Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach

Applied Energy xxx (2017) xxx–xxx

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Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach Zheng Rong Chong a, Zhenyuan Yin a,b, Jun Hao Clifton Tan a, Praveen Linga a,⇑ a b

Department of Chemical and Biomolecular Engineering, National University of Singapore, 117585, Singapore Lloyd’s Register Global Technology Centre Pte Ltd, 138522, Singapore

h i g h l i g h t s
Water saturated hydrate bearing sediment was depressurized to 5.0–2.1 MPa.
Cumulative gas production increased with decreasing bottom hole pressure.
Aqueous product was produced continuously at higher bottom hole pressure.
Thermal buffering at subzero temperature enhanced gas production.

a r t i c l e

i n f o

Article history: Received 11 January 2017 Received in revised form 27 March 2017 Accepted 13 April 2017 Available online xxxx Keywords: Energy recovery Methane hydrates Unconventional gas Hydrate bearing sediment Depressurization Gas production

a b s t r a c t A huge amount of natural gas hydrates remains untapped in permafrost and continental margin. While several short term field production tests have been carried out, the underlying challenges during hydrate dissociation in porous media, such as the interdependent production behavior of gas and water, is still not well understood. In this work, we employed depressurization technique to recover natural gas from a water saturated hydrate bearing sediment (40% SH, 50% SA and 10% SG) at 281.5 K surrounding temperature. During depressurization, the bottom hole pressure (BHP) was maintained at constant pressures of 5.0, 4.0, 3.0 and 2.1 MPa respectively to evaluate its effect on gas and water production. As expected, a higher BHP (corresponding to a lower dissociation driving force) resulted in a slower gas and water production. At a BHP of 2.1 MPa, thermal buffering was observed below ice point (272.7 K), accompanied by enhanced gas production. By lowering BHP from 5.0 MPa to 2.1 MPa, the percentage methane produced increased from 45.5% to 83.0%; whereas the cumulative water production decreased from 217 mL to 157 mL. The difference in gas and water production was attributed to the preferential production of aqueous phase at higher BHPs (5.0 and 4.0 MPa) nearing the end of hydrate dissociation whereby less hydrates were present. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Clathrate hydrates are inclusion compounds comprising of guest molecules and water, whereby the guest molecules are entrapped within the ice-like crystalline structure formed by water molecules under low temperature and high pressure conditions [1–3]. The most abundant species of clathrate hydrates in nature is methane hydrate, which is found in the continental margin and permafrost [1,4]. While there are uncertainties on the exact amount of methane hydrates in nature, the amount of gas that is sequestered in hydrate form is estimated to be around 3000 TCM, which is about an order of magnitude higher than the esti⇑ Corresponding author.

mate for conventional gas [4,5]. Driven by the abundance of hydrate resources, several governmental agencies, research institutes and industrial corporations have formed research programs to conduct hydrate coring and production tests in various parts of the world since 1990s [6–8]. Although field production tests have been conducted to elucidate the production behavior from naturally occurring hydrate reservoirs, there are several limitations, including the extensive amount of time, labor and cost required and the operational difficulties to control the production conditions [7,9]. Owing to these challenges, the field production tests to date are typically short-termed (<6 weeks) [5,7], and the results acquired were used to match with reservoir simulators to improve the reliability of simulators in modelling hydrate dissociation [10].

E-mail address: [email protected] (P. Linga). http://dx.doi.org/10.1016/j.apenergy.2017.04.031 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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Nomenclature C Nhyd n P R

q

S T t t90 V Vm x z

molar density hydration number number of moles pressure universal gas constant mass density saturation temperature time time taken to recover 90% of product volume molar volume of gas at STP fractional conversion of methane compressibility factor

To recover energy from hydrate bearing sediments (HBS), the commonly cited approaches are depressurization, thermal stimulation, inhibitor injection and gas exchange. Among these approaches, depressurization is regarded most promising due to its high energy efficiency [11]. This approach has received significant research attention over the years, and the production profile via depressurization have been investigated from experimental works [12,13], numerical simulation [14–16] and field production tests [17,18]. Experimental depressurization studies in the open literature have been conducted in different procedures, scales [9,19–21] and hydrate contents [12,13]. In an early study, Yousif et al. investigated the dissociation kinetics of hydrates in Berea Sandstone through depressurization and developed an analytical model to describe hydrate dissociation as a moving boundary problem [22]. Through the measurement of electrical resistance and the inlet and outlet pressures of a cylindrical reactor, Sung et al. reported that the mobility of gas is increased after the gas saturation is above a critical limit [23]. To acquire in-situ information during methane hydrate dissociation, Kneafsey et al. employed Xray computed tomography to elucidate the density changes within a 26.7 cm length, 7.62 cm diameter core [24]. The density changes during hydrate dissociation revealed that hydrate dissociation occurred in a radially inward direction; and water moved from near wall region to central region due to capillarity. In the past decade, several middle (1 to 10 L [21,24–28]) and large scale reactors (>10 L [9,12,13,20]) were built in various institutions to investigate the dissociation behavior of hydrates in large scale sediments with the hope of simulating flow limited production cases similar to those in hydrate reservoirs. In 2009, Zhou et al. demonstrated the capability of forming and dissociating hydrates in a high pressure reactor with an internal volume of 59.2 L [13]. The study reported the formation of 11% hydrates in the sediment pore space, with hydrates first formed from the top of the reactor (near the gas cap) and progressed to various locations within the sediment. During depressurization test, the authors observed an increase in gas production within a transition regime below 273.2 K [13], which is in line with the mathematical model prediction by Tsypkin [29]. From 2011 onwards, Li and his co-workers performed a series of investigations from a 5.8 L cubic hydrate simulator (CHS) [21,30,31] and a 117.8 L pilot-scale hydrate simulator (PHS) [20,32–36] to elucidate the production profile from HBS under various production schemes, well configurations, phase saturations etc., yielding important information for the development of technology to recover energy from HBS. During the production phase in some studies, the depressurization period

