oil) is commonly employed worldwide in regions lacking gas collection, processing and transportation infrastructure. Estimates show that 140 billion cubic meters of natural gas was flared worldwide in 2015 [2], which is equivalent to 4% of the global production and 20% of domestic gas consumption in the US. Flaring in the US has increased by 4 times since 2000 [1], and the US is presently the 4th
* Corresponding author. E-mail address: [email protected] (V. Bahadur). https://doi.org/10.1016/j.energy.2020.117145 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
largest flaring country. The surge in flaring in the US can be attributed to the widespread use of hydraulic fracturing to produce oil from shale formations. Amongst US shale fields, the Bakken in North Dakota (primarily) and the Eagle Ford in Texas are responsible for 40% and 15% respectively of total flaring. Locally, the flaring percentages are significantly higher [3]. Another important consideration is the venting of natural gas during completion of gas wells. Vented gas emissions from shale wells are estimated to be two orders of magnitude higher than the emissions associated with conventional wells [4]. This is significant since methane is a much more potent greenhouse gas than CO2 (product of flaring). The second issue addressed by this work is the water requirement associated with hydraulic fracturing. The fresh water requirement per well ranges from 7.5 to 34 million liters [5], with an average of 9.5 million liters [7]; this is enough to fill four Olympic sized swimming pools [6]. Sourcing water is challenging since many shale fields are located in acute water stress regions [8]. Fifty percent of US Shale wells lie in extreme stress regions, where freshwater procurement and transportation costs can reach 3.2 cents/liter [9]. Fresh water is a critical bottleneck for shale oil production in many regions [2]. The use of brackish groundwater is a possible alternative to freshwater use; however, groundwater
2
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Nomenclature
Symbols M A S Dp
Dp b
MR w C
r k ED
h I
Water flow rate (kg/s) Permeability coefficient (kg/m2-s-kPa) Membrane area (m2) Applied pressure across the membrane (MPa) Osmotic pressure across the membrane (MPa) Concentration polarization factor (no units) Membrane rejection (no units) Water salinity (g of salt/kg of water) Modified Van’t Hoff coefficient (kPa-kg/g) Density (kg/m3) Mass transfer coefficient across membrane (m/s) Energy density of natural gas (MJ/m3) Overall efficiency (no units) Іncome (million $)
extraction is stringently regulated. Furthermore, saline water reduces the efficiency of chemical proppants mixed with water [4]. The third issue addressed by this work is the flowback water from the well after hydraulic fracturing (within a few days). The amount of flowback water can range from 10% to over 100% of the injected water [6], and varies from well-to-well [10]. Interestingly, most of the flowback water and excess gas production (which is flared) occurs in the days to weeks following well completion. The total dissolved solids (TDS) concentrations in flowback water range from 10,000 parts per million (ppm) (brackish water levels) to over 200,000 ppm (hypersaline). Water with TDS levels at the higher end of this range is beyond the operating regimes of conventional water treatment techniques [4] like thermal desalination (eg. mechanical vapor recompression), thermal evaporation, crystallization etc [11]. Presently, large volumes of flowback water are trucked or piped from shale fields to disposal sites. Many shale fields lack suitable injection sites in close proximity, which significantly increases the cost of wastewater disposal [12]. From an environmental perspective, improper disposal can contaminate surface and groundwater aquifers [13]. There is recent and growing focus [14] on the need for treatment and reuse of flowback water [15], to prevent irremediable long-term environmental problems [16]. One possible solution to all the three problems is to use the natural gas (currently being flared or vented) to treat flowback water onsite, thereby producing freshwater for hydraulic fracturing. Glazer et al. [6]. analyzed flared gas-powered thermal desalination techniques for freshwater production. The present group has conducted extensive studies on the use of excess natural gas and landfill gas-powered atmospheric water harvesting for producing water for oilfield operations [2]. These technology concepts involve the use of natural gas-powered refrigeration systems to produce cooling capacity to achieve large scale dehumidification of ambient air [17]. Thermal desalination and atmospheric water harvesting each have their inherent advantages and drawbacks, and their viability depends on the local conditions and the quality of flowback water streams [18]. Tavakkoli et al. [19]. conducted a technoeconomic analysis of membrane distillation-based reuse of flowback water and reported favorable numbers of $ 0.74/m3 of freshwater. Thiel et al. [20]. compared the energy consumption of various desalination methods for flowback water generated in shale fields. In general, utilization of waste gas streams for desalination or electricity generation has been widely studied [21]. Eldean & Soliman [22] studied the use of waste gases from oil refinery
T M D r Cc Q e MTU ε
Payback Period (years) Мaintenance costs (million $) Depreciation (million $) Discount rate of the project (no units) Capital investment costs (million $) Freshwater output (m3 water=m3 gas) Specific energy input (MJ=m3 ) Mass Transfer Units (no units) Effectiveness (no units)