Subscript/Superscript A aqueous phase CR crystallizer CP constant pressure DP depressurization G gaseous phase GR gas receiver H hydrate phase j jth injection stage m methane w water

was differentiated into two stages: the depressurizing stage (tDP) which comprises the time interval from the final pressure of hydrate formation to the preset bottom hole pressure (BHP); and steady pressure stage (tCP) [33,35]. The depressurizing stage for large scale reactors typically takes more than 30 min [20,32–36]. By comparing the results obtained from PHS and CHS, Li et al. postulated that the scale of the hydrate simulator affected the duration of the gas production during constant pressure period significantly (tCP), but has little effect on tDP as tDP is affected by the depressurization rate [20]. In 2014, gas production from the world’s largest hydrate simulator to date, High-pressure Giant Unit for Methane-hydrate Analyses (HiGUMA) with an internal volume of 1710 L was investigated by Konno et al. [9]. The authors reported that the gas production stabilized at an undesirably low rate after sensible heat was consumed, and ‘‘deep depressurization” below ice point can significantly enhance gas recovery factor. In a recent study conducted by Heeschen et al. [12], a large-scale reservoir simulator (LARS) of 210 L capacity was used to simulate gas production through multistage depressurization of high hydrate saturation HBS (90%) similar to that of Mallik permafrost hydrate site. The large capacity of the LARS allowed the depressurization experiments to simulate production behavior of HBS similar to that obtained from Mallik site test. It was reported that while the experimental data closely matched the gas production profile from Mallik test at low and moderate levels, they differed at high rates of gas production, possibly due to the absence of water flux in LARS which affected the flow behavior. Another interesting aspect that is not well elucidated is the effect of ice formation on the production behavior from HBS. The formation of solid ice during production stage can reduce the permeability of the sediment, making depressurization more difficult to achieve. On the other hand, the dissociation heat of hydrates in ice formation regime (qhi) is around three times lower than that in water formation regime (qhw) due to heat released from ice formation [29]. As less heat is required to dissociate hydrates, the gas recovery can be enhanced. Before the effect of ice formation was elucidated experimentally, Tsypkin formulated a mathematical model of hydrate decomposition taking into account ice formation and reported a sharp increase in gas recovery within a transition regime whereby both ice and water were produced simultaneously [29]. The enhancement was due to an anomalous increase in the velocity of the dissociation front when hydrate-water regime is replaced by hydrate-ice regime [29]. A number of works reported that the formation of ice, or a subzero temperature condition in

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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the core sample, promotes hydrate dissociation [9,29,35,37], while a few other studies reported that formation of ice reduced the rate of gas production/hydrate dissociation [27,38,39]. To evaluate long term production scenarios in hydrate reservoirs, it is essential to improve the reliability of numerical models through well controlled laboratory conditions like bottom hole pressure during depressurization. In this work, we first synthesized water saturated HBS in sand, the porous media of highest recoverability [4], of similar phase saturations. Upon forming quantitatively similar HBSs, we decomposed these HBSs under a constant production pressure of 2.1 MPa, 3.0 MPa, 4.0 MPa and 5.0 MPa with a constant surrounding temperature of 281.5 K to elucidate the effect of pressure on the gas and water production profile during hydrate decomposition. To simulate a constant pressure production scenario, the transition interval whereby pressure was brought down to the BHP (tDP) is minimized. In addition, the lowest pressure studied in this work (2.1 MPa) is below the quadruple point of methane hydrates, enabling us to investigate the production behaviors from water saturated HBS under a subzero temperature condition.

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troller (OMEGA CN2120), controls the pressure of the HBS during depressurization and distinguishes the Crystallizer Compartment from the Receiver Compartment. A 60 lm filter was included in the crystallizer outlet to prevent sand production into the Receiver Compartment. The Receiver Compartment consists of a 400 ml gasliquid separator (GLS) made of stainless steel and a 1 L gas receiver (GR) made of 316 Stainless Steel. The GLS is designed to collect liquid water during the production phase while allowing gas to flow towards the GR. A KERN 572-57 precision balance (±0.1 g) is used to quantify the amount of water collected within the GLS. The GR is designed to collect the methane gas produced during the depressurization phase. Another Rosemount pressure transmitter and thermocouple are used to measure the pressure and temperature of the GR respectively. A Vacuubrand MZ 2C NT vacuum pump is used to ensure the GR is in a state of vacuum before the depressurization stage. The data acquisition system from National Instruments, along with the LabView software, is used to record data from the experimental setup into a personal computer.