Subscripts p f f,in GT pump t¼j s
Permeate Concentrating Inlet feed Gas turbine Pump For the specific jth year Sea water
plants for desalination and electricity generation. Osipi et al. [23]. conducted a techno-economic analysis of different desalination methods for produced water treatment. The use of excess natural gas-powered reverse osmosis (RO) for treatment and reuse of flowback water has not been considered in any available literature, and is the focus of this work. RO is a well understood membrane-based water treatment technique and has been widely commercialized since the 1960s. In this work, we present technical and techno-economic analysis of excess natural gas-powered RO systems for top shale fields in the US. The first section of the manuscript details an ε-MTU (effectiveness-mass transfer units) model. Our work extends a previously developed model and uses it to estimate the water production via RO powered by excess gas. Water production from RO systems is compared with two other methods of producing water for eight shale plays in the US. A detailed techno-economic analysis is conducted to quantify the economic viability of RO technology in shale plays, where it is technically feasible. It is noted that the term ‘water production’ in this study refers to any technique which can produce freshwater (thermal desalination, reverse osmosis, atmospheric water harvesting). The novelty of this work lies in the conceptualization, and in the technical and techno-economic analysis of excess natural gaspowered RO for flowback water treatment for shale fields. We note that there is no existing study on the use of excess natural gas for reverse-osmosis-based treatment of flowback water. Modeling frameworks for quantifying the technical and economic impact of this technology are established and are used to assess the benefits for US shale plays. It is stressed that the methodology developed in this work can be applied globally keeping in mind that shale extraction is being explored in many other regions worldwide. 2. Description of excess natural gas powered reverse osmosis system Reverse osmosis relies on the application of external pressure to overcome the osmotic pressure gradient across a membrane. The membrane is semi-permeable which allows water molecules to pass through but traps the relatively larger salt molecules, thereby purifying water as it passes through the membrane [24]. Desalination produces two separate water streams, where one of the streams is made more dilute and the other more concentrated. In commercial RO-based desalination applications, spiral wound
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membranes are commonly used, due to their high packing efficiency [25]. Water moves across the membrane into the permeate stream, which is desalinated water. The remainder of the flow stream is concentrated due to the accumulation of salts and is collected separately for disposal. Fig. 1 shows a detailed schematic of an excess natural gaspowered RO system for flowback water treatment. Excess natural gas from the wellhead is fed to a gas turbine after cleanup in a gas conditioning module which could include knockout drums and scrubbers. The gas turbine powers the pump driving the RO system. Alternatively, a gas engine could also power the pump and the analysis will not differ significantly. Flowback water is pre-treated before it is pressurized and fed through the membrane. The pretreatment methods depend on the feed water quality. Coagulation, flocculation and filtration are the most common forms of primary pre-treatment [29]. It is important to note that modern pretreatment methods like ultra-filtration (UF) or nano-filtration (NF) membranes can significantly mitigate membrane fouling associated with inadequate pre-treatment of feed water [30]. The quantity and quality of desalinated water depends on flowback water volume and quality, performance parameters associated with the RO membrane, availability of excess gas and the efficiency of the gas turbine.
3. Extended ε-MTU model for reverse osmosis In this work, we extend a previously developed model by Banchik et al. [26]. which used the ε-MTU approach to obtain the performance curves associated with RO-based desalination. The ε-MTU model obtains a relation between the dimensionless parameters ε and MTU. ε is the effectiveness of the mass exchanger and is the ratio of the operating recovery ratio of the system to the maximum achievable recovery ratio. Recovery ratio is defined as the fraction of the feed water recovered as desalinated water. The effectiveness can be considered analogous to the second law (exergy) efficiency in thermodynamics, which is defined as the ratio of the operating efficiency to the maximum Carnot efficiency. Effectiveness is a key measure of system performance and is bounded by 0 and 1. The parameter MTU (Mass transfer Units) is a measure of the size of the RO system. Higher values of MTU suggest larger membrane areas. It is noted that this modeling approach is analogous to the ε-NTU approach [27] which is widely used in the design of heat exchangers. Fig. 2 schematically shows the mass flow streams associated with membrane-based RO. Mass transfer occurs through the membrane from the concentrated water stream to the permeate stream. Banchik et al. [26]. developed an analytical model for this system under certain assumptions, which are detailed below.