3. Experimental procedure 2. Experimental section 3.1. Methane hydrate formation 2.1. Materials The gas used in this study is CH4 at 99.9% purity, supplied by Air Liquide Singapore Pte Ltd. Unconsolidated silica sand with particle size ranging from 0.1 to 0.5 mm is supplied by River Sands (W9). Deionized water is applied to create the excess water environment during hydrate formation. 2.2. Experimental apparatus The apparatus used in this study is the same as our previous work [40]. Fig. 1A presents an illustration of the apparatus setup. The setup consists of a 980 ml fixed bed crystallizer, made of 316 Stainless Steel, with two Omega copper-constantan T-type multipoint thermocouples (±0.1 K) installed to acquire temperatures at various locations in the crystallizer (Fig. 1B). Pressure measurements at the top and bottom of the crystallizer (PCTop and PCBtm) are acquired by two Rosemount SMART pressure transmitters (±15 kPa). Water injection during the hydrate formation phase is done via a Teledyne ISCO, 500D syringe pump. A valve (V3) and a Fisher–Baumann control valve, together with a self-tuned PID con-

Under excess water hydrate formation approach, the crystallizer was first filled with dry porous media (silica sand) and a predefined amount of gas was loaded according to the desired hydrate saturation. Subsequently, water was added in excess to the crystallizer [41–43]. Accordingly, 980 mL crystallizer was first filled with 1480.5 g of dry sand and connected to the lines as shown in Fig. 1A. Subsequently, the crystallizer was purged with methane gas to a pressure of 1 MPa three times (by opening and closing of V1 and vent valve) to ensure the gas occupying the void spaces within the crystallizer was purely methane. The crystallizer was then pressurized to 6.3 MPa (Point A in Fig. 2) with methane gas to ensure the predefined amount of methane (1.53 mol) was introduced. The crystallizer temperature was set to 288.2 K, which is outside the hydrate stability zone, and the system was left to stabilize. After the pressure of the system had stabilized, water was injected into the system at 50 ml/min via V2 until a pressure of 9.5 MPa was reached (Point B). Subsequently, the crystallizer was cooled to 274.2 K to initiate hydrate formation. During this period, pressure drop was observed along with some temperature spikes, which were indicative of methane hydrate formation (B to C). Once

Fig. 1. Schematics of (A) the block flow diagram for experimental apparatus applied in this study and (B) the locations of temperature measuring points within CR. Modified from Chong et al. [40].

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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Fig. 2. Schematics of pressure-temperature trajectories during (A) hydrate formation in excess water environment; and (B) hydrate dissociation via depressurization.

the pressure drop within the crystallizer levelled at 10 kPa/h, subsequent round of water injection was performed, pressurizing the crystallizer to 9.5 MPa once again. Three rounds of water injection were performed to deliver the predetermined amount of water (412 mL). After the third injection, when the pressure of the crystallizer approached 8 MPa (Point E), the crystallizer temperature was gradually raised to 281.5 K (Point F). Once the temperature and pressure within the crystallizer stabilized at the predetermined pressure (i.e. the desired saturation was achieved), the hydrate formation phase was deemed complete.

3.2. Methane hydrate dissociation Prior to the depressurization stage, the receiver section was evacuated by vacuum pump to ensure the receiver section was free of residual air. After the completion of hydrate formation stage, the pressure of the crystallizer was slowly decreased by opening V3 and the control valve. The control valve was set to 6.1 MPa (Point G in Fig. 2B) to slowly decrease upstream pressure within 0.16 h prior to the start of hydrate dissociation. During this period,

hydrates were stable as it was within hydrate stability zone (Peq at 281.5 K is 6.0 MPa), and only minute amount of gas and water was produced. At the end of 0.16 h, the crystallizer was immediately depressurized to various bottom hole pressures across different runs ranging from 2.1 MPa to 5.0 MPa (corresponding to runs S - P), controlled by the control valve. The temperature and pressure data were logged every 10 s, along with the water production data from the weighing balance. The dissociation experiment continued until no observable increase in gas (1 kPa/hr) and water production (<0.1 g/hr) was detected. 3.3. Methods of calculation 3.3.1. Methane hydrate formation As mentioned in Section 3.1, a predetermined amount of methane gas was introduced into the crystallizer for hydrate formation under excess water approach. The initial amount of moles of methane gas, nm,0 is calculated using Eq. (1):

nm;0 ¼

Pc;av g V CR zRT c;av g

ð1Þ

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where PC,avg is the average pressure between PCTop and PCBtm in the crystallizer, VCR is the volume of the crystallizer that is occupied by methane gas, z is the compressibility factor estimated at PC,avg and Tc,avg using the Pitzer correlations, R is the ideal gas constant and Tc,avg is the average of the temperatures measured by the thermocouples within the crystallizer. During hydrate formation, water was injected into the crystallizer at various points in time. The amount of moles of water injected in the jth stage is calculated using the recorded volume of water injected, Vw,j, and Eq. (2):