3
Membrane rejection is 100%, i.e. no salt passes through the membrane. Permeability coefficient (A) is constant across the membrane and is independent of feed water quality. Concentration polarization factor is constant. Pressure drop due to frictional losses along the channel is negligible. Osmotic pressure follows the van’t Hoff’s equation independent of the feed water quality. The present work extends the previous work by relaxing the 100% membrane rejection assumption, thereby allowing the possibility of salt passage through the membrane. Also, this work considers the membrane rejection to be independent of the recovery ratio of the flow. Membrane rejection is an important parameter in the consideration of flowback water desalination. While it is possible to have membrane rejections of 97e99% [28] for seawater, flowback water can include a wider variety of dissolved salts which may be permeable through the RO membrane. Relaxing the 100% membrane rejection assumption in the model is thus a significant improvement. Also notable is the fact that the model can still be analytically solved. In this study, we use a membrane rejection of 90%, however the model can easily incorporate any other membrane rejection numbers. Next, the mathematical formulation of the presently developed model is discussed. The differential permeate flow rate is due to the difference between applied pressure and osmotic pressure which can be estimated as:
dm_ p ¼ AðDp bDpÞdS
(1)
where m_ p is the permeate mass flow rate, A is the permeability coefficient, Dp is the applied pressure across the membrane, b is the concentration polarization factor, Dp is the osmotic pressure due to the feed water salinity and S is the area of the membrane. The balance of solute in the concentrated feed stream and the permeate stream with the inlet feed water stream, respectively, can be captured as:
ðMRÞm_ f ;in wf ;in ¼ m_ f wf
(2)
ð1 MRÞm_ f ;in wf ;in ¼ m_ p wp
(3)
where MR is the membrane rejection which is defined as the fraction of total dissolved solids that is rejected into the concentrated stream. m_ f ;in is the mass flow rate of the input feed stream, wf ;in is the input feed water salinity, wf is the salinity of the feed water stream as it flows through the spiral-wound membrane and wp is the permeate feed water salinity. The equation corresponding to the overall mass balance of the
Fig. 1. Schematic of a natural gas-powered reverse osmosis (RO) system for treatment of flowback water. A natural gas-fired turbine powers the pump driving the RO system. Flowback water is separated by the membrane into a permeate stream and a concentrated stream.
4
A. Kar, V. Bahadur / Energy 196 (2020) 117145
Fig. 2. Schematic depicting various flow streams and mass transfer across the RO membrane. The governing differential equation is formulated for the control volume indicated by the dotted lines.
SR0f can be defined as [26]:
solute can be obtained by adding equations (2) and (3) as:
m_ f ;in wf ;in ¼ m_ p wp þ m_ f wf
(4)
In the above equation, the left-hand side represents the total solute quantity in feed water and the right-hand side is the sum of total solutes in the permeate and concentrated feed streams. Similarly, a mass balance for water yields:
m_ f ;in ¼ m_ p þ m_ f
(5)
The osmotic pressure can be estimated from the modified van’t Hoff equation [26]:
Dp ¼ C wf wp
(6)
where C is the modified van’t Hoff coefficient. The differential equation for the permeate flow rate (equation (7), below) can be obtained by combining the above equations as:
dm_ p ¼ A Dp bC m_ f ;in wf ;in
!! MR 1 MR dS m_ f ;in m_ p m_ p
(7)
In the above equation, all other parameters except m_ p and S can be treated as constants. As per Fig. 2, this differential equation is valid over the length of the channel. The permeate flow rate increases with the increasing area of contact of the flow with the membrane. Next, certain dimensionless parameters are introduced as per the model of Banchik et al. [26]. Firstly, the recovery ratio RR is defined as [26]:
RR ¼
m_ p m_ f ;in
ASDp m_ f ;in
(9)
MTU represents the size (area) of the RO membrane. Next, the osmotic pressure ratio is defined as [26]:
pf ;in Cwf ;in ¼ SRf ¼ Dp Dp
(11)
In the above equation, the factor b captures the influence of concentration polarization, which is a non-linear effect observed in membrane-based purification systems [8]. It occurs due to excess accumulation of salts on the high-pressure side of the membrane [24], resulting in an increase in the effective osmotic pressure [12]. b can be estimated as [25]:
! RRoperation ADp m_ p ¼ exp b ¼ exp krs S MTUoperation krs
(10)
It is noted that the membrane performance increases for smaller osmotic pressure ratios. Finally, the modified osmotic pressure ratio
(12)
RRoperation and MTUoperation are the operating recovery ratio and mass transfer units respectively, which is mathematically equivalent to the recovery ratio and the mass transfer units at the outlet of the spiral wound membrane. The value of b can be obtained by nonlinearly solving equations (12) and (14). A value of b greater than 1 increases the effective osmotic pressure ratio SR’f and reduces membrane performance. Introduction of the above four dimensionless parameters transforms equation (7) into a differential equation in RR and MTU as:
dðMTUÞ ¼
SR0f ð1
RRð1 RRÞ dðRRÞ MRÞ þ 1 SR0f RR RR2
(13)
The right-hand side of the above equation can be analytically integrated, which yields:
(8)
The recovery ratio is a measure of the fraction of water recovered from the inlet feed stream. Secondly, the Mass Transfer Units (MTU) is defined as [26]:
MTU ¼
SR0f ¼ bSRf
MTU ¼ RR SR0f
R1 RR R2 RR þ ln 1 ln 1 R1 R2 R1 R2 R1 R2 (14)
whereR1 and R2 are the roots of the quadratic equation with R1 being the positive root:
x2 ð1 aÞx ab ¼ 0 a ¼ SR0f
(15)
b ¼ 1 MR The physical significance ofR1 is that it represents the maximum achievable recovery ratio for a given membrane and inlet feed. The effectiveness of the mass exchanger can be defined as:
A. Kar, V. Bahadur / Energy 196 (2020) 117145
ε¼
RR R1
(16)
Putting everything together, the modified model is obtained as:
MTU ¼ εR1 a
R1 R2 R lnð1 εÞ þ ln 1 ε 1 R1 R2 R1 R2 R2 (17)
Fig. 3 illustrates the results of the model for multiple modified osmotic pressure ratios SR’f and membrane rejection (MR) values. The plot shows the operation curves at different modified osmotic pressure ratios with MRs of 100% and 90%. With a MR of 90%, the MTU is lesser than the one with a MR of 100%. This is expected as we are over-estimating the osmotic pressure with a MR of 100%; the required membrane area (quantified by MTU) will be lower for the case of 90% MR. In general, the influence of MR on MTU is larger (for any SR’f ) as the effectiveness increases. However, for low SR’f (eg. SR’f ¼ 0:25), the MTU is seen to be fairly insensitive to the MR. It is noted that all the results reduce to those derived by Banchik et al. [26]. in the limit MR/ 1. The above model was presently used to estimate the freshwater output of RO-based desalination of flowback water. The following section details the modeling framework for estimating freshwater output of the RO system, which is powered by natural gas. 4. Estimating freshwater output from excess gas-powered reverse osmosis of flowback water Presently, the energy requirements for RO are met by combustion of excess natural gas (from the wellhead) in a gas turbine, which then powers the pump in the RO system. The operating parameters for this system are listed in Table 1. Implementation of the presently developed ε-MTU model requires estimation of the effectiveness and the mass transfer units. Commercially, spiralwound membranes are widely used because of their high packing density of 500e1200 m2/m3 [28]. A typical industrial spiral wound module 20 cm diameter, and 1 m long with a membrane area of 30 m2 can handle feed water flow rates on the order of 1 kg/s [28]. Using a permeability constant of 3.61 g/m2-s-MPa (Table 1) and an applied membrane pressure of 10 MPa, the MTU is estimated to be
5
1.1. The effectiveness can be estimated from the non-dimensional curves in Fig. 3. For flowback water salinity in the seawater range, and a membrane rejection of 90%, the effectiveness is 0.85. In this work, we conservatively choose an operating effectiveness of 0.8, since operating at higher effectiveness will require high membrane areas which might not be feasible for flowback water applications. The efficiencies of the gas turbine and the pump are key parameters in determining the energy available for water treatment; this study utilizes a pump with an efficiency of 85% (Table 1). The efficiency of the gas turbine depends on the rated output power of the turbine. In the following section, we develop a framework for techno-economic analysis which utilizes micro to small-scale turbines with rated output capacities less than 600 kW and efficiencies ranging between 15e20%. Presently, we use a turbine efficiency of 18% (Table 1). The energy input to the pump per unit of natural gas is estimated as:
epump ¼ ED hGT
(18)
The feed water volume output of the pump per unit of gas utilized can be obtained from the energy input estimated by equation (19). The freshwater output per unit of utilized natural gas is estimated using Q which is the input to the RO module per unit of excess gas.
Q ¼
epump hpump Dp
(19)
The feedwater salinity (as measured by TDS) imposes limitations on the ability to implement RO. From a practical perspective, it is accepted [24] that RO can only treat water with a maximum TDS of 50,000 ppm. Hence, it was assumed that RO will only be implemented for flowback water streams with TDS levels less than 50,000 ppm. For higher TDS levels, the external pressure required to overcome the osmotic pressure exceeds the maximum tolerable membrane pressure [25]. Membrane fouling is also a significant problem at high TDS levels and membrane replacement can be expensive [24]. It is also important to note that this analysis assumes the flowback water to be of similar composition to that of sea water. While this is true to an extent considering that the composition of flowback water is dominated by chloride ions, the overall composition could vary significantly across different shale regions [31]. 4.1. Results- freshwater output
Fig. 3. ε-MTU curves for various modified osmotic pressure ratios. The solid lines represent the performance for a membrane rejection of 100%. Dotted lines represent a membrane rejection of 90%.
Fig. 4 shows the amount of desalinated water obtained per cubic meter of natural gas and the corresponding operating recovery ratio, as a function of the TDS level of flowback water. Feed water salinity is a key parameter in determining the overall performance of a RO membrane. Fig. 5 shows results till TDS levels of 50,000 ppm, beyond which RO will become infeasible. The amount of water recovered is plotted for applied membrane pressures of 6, 8 and 10 MPa. These are typical of the membrane pressures utilized in existing RO installations [28]. A higher membrane pressure reduces membrane fouling over time; however, it increases energy consumption and requires better membrane quality. For flowback water with TDS between 20,000e50,000 ppm, it is possible to obtain 0.3e0.6 cubic meter of freshwater per cubic meter of gas. Note, that the operating recovery ratio decreases linearly with higher salinities of the feed water, which implies a reduction in the volume of permeate water. The above analysis predicts the water production resulting from the use of excess gas for RO-based treatment of flowback water. To quantify the advantages of this concept it is essential to compare the water production with other technologies which also use
6
A. Kar, V. Bahadur / Energy 196 (2020) 117145 Table 1 Performance-related parameters for the RO system, pump and gas turbine. Parameter
Value
Reference
A ε
3.61 106 kg/m2-s-kPa 0.8
MR C
0.90 73.45 kPa-kg/g 1060 kg/m3 39 MJ/m3 18% 85%
[26] Estimated value for spiral-wound membranes (using the present model) Selected value [26]
rs ED
hGT hpump k
3 104 m/s
[32] [33] [26]
increases due to higher energy requirements. The key takeaway from Table 2 is that RO is the most effective technology for freshwater production, if the TDS levels allow its use. 5. Applications of the three water production technologies in US shale fields
Fig. 4. Water production from RO-based flowback water treatment versus feed water salinity, for various applied pressures.