nw;j ¼

qw V w;j 18:015

ð2Þ

The total amount of moles of water injected, nw,total is the sum of the amounts of water injected at each stage:

nw;total

j X qw V w;j ¼ 18:015 0

ð3Þ

During methane hydrate formation, methane within the crystallizer exists in one of three phases: as free gas (nGm), dissolved in aqueous phase, (nAm) or converted into hydrates (nH m). Assuming negligible amount of water is present in the vapor phase, water within the crystallizer were distributed in aqueous phase (nAw) or hydrate phase (nH w). The mole balance of methane and water can then be calculated as shown in Eqs. (4) and (5) respectively:

nHm þ nmA þ nGm ¼ nm;0

ð4Þ

nHw þ nwA ¼ nw;total

ð5Þ

The amount of moles of methane hydrate formed at the end of each stage can be calculated using Eq. (6):

nH;j ¼ nHm ¼ xj nm;0

ð6Þ

where xj is the fractional conversion of methane into methane hydrates at the end of stage j. During hydrate formation, the amount of moles of water consumed is dependent on the hydration number, Nhyd, which is defined as the ratio of water molecules to methane molecule in the hydrate crystal. Using Eq. (7), the amount of moles of water present in the hydrate phase can be calculated:

nHw;j ¼ Nhyd xj nm;0

ð7Þ

The hydration number used in this study is 6.1 [44,45]. Using Eq. (5), the amount of moles of water in aqueous phase, nAw, can then be calculated. The unreacted methane in the crystallizer exists in either one of the two remaining phases – gaseous or aqueous phase. In this study, the amount of moles of methane dissolved in aqueous phase was estimated from hydrate-liquid equilibrium model proposed by Duan’s group [46,47]. The amount of moles of methane present as free gas can be calculated using Eq. (8):

nGm;j ¼

P c;av g V G;j zRT c;av g

ð8Þ

Since Pc,avg and Tc,avg are measured variable during the formation run, VG,j is the only unknown on the right hand side of Eq. (8). As the system is isochoric in nature, VG,j, which represents the volume occupied by free gas in the crystallizer, can be obtained from Eq. (9):

V G;j ¼ V CR  V A;j  V H;j ¼ V CR 

A nw;j

Cw



nH;j CH

ð9Þ

where VCR is the total volume of the crystallizer and VA,j and VH,j are the volumes of aqueous and hydrate phases respectively. VA,j and VH,j are functions of xj as seen from Eqs. (6) and (7) while Cw and CH are the molar densities of water (0.0555 mol/cm3) and methane hydrates (0.00735 mol/cm3) respectively [48,49]. Solving Eqs. (4),

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(8) and (9) gives a unique value of xj, which is representative of the amount of hydrates formed in jth injection stage at the pressure and temperature conditions. This quantifies the amount of hydrate formed as well as other phases present at stage j. Upon successfully quantifying the volume of hydrates formed, we characterized the HBS using their phase saturation (i.e. volumetric ratio of phases within the void space in sediment). It was noted that a portion of the total volume of the crystallizer consisted of gas lines (Vtube), which will not form hydrates. Thus, Vtube is excluded from the calculations involving volume of the crystallizer. The reactive volume for the characterization of phase saturation is therefore the pore volume within the crystallizer, Vpore, which can be calculated by Eq. (10):

V pore ¼ V CR  V tube

ð10Þ

Saturation of a phase is defined as the volumetric ratio of the phase against the total pore volume of the crystallizer as can be seen in Eq. (11) and the relationship between the phase saturations is demonstrated in Eq. (12):

SH ¼

VH xnm;0 ¼ V pore V pore

SG þ SA þ SH ¼ 1

ð11Þ ð12Þ

Since gaseous and aqueous phase can present in both Vpore and Vtube, it is necessary to estimate the volume of each phase within each of the locations. The volumes of water and gas in pore space was estimated using Eqs. (13) and (14) respectively, assuming a proportional distribution of each phase between the two locations based on the ratio between the volumes of the pore and gas lines:

V A;pore ¼

V pore  V H VA V CR  V H

ð13Þ

V G;pore ¼

V pore  V H VG V CR  V H

ð14Þ

The saturation of aqueous and gaseous phases in the pore volume can then be calculated with Eqs. (15) and (16) respectively:

SA ¼

V A;pore V pore

ð15Þ

SG ¼

V G;pore V pore

ð16Þ

3.3.2. Methane hydrate dissociation During the process of hydrate dissociation, the gas and water produced passed through a control valve which maintained the crystallizer at the preset BHP varying from 2.1 MPa to 5.0 MPa across different runs. The mass of aqueous phase collected within the gas liquid separator was recorded with a weighing balance, and the amount of moles of methane gas within the receiver section, nGm,GR, can be quantified using Eq. (17):

nGm;GR ¼

PGR ðV GR  V A;GR Þ zRT GR

ð17Þ

where PGR was the absolute pressure of the receiver section, VGR was the total volume of the receiver section, VA,GR was the volume of aqueous phase collected in the gas liquid separator (assuming the density of water as 1 g/cm3), z is the compressibility factor of methane gas at PGR and TGR (the temperature of the receiver section), and R is the ideal gas constant. The amount of moles of methane gas, nGm,GR was then converted into standard volume, Vm,GR, according to the IUPAC definition of gas at standard temperature and pressure (STP, 273.15 K and 105 Pa).

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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V m;GR ¼ nGm;GR V m

ð18Þ

where V m was the molar volume of gas at STP (22.71 L/mol). The percentage of methane produced into gas receiver after depressurization is computed from Eq. (19)

CH4 Produced ð%Þ ¼

nGm;GR  100 nm;0

ð19Þ

To validate the results of the study, the total amount of moles of methane in the final state is computed and compared against the initial amount of moles of methane gas introduced during the formation phase. Having calculated the amount of moles of methane gas in the receiver section (nGm,GR), the amount of moles of methane gas left in the crystallizer, nGm,CR, can be quantified:

nGm;CR ¼

Pc;av g V G;CR zRT c;av g

ð20Þ

where PC,avg is the average pressure between PCTop and PCBtm in the crystallizer, z is the compressibility factor of the methane at Pc,avg and Tc,avg, and R is the ideal gas constant. The volume occupied by the methane gas, VG,CR can be calculated using the volume balance of the water within the system since both the aqueous phase volume collected in the receiver section, VA,GR, and the total amount P of water injected during the formation phase, 0j V W , are known:

V G;CR ¼ V CR 

j X V W þ V A;GR

ð21Þ

0

Using the NIST solubility correlation, the amount of methane gas dissolved in the gas liquid separator, nAm,GR, and the crystallizer, nAm,CR, can be calculated. Hence, the final amount of methane gas remaining in the system at the end of the dissociation phase (nm,f) can be calculated as the sum of the dissolved methane in the gas liquid separator and the crystallizer, the amount of moles of methane gas collected in the receiver section, nGm,GR, and the amount of moles of methane gas remaining in the crystallizer, nGm,CR: A A nm;f ¼ nGm;GR þ nGm;CR þ nm;GR þ nm;CR

ð22Þ

The percentage methane mole balance can then be computed using Eq. (23):

CH4 Balance ð%Þ ¼

nm;f  100 nm;0

ð23Þ

4. Results and discussion 4.1. Methane hydrate formation The experimental conditions, including the initial amount of methane injected, total amount of water injected and corresponding experimental results are summarized in Table 1. The evolution

of pressure and temperature during hydrate formation for experiment R1 is illustrated in Fig. 3. After the first injection stage, cooling of HBS from 288.2 K to 274.2 K (B to C) causes a significant hydrate formation, as evidently seen from the rapid pressure drop and temperature increase. The final pressure before second injection is 3.0 MPa, which is close to the equilibrium pressure at a formation temperature of 274.2 K. The decrease of pressure to the equilibrium pressure shows that the system is in an excess water environment, as opposed to water limited environment whereby gas in excess can maintain the pressure of the system above the equilibrium pressure [50,51]. Therefore, to further allow hydrate formation and simultaneously create water saturated sediment, water is further injected to pressurize the system to an elevated pressure of 9.5 MPa (C to D1). After the 2nd water injection, the unreacted free gas continues to form hydrates or to a smaller extent, dissolves in water, causing a continual pressure decrease. In Fig. 3, it can be observed that a discrepancy in the pressure readings between the top and bottom of crystallizer was detected at specific intervals (37–86 h), and the gap narrowed at 49th hr. Such observation implies that (1) hydrate formation reduces the permeability of the sediment, impeding the flow of free phases in the void volume and (2) hydrate distribution within the sediment may not be homogeneous – i.e. pockets/clumps of high hydrate saturation may be present within the crystallizer. The difference in pressure between measured locations during hydrate formation has also been observed in several runs of this study and other experimental works [12,22]. After second injection, when the crystallizer pressure drop was less than 10 kPa/hr, more water was injected to pressurize the system and further increase the aqueous phase saturation (E1 to D2). During this final injection, a small amount of water (20 mL) can cause a huge increase in pressure, which shows that the compressible volume in crystallizer, mainly contributed by presence of gaseous phase, is minimal. After 3rd water injection, further pressure decrement was observed due to hydrate formation or gas dissolution. Once the final gas pressure approached the predetermined set pressure, the sediment temperature was slowly increased to 281.5 K (E2 to F), mimicking the condition of 130 m below seafloor (assuming a surface water temperature of 277.2 K and a geotherm of 3 K/100 m [52,53]). The final phase saturation of HBS in different experimental runs are shown in Fig. 4. Similar to our previous work [40], the targeted saturations of the HBS is 40% hydrate, 50% aqueous and 10% gas. It can be seen that in our study, the deviation in phase saturation was within 1%. The deviation is mainly due to the fact that hydrate formation can occur in any region within the unconsolidated sand bed – some of which may cause the segregation of methane and water. For instance, the run with the lowest hydrate saturation was Q1, which could not achieve a 40% hydrate saturation despite a longer experimental duration. Although hydrate distribution cannot be measured directly in our current study, the formation of HBS of quantitatively similar phase saturations allows us to compare the production behavior under different BHP, as discussed in the next section.