natural gas for water production. Two such competing technologies are considered; thermal desalination (TD) and atmospheric water harvesting (AWH). Glazer et al. [6]. studied excess natural gaspowered TD of flowback water. Thermal desalination includes a broad class of methods such as Multi-Stage Flash (MSF), MultiEffect Distillation (MED), Mechanical Vapor Compression (MVC) and Membrane Distillation (MD). Glazer et al. [11]. studied the use of excess natural gas powered-MVR, as it can withstand higher TDS levels than other techniques. The concept of excess natural gas-powered AWH was developed by the present group and involves gas-powered refrigeration cycles to condense water from humid air [34]. The system uses a gas turbine to power the compressor of the refrigeration cycle, and the cooling produced is used to condense atmospheric moisture [2]. This technology does not require a water source, which is advantageous in some situations [17]. However, the water harvest depends strongly on ambient weather; condensation rates are higher in hot-humid conditions as compared to cold-dry conditions [18]. Importantly, as this work shows, AWH is much more energy intensive than RO and TD and can be attractive only in hot-humid climates where there is no flowback water. The amount of water produced per cubic meter of excess gas using RO, TD and AWH is listed in Table 2. RO will maximize water production, while AWH has the lowest water production. Clearly, the low specific energy consumption of RO results in significantly higher water production than the other two technologies. The range of water produced by TD and RO in Table 2 is due to varying recovery rates (salinity and process dependent). The amount of water recovered via RO reduces as the salinity of flowback water
The use of excess natural gas-powered RO, TD and AWH was analyzed for eight of the biggest US shale fields. All these fields face challenges associated with flaring/venting gas, flowback water disposal and freshwater availability. These fields are shown in Fig. 5 and are Bakken, Niobrara, Marcellus, Utica, Permian Basin, Barnett, Eagle Ford, and Haynesville-Bossier. Notably, four of these fields are in Texas, which is responsible for significant US flaring and has significant water-related challenges. In this study, the Marcellus and Utica are analyzed together due to their geographic continuity, which is a common practice. Data on flared gas volume, flowback water volume and flowback water quality for the first seven shale fields was obtained from Glazer et al. [11]. Data relevant to the Barnett shale (Texas) was acquired from multiple sources. Flared gas volumes in Texas are reported by the Railroad Commission of Texas (RRC) [35] and the Energy Information Administration (EIA) [36]. The quantity and quality of flowback water was obtained from Environmental Protection Agency (EPA)’s compilation [37] of FracFocus data [38]. Next, the criteria for deciding the use of specific water production techniques (between RO, TD and AWH) for these shale fields is outlined. As per Table 2, RO should be the preferred technique owing to its highest freshwater yields. However, RO can be used only with flowback water streams with TDS levels less than 50,000 ppm. TD will work for TDS levels between 50,000 and 200,000 ppm. Water with TDS levels higher than 200,000 ppm is too saline for treatment using conventional techniques [11]. AWH can be attractive in such scenarios. While the use of AWH does not solve issues related to flowback water, it can still utilize the excess gas to produce onsite freshwater. 5.1. Results of analysis of water production technologies in US shale fields Table 3 shows the total annual volume of flowback water, the average TDS level and excess natural gas volumes for the eight shale fields of interest. It is noted that the quality and flowrate of flowback water and excess gas production vary within these fields. Table 3 shows that average TDS concentrations exceed 80,000 ppm in the Marcellus, Utica, Haynesville-Bossier, Permian Basin and Barnett shale fields [11]. The use of RO is ruled out and TD processes like MVC can provide desalination solutions. Average TDS concentrations of the flowback water from the Eagle Ford and Niobrara shales are below 50,000 ppm [11]. RO can be used in these fields and should be the preferred option compared to TD. Flowback
A. Kar, V. Bahadur / Energy 196 (2020) 117145
7
Fig. 5. Location of the eight shale fields considered in the present study.