Table 1 Summary of experimental conditions during MH formation and saturations of the hydrate bearing sediments. Exp. run

Exp. duration (hr)

nm,0 (mol)

P VW,j (mL)

xCH4

SH

SA

SG

P1 P2 P3 Q1 Q2 R1 R2 S1 S2

164.99 97.46 145.10 330.55 214.09 209.69 162.07 259.51 93.58

1.5343 1.5324 1.5336 1.5334 1.5324 1.5327 1.5325 1.5316 1.5330

413.10 414.68 412.25 411.20 411.34 412.55 416.94 413.23 411.98

82.23 83.66 83.40 80.25 83.10 82.25 83.18 82.04 83.16

40.3 40.9 40.9 39.3 40.7 40.3 40.7 40.2 40.7

50.0 50.3 49.4 50.5 49.4 49.9 50.4 50.2 49.5

9.7 8.7 9.8 10.2 10.0 9.8 8.9 9.7 9.8

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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Fig. 3. Pressure and temperature evolution during hydrate formation for experiment R1.

Fig. 4. Overall phase saturations of 9 samples synthesized in this study.

4.2. Hydrate dissociation at different bore hole pressures A summary of the experimental conditions and findings during the depressurization of HBS is tabulated in Table 2 and will be elaborated in this section. As schematically shown in Fig. 2B, after hydrate formation, the pressure of the crystallizer was first brought down to 6.1 MPa (Point G in Figs. 2B and 5), after which the crystallizer pressure was reduced to the desired BHP of

5.0 MPa, 4.0 MPa, 3.0 MPa, and 2.1 MPa. As shown in Fig. 5, time zero for dissociation experiment was the instance when the control valve was activated and BHP was reduced to the targeted pressure. The time interval before the HBS reached the constant production pressure is termed as tDP, whereas the duration for constant pressure production is termed as tCP. As can be seen from Fig. 5 and Table 2, the tDP in this study was around 1–5 min, which is minimal as compared to the duration of tCP (>3 h). The minimization of tDP is

Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031

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Table 2 Summary of the conditions and results for hydrate dissociation.

P1 P2 P3 Q1 Q2 R1 R2 S1 S2

BHP (MPa)

tDP (min)

tCP (hr)

CH4 produced (%)

Vm,GR (SL)

VA,GR (mL)

GWR (SCC/mL)

CH4 balance (%)

t90,G (hr)

t90,A (hr)

5.0 5.0 5.0 4.0 4.0 3.0 3.0 2.1 2.1

1.2 1.8 2.9 2.2 1.4 2.4 2.2 4.8 5.0

64.70 63.19 65.85 20.03 49.70 23.38 23.17 5.20 3.85

46.44 43.84 46.26 63.06 58.67 73.95 73.75 83.24 82.77

16.18 15.26 16.11 21.96 20.42 25.74 25.67 28.95 28.82

225.5 214.3 210.0 187.0 214.0 177.6 180.6 158.0 156.6

71.75 71.19 76.72 117.43 95.41 144.94 142.13 183.24 184.01

98.89 93.97 96.39 98.81 97.99 98.81 99.53 99.20 99.92

6.97 4.96 4.70 5.72 3.05 2.15 2.05 0.55 0.54

17.73 10.24 13.74 9.58 6.79 3.03 2.71 0.92 0.41

Fig. 5. Crystallizer pressure profile during the first 10 min of dissociation stage.

desired as the production profile captured is representative of the case when the HBSs were exposed to a constant BHP. In addition, the rapid decrease in sediment pressure achieved in this study is attributed to a low gas saturation, which has been discussed in a previous work [35]. In some previous experimental investigations, the presence of large gaseous phase posed challenges in controlling the production pressure during hydrate dissociation [37]. As the reduction of pressure causes the Joule-Thomson effect which leads to an instantaneous drop in temperature and potential formation of hydrates, the depressurization rate has to be carefully monitored in such systems. In addition, the gas production profile captured in these cases does not arise solely from hydrate dissociation, but also due to the flow of the free gas phase, which resulted in the conclusion that the rate of gas production is dependent on the depressurization rate – with faster depressurization rate resulting in a rapid gas production in the initial stage [30,39,54]. We also summarized the time taken to produce 90% of gaseous phase (t90,G) and aqueous phase (t90,A) products under each BHP in Table 2. 4.2.1. Pressure-temperature curve during hydrate dissociation The changes in pressure in the receiver compartment (PGR), amount of water collected (VA,GR), and temperature at various location within the crystallizer during the first 3 h for R1 and S1 are presented in Fig. 6. During the first 0.16 h whereby the crystallizer pressure was reduced to 6.1 MPa (light blue shaded area), only a minute amount of water and gas was collected in the gas receiver compartment. The temperature change within the crystallizer was mild during this period, indicating that a low saturation of gas was present in the crystallizer.

Right at the instance of the activation of control valve to the set BHP (t = 0 h), the temperatures at various locations within the crystallizer decreased instantaneously due to the consumption of sensible heat within HBS for hydrate dissociation. As expected, if the localized amount of hydrate is high, more sensible heat is consumed at this instance compared to those locations with less amount of hydrates, resulting in a larger temperature decrease [55]. In both Fig. 6A and B, the magnitude of decrement varies according to location in a sequence of Ta1 < Tb3 < Ta4 < Tb6, which suggests a non-uniform distribution of phases within the HBSs. The finding of heterogeneous hydrate distribution during dissociation is similar to that described by Heeschen et al. in a 210 L hydrate sample [12]. As hydrate dissociation progressed, the temperature across the sediment increased due to heat transfer from the surrounding (constant surrounding temperature of 281.5 K). As heat is transferred in a radially inward direction, the temperatures at outer radii (Tb3 and Tb6) increased more rapidly than the inner radii (Ta1 and Ta4). It has been reported extensively that hydrate dissociation via depressurization for a highly permeable sediment is a heat transfer limited process [7,9,56–58], and the heat transfer rate is dependent on the composite thermal properties of the sediment, the temperature gradient and the convective heat transfer from fluid flow. To compare the temperature profiles across different BHPs, we selected Ta4, which shows a more distinctive temperature response, as the location of interest and plotted the temperature evolution in Fig. 7. As can be seen, the temperature in each case dropped to near equilibrium temperature at the BHP due to the consumption of sensible heat to supply heat for endothermic

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Fig. 6. Pressure, temperature and water production profile during the first 3 h of depressurization test for (A) R1 at 3.0 MPa; and (B) S1 at 2.1 MPa.

Fig. 7. Comparison of temperature progression during depressurization production under different bottom hole pressures. Enlarged figure shows the first 0.2 h during depressurization of experiment S1, whereby a sharp temperature decrease followed by a sudden temperature increase was observed before temperature buffering at 272.7 K.

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hydrate dissociation. Comparing the lowest temperatures attained in Fig. 7, we found that while Ta4 in cases P and Q decreased right to the equilibrium temperature at the prevailing BHPs, the lowest temperature attained in case R and S were above the equilibrium temperature - with case R slightly above the equilibrium temperature (0.3 K difference) and case S far above the equilibrium point at 2.1 MPa (266.9 K, not shown, corresponding to 5.5 K difference). For cases P, Q and R, after the initial decrease to the lowest temperature, the temperature gradually increase between 0.1 and 1 h, with cases R (3.0 MPa) achieving the highest average temperature increase rate of 4.1 K/hr, which is much higher than those of cases Q (4.0 MPa, 2.2 K/hr) and cases P (5.0 MPa, 0.2 K/hr). During the temperature uptake, it is noteworthy that several dips in temperature profiles were observed in cases Q and R; which were not seen in cases P and S. For instance, a sudden temperature decrease of 1.1 K was observed in R2 at t = 0.3 h, and similarly a 0.7 K temperature dip was seen in R1 at t = 1.3 h. As compared to case R, the magnitudes of temperature dips observed in case Q is more subtle. Similar observation has also been reported in a previous work employing depressurization to hydrates formed in sands [55]. The possible explanations behind the temperature dips observation are: (1) sudden expansion of gas pockets entrapped within hydrate bearing sediments; (2) movement of pore filling hydrates during this stage due to a rapid production of gas and water, which consumed localized heat for hydrate dissociation; and (3) flow of ‘‘cold” fluid from hydrate dissociation which passed through the measurement point. While the underlying principles which caused the temperature dips cannot be derived from our current apparatus, more advanced analytical tools capable of mapping the distribution of hydrates within porous media (such as magnetic resonance imaging) may aid in elucidating this phenomenon [59]. As hydrate dissociation progressed (t > 2 h), the dissociation interface gradually shrank from the boundary to the inner core, increasing the heat transfer resistance, thereby causing a slower heat transfer rate to the hydrates at the core centre [56]. On the other hand, Cases S (2.1 MPa) shows a distinctive temperature profile as compared to the others. After the system reached the lowest temperature (see enlarged image in Fig. 7), the temperature measured at Ta4 remained constant at 272.7 K for 0.2 h in S2 and 0.5 h in S1, which is identical to the ‘‘thermal buffering” observed by Circone et al. [55] and Zhou et al. [13]. It is noticed that this temperature was below ice point (273.2 K) but far above equilibrium temperature of methane hydrates at 2.1 MPa (266.9 K). It is possible that apart from consuming sensible heat for hydrate dissociation, the latent heat of ice formation (an exothermic reaction) was also incurred for hydrate dissociation, as can be seen from the sudden increase in temperature at t = 0.06 h from the enlarged section in Fig. 7. Therefore, this constant temperature zone occurred below ice point, yet significantly above the equilibrium temperature. The formation of ice at pressure below 2.6 MPa has been affirmed from a microscopic MRI investigation [39]. Despite the possibility of ice formation, the lowest temperature observed in this study was 272.3 K, which is above the range reported for anomalous ‘‘self-preservation” effect of pure methane hydrate (242–271 K) [60]. After the short period of constant temperature below ice point, the temperature quickly rose to the surrounding temperature. It is noted from Fig. 7 that the rate of temperature rise for case S (2.1 MPa) BHP is significantly faster than the other cases, which implies that significant fraction of hydrates has dissociated before the temperature uptake. For the other BHPs (Case P, Q, R), the presence of un-dissociated hydrates continued to consume heat, causing a slower temperature increase to the surrounding temperature of 281.5 K. Comparing the temperature achieved at the 5th hr, it can be clearly seen that the higher the production pressure, the lower the local temperature at Ta4,