Table 2 Comparison of water production using three technologies which use excess natural gas. Method
Water produced per m3 gas
Reverse osmosis Thermal desalination Atmospheric water harvesting
340e760 L 115e380 L 2e10 L
water from the Bakken shale is hypersaline with TDS levels exceeding 250,000 ppm [11]; this precludes the use of conventional desalination techniques. Importantly, the Bakken accounts for 40% of the total US gas flaring [3]. The significant volume of excess gas can make it economically viable to implement AWH. It is noted AWH is very energy intensive and can be used only in the summer
for the climate in the Bakken. However, it can still generate significant quantities of freshwater in relation to the amount of water used for hydraulic fracturing operations [17]. Table 3 shows the freshwater production achievable in each of the eight shale fields, based on the quantity/quality of flowback water and availability of excess natural gas. The amount of freshwater generated using RO is estimated using the model developed in this work. The freshwater obtained using TD and AWH was obtained from Glazer et al. [11]. and Ozkan et al. [42], respectively. Importantly, our analysis shows that the use of excess gas for flowback water treatment will not lead to 100% gas utilization or treatment of 100% of flowback water. Instead, these numbers depend on the relative volumes of excess gas and flowback water. Table 3 shows the unused gas, and untreated flowback water in each field. As an illustration, RO-based treatment in the Eagle Ford
Table 3 Summary of the performance of various water production techniques for eight shale fields in the US. The volumes reported are cumulative annual volumes. RO, TD and AWH represent reverse osmosis, thermal desalination and atmospheric water harvesting, respectively. Shale field
Average TDS of flowback water (ppm)
Flowback water Excess gas Technology Freshwater production (million m3), Remaining wastewater (million (million m3) (% of total water requirement) m3), (% of initial flowback water) (million m3) used
Unutilized gas (million m3)
Bakken Marcellus/ Utica Eagle Ford Niobrara HaynesvilleBossier Permian Basin Barnett
250000 130,000
51.6 5.5
2730 449
AWH TD
(6.7), (26%) (2.75), (5%)
(51.6), (100%) (2.75), (50%)
None 417.3
40,000 25,000 120,000
252.6 3.3 48
1060 160 17
RO RO TD
(145.2), (200%) (2.2), (23%) (1.7), (34%)
(107.4), (42%) (1.1), (33%) (46.3), (96%)
645.2 155.4 None
140,000
686.4
621
TD
(58.8), (149%)
(627.6), (98%)
None
85000
24.3
310
TD
(12.15), (137%)
(12.15), (50%)
188.5
8
A. Kar, V. Bahadur / Energy 196 (2020) 117145
shale will generate 145.2 million m3 of freshwater from an initial 252.6 million m3 of flowback water. The concentrated waste stream from the RO process, which is 107.4 million m3, still needs to be disposed. Note that the volume of water that needs disposal is only 42% of the initial volume. 223.4 million m3 of gas will be utilized and 836.6 million m3 of gas will be left unutilized. Overall, gas availability exceeds (in a relative sense) the flowback water quantity in the Eagle Ford shale. The situation is reverse in the Bakken, Haynesville-Bossier and Permian Basin, where the flowback water is in excess (relatively), and 100% of excess gas is consumed to run TD processes. In such places, wastewater will be left behind, which consists of untreated flowback water and the concentrated waste stream from the TD process. Table 3 also quantifies the amount of freshwater generated as a percentage of the total freshwater requirement in the shale play. The total freshwater requirement was obtained from a recent report by Freyman [7]. Data for the Haynesville/Bossier and Barnett shale fields was not available in this report. The freshwater requirement was therefore estimated from the total number of drilling permits issued annually multiplied by the average water consumption per well. Data on the total number of drilling permits was obtained from the Railroad Commission of Texas [35]. Table 3 shows that the use of waste gas for flowback water treatment can produce significantly more freshwater than current requirements in the Eagle Ford (via RO), Permian Basin (via TD) and the Barnett (via TD) shales. This concept is particularly attractive for the Eagle Ford shale and can produce twice the freshwater requirements. Water production is also significant in the Niobrara (via RO) and Haynesville-Bossier (via TD) shales. Table 3 also reports the remaining wastewater as a percentage of the initial wastewater volume. It is seen that wastewater volume is reduced by at least half in Marcellus/Utica (via TD), Eagle Ford (via RO), Niobrara (via RO) and Barnett (via TD) shales, which will significantly reduce disposal-related costs. As an extension of the current concept, it is noted that the large volumes of unutilized gas in the Eagle Ford, Marcellus/Utica, Barnett and Niobrara shales can be used to power AWH cycles to further produce freshwater. While this can be an attractive proposition, the implementation of two separate water production projects at one location might be challenging from an economics and logistics standpoint. Another possible application of the unutilized gas is its use in RO-based treatment of brackish groundwater to produce freshwater for oilfield operations. While there are other such applications and technologies, final decisions depend on techno-economic analysis of the suite of technologies discussed in this work. The next section includes a detailed techno-economic assessment of excess gas-powered RO-based treatment of flowback water for the Eagle Ford and Niobrara shales. 6. Techno-economic analysis of reverse osmosis-based desalination of flowback water The economic viability of any RO-based flowback water treatment project will be contingent on a favorable techno-economic assessment. This section details a techno-economic study of excess natural gas-powered RO-based flowback water treatment for the Eagle Ford and Niobrara shales. It is noted that the present group conducted a similar study [2] on landfill gas-powered AWH for water production for oilfield operations. While several techniques can be employed to estimate the economic benefits of a project, this work uses the payback period (PBP) as the criteria of interest. The PBP is the time required to recover investment and is widely used in decision making. A shorter PBP indicates a faster return on investment and reduced risk, which is attractive to investors.
Presently, analysis was conducted from the standpoint of an enterprise developing and using the infrastructure to treat flowback water via excess gas powered RO, and supplying water to nearby oilfields. Separate analysis was conducted for the Eagle Ford and Niobrara shales. Presently, shale operators source water (via pipelines or trucking) and truck the flowback water to disposal sites. Large transportation distances imply significant costs [39] associated with water management, given the large volumes of freshwater and flowback water being handled. A new enterprise which provides complete water management services (water supply, flowback water handling and disposal) and excess natural gas handling service would be of interest to present-day shale operators if the costs associated with water and excess gas management were lowered. The concept introduced in this study can be technologically deployed via the development of mobile, small-scale desalination (RO-based) units. Having mobile infrastructure is key due to the short-term nature of both excess gas and flowback water production. Centralized water treatment facilities in the shale plays, while technically attractive will not be economically viable owing to the costs of trucking water from individual wellsites (or using pipelines to transport water). Mobile units offer the advantages of onsite water treatment, and onsite water production, which is attractive from a logistics standpoint. Importantly, small-scale mobile desalination units are commercially available from several water management companies. Next, we discuss the capacity and costs associated with mobile RO-based desalination. Current mobile RO trailer units are primarily designed [40] to treat brackish water, with TDS levels less than 10,000 ppm. However, it is possible to custom-make trailerbased units [40] for treating water with salinity close to that of seawater (TDS level >30,000 ppm), and to integrate a small-scale gas turbine. Such trailer-based desalination trailer units can have capacities upto 1000 gallons per minute (gpm). Based on the volume of flowback water, the trailer capacity for use in the Eagle Ford and the Niobrara shales was selected as 400 gpm and 50 gpm, respectively. For the economic analysis we obtained [40] an estimate of $ 2.5 million for a custom-made 400 gpm capacity trailer (targeted at Eagle Ford), excluding the costs of the gas turbine and its integration. For the 50 gpm trailer (targeted at Niobrara), the price was estimated to be $ 1 million [40]. It is noted that these costs are conservative estimates. Also, multiple such mobile units will be needed to treat flowback water in the entire shale play. Based on flowback water volumes, it is estimated that 365 and 45 units will be needed in the Eagle Ford and Niobrara, respectively. Also, a utilization efficiency of 50% is assumed in this study (unit is used for 50% of time). All these numbers are detailed in Table 4. It is important to note that the waste stream from the desalination process is included in the total wastewater disposal volume, and is accounted for in the analysis. The cost of the gas turbine unit was estimated using a handbook [32]. Desalination units require turbines with rated output capacity ranging from 50 to 600 kW, depending on the salinity of flowback water and the capacity of the desalination unit. The rated output capacity falls in the class of micro to small-scale turbines which are compact-enough to be mounted on a trailer and integrated into a desalination unit. The running and maintenance costs of the desalination unit and the gas turbine were assumed to be 20% of the capital cost. This is a conservative estimate as other studies [41] on maintenance costs of RO plants and turbines showed maximum costs of 15% and 12% of the capital cost, respectively. All these parameters are listed in Table 4. The revenue for the water treatment enterprise depends on the price charged for desalinating flowback water and providing freshwater to the shale operator. Since prices fluctuate, this study
A. Kar, V. Bahadur / Energy 196 (2020) 117145
9
Table 4 Key parameters and cost inputs for the techno-economic assessment of RO-based flowback water treatment in the Eagle Ford and Niobrara shales. These numbers correspond to a single mobile RO unit powered by excess gas. Parameter
Eagle Ford
Niobrara
Details
Value
Details
Capacity e 400 gpm
Cost of mobile desalination unit
$ 2.5 Capacity- 50 gpm million [40] Calculated using TDS levels of 40,000 ppm and 530 kW Calculated using TDS levels of 25,000 ppm and capacity of RO unit. capacity of RO unit. $1000/kW [32] $ 0.53 $ 1400/kW [32] million 10% of total cost of turbine & desalination unit $ 0.3 10% of total cost of turbine & desalination unit million Onsite equipment $ 0.1 Onsite equipment million $ 3.43 million
Capacity of gas turbine Cost of turbine System integration cost Infrastructure for handling excess gas Total capital cost per unit
Operating and maintenance costs
20% of total capital cost
$ 0.69 million Depreciation 10% of capital cost $ 0.34 million Operator-related expenses per unit 5 operators per unit with mean compensation $ 0.5 of 100,000 $ per year million Total operating, maintenance and depreciation $ 1.33 costs per year per unit million Unit utilization efficiency Assumption 50% Total number of units across shale region 365 Maximum freshwater that can be produced Capacity of RO trailers Utilization Efficiency Discount rate of project 10%
Value $ 1 million [40] 47 kW $ 0.07 million $ 0.11 million $ 0.1 million $ 1.28 million
20% of total capital cost
$ 0.26 million 10% of capital cost $ 0.13 million 5 operators per unit with mean compensation $ 0.5 of 100,000 $ per year million $ 0.69 million Assumption 50% 45 Maximum freshwater that can be produced Capacity of RO trailers Utilization Efficiency 10%
quantifies the benefits for various price levels. The freshwater production rates were estimated using the model described in Section 4. The payback period T for this RO-based project can then be estimated by solving the below equation:
0
T X
It¼j
j¼0
ð1 þ rÞj
@Cc þ
T X Mt¼j þ Dt¼j j¼0
ð1 þ rÞj
1 A¼0
(20)
where It¼j, Mt¼j, and Dt¼j represent the income in a specific year, the maintenance costs and the depreciation, respectively. Cc is the capital (initial) investment in the project. A discount rate r of 10% was presently used and is typical of desalination projects [43]. 6.1. Results of techno-economic analysis Fig. 6 shows the PBP for RO-based flowback water treatment in the Eagle Ford and Niobrara shales as a function of the price charged for treatment (and consequent freshwater production) of flowback water. Firstly, the results indicate that the economic viability of this technology is more favorable in the Eagle Ford shale than the Niobrara shale. The Eagle Ford has much higher volumes of flowback water and excess gas as compared to the Niobrara, which translates to better economic benefits. The payback period is just over a year for prices of 1.1 cents/liters in the Eagle Ford. This price is much lower than current costs associated with flowback water disposal and freshwater sourcing in the Eagle Ford. Wastewater disposal averages 1.5 cents/liter [9] and freshwater costs average 0.9 cents/liter in the Eagle Ford [39]. Shale operators are therefore currently paying an average of 2.4 cents/liter to obtain a liter of freshwater and dispose a liter of flowback water. With this background, payback period of a year or less is a very attractive proposition from an investment standpoint. Overall, this analysis suggests that this technology has significant economic potential in the Eagle Ford which is responsible for a significant fraction of US
Fig. 6. Payback period versus the price charged for RO-based flowback water treatment projects in the Eagle Ford and Niobrara shales.
flaring and where freshwater sourcing can be a deal breaker. The PBPs for the Niobrara shale are noticeably higher than those in the Eagle Ford; a 3-year PBP corresponds to a price of 1.8 cents/ liter. While there is no publicly available reliable data on the flowback water disposal and freshwater procurement costs in the Niobrara, it is expected to be higher than 1.8 cents/liter. In most shale fields like the Marcellus, Bakken and Permian Basin, flowback water disposal costs are much higher than those in the Eagle Ford and are at least 1.6 cents/liter [39]. Freshwater procurements costs typically exceed 1.3 cents/liter [9]. Based on these discussions, a price of 1.8 cents/liter (with a 3 year PBP) represents an attractive
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economic proposition for the Niobrara shale. It is noted that this analysis does not consider other factors which will increase the value proposition of this technology. While flowback water disposal is carefully regulated, there are usually no financial repercussions on venting or flaring gas. Any future regulatory push towards taxing emissions will enhance the economic proposition of this emissions reducing technology. This technology also offers several intangible benefits such as reduced water disposal requirements, reduced pressure on surface and groundwater resources, reduction in pollution (from flares) and reduced truck traffic. The carbon footprint associated with oil-gas production can be reduced via large scale adaptation of this technology. All such benefits are not easily monetized, but they contribute towards decision making of projects. 7. Challenges associated with RO-based treatment of flowback water From a technology standpoint, the proposed concept can be implemented using existing technology, with all components of the system in Fig. 1 being commercially available. Treatment and reuse of flowback water is increasingly becoming a common practice, and the use of excess natural gas to power RO systems will be of interest to the vast and growing water management industry that caters to shale operators. It is noted that while the analysis focused on the Eagle Ford and the Niobrara shales (owing to lower salinities of flowback water in these fields), this technology can be used in other shale plays where the local TDS levels permit the use of RO. While the low specific energy requirement of RO gives it a competitive advantage over other desalination methods [24], the current technological state of RO makes desalination of water with TDS levels higher than 50,000 ppm unfeasible [25]. This is a major limitation to the adoption of RO in many shale fields, which have much higher TDS levels. There do exist methods which allow the RO process to tolerate higher TDS levels like Vibration Shear Enhanced Processing (VSEP), RO integrated with nanofiltration and microfiltration (RO-NF, RO-MF) and others [44]. There is significant ongoing research on novel membrane-based technologies and improvements in the conventional methods to tolerate higher TDS levels [45]. Another challenge for flowback water treatment using RO is the handling of organic contaminants, which are difficult to remove using conventional pre-treatment methods like coagulation or flocculation, and lead to membrane fouling [24]. Also, the composition of flowback water varies across different wells which increases the challenges associated with membrane design. 8. Conclusions Water management-related issues can be a deal-breaker for future development of shale resources, and this study proposes a novel solution which can impact the energy, water and environment sectors. The first part of the study develops an analytical model to quantify the performance of RO-based flowback water treatment systems, which are powered by onsite excess natural gas. Comparison with other freshwater production techniques clearly highlights the low specific energy requirements of RO (upto 760 L of freshwater per m3 of gas). The TDS level requirements (<50,000 ppm) of flowback water to be compatible with RO impose significant restrictions on its geographic applicability. However, in shale plays where RO is technically feasible, freshwater production can be very significant (eg. 200% of water requirements of the Eagle Ford). Importantly, the techno-economic analysis indicates that RObased projects would be economically attractive (payback period: 1 year), owing to the high current costs of wastewater disposal and freshwater procurement. Overall, this waste-to-value conversion
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