signifying that hydrate dissociation is still incomplete at these time points. 4.2.2. Gas production profile at different depressurization extent A comparison of the cumulative gas production is shown in Fig. 8. As can be seen, the gas production rate during the first hour of dissociation is more rapid at a lower BHP. The enhanced gas production rate at low BHP was due to a higher dissociation driving force, and such observation is in line with a number of experimental studies [9,33,58]. In addition, our experimental results suggest that the cumulative gas production is higher at a lower BHP [28]. Regarding gas production below ice point, it can be clearly seen in Fig. 8 that the cumulative gas production was enhanced by 12.5% from 25.7 SL (3.0 MPa) to 28.9 SL (2.1 MPa), which aligns with Tsypkin’s mathematical model [29]. In that study, Tsypkin postulated the presence of a ‘‘transition regime” during hydrate decomposition whereby both water and ice are formed simultaneously, and gas production increases anomalously within this regime (for SH = 0.4, the transition regime was predicted to lie between 1.8 MPa and 2.3 MPa) [29]. It should also be noted that since ice nucleation is a stochastic process and depends on a number of experimental factors, whether ice indeed formed in case S just below ice point requires further investigation. Nonetheless, a clear trend observed from Fig. 8 is that both the cumulative gas production and the rate of gas production increase with increasing driving force. In addition, a simulation work reported that gaseous phase convective heat transfer enhances hydrate dissociation more significantly compared to aqueous phase flow [61], which aligns with our experimental finding that the temperature increase after consumption of sensible heat (the temperature ‘‘trough”) is more rapid for cases with higher gas production (refer to Figs. 7 and 8). However, some experimental works reported a decrease in gas production rate under higher depressurization extents [28,62]. An investigation on gas production from hydrates formed in Berea sandstone via depressurization to 1.72–2.21 MPa reported severe hydrate plugging at the lowest BHP, as evident from the distribution of pressure within the core [62]. Due to the regeneration of hydrates within the core at the lower pressure experimented, the study reported an increased gas production rate at a higher HBP (i.e. lower pressure driving force) [62]. It should be noted that the permeability of Berea sandstone in the said study [62] was 329 mD, which is 10 times lower than the permeability of unconsolidated sand (3 D) employed in our study. Hence, the pressure difference observed between both sides of the crystallizer (PCTop and PCBtm) were insignificant in our study. In another experimental study by Li et al. using 117.8 L PHS, the depressurization production profile of a water saturated HBS was investigated [33]. In contrast with our finding of higher cumulative gas production at lower BHP (Fig. 8), Li et al. reported similar cumulative gas production at BHPs of 3.7 MPa, 4.2 MPa, and 4.7 MPa [33]. The difference in cumulative gas production profile between the two studies may be due to (1) a difference in reactor scale, which can significantly affect heat and mass transfer captured in the two studies; and (2) the incorporation of a vertical well in the PHS study. It is expected that incorporation of wellbore changes the flow path of gas and water – for a vertical well, fluid is collected in radial direction; whereas fluid is collected from the top of HBS in our current configuration (axial direction). This presents an opportunity in our future study to elucidate how gas production can be facilitated by varying well configuration/flow path of the dissociated fluids. 4.2.3. Water production profile at different depressurization extent Apart from analyzing gas production, it is also essential to elucidate water production profile during hydrate dissociation for energy recovery studies, since water is expected to be produced

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Fig. 8. Comparison of the gas production profile during the first 10 h of production stage between different extents of depressurization.

along with gas. A comparison of water production profile across different experimental BHP is presented in Fig. 9. As can be seen from the enlarged part of Fig. 9, the initial rate of water production (1st hr) is directly proportional to the extent of depressurization – with 2.1 MPa BHP yielding the highest initial rate of water production. It should be noted that apart from water dissociated from hydrates, free water was also produced during this period of time. For a water saturated HBS formed in HiGUMA (1710 L), Konno et al. reported that during depressurization, water production rate reached a peak before gas production rate peak due to the drainage of pore filling water in the beginning of depressurization [9]. Subsequently, water production plateaued off for 2.1 MPa and 3.0 MPa within 10th hours; whereas they were continuously produced even after 10th hr for experiments conducted at 4.0 MPa and 5.0 MPa. The continuous production of water, despite at a low rate, caused the cumulative water production at 4.0 MPa and 5.0 MPa to

be higher than those at lower BHP of 2.1 MPa and 3.0 MPa in the end of depressurization test. Our finding of reduced water production at lower BHP is contradictory to a numerical simulation study which reported an increase in cumulative water produced when the BHP was reduced from 3.0 MPa to 2.2 MPa [56] and another experimental result at comparable phase saturation from 3.7 MPa to 4.7 MPa [33]. A notable difference we observed from the simulation study is that the simulation result predicted that more than 95% water is produced in the first 10 min, which is much shorter than what we obtained from our study (refer to Table 2, t90,A). In fact if we compare the cumulative water production for the first hour in Fig. 9, the cumulative water production was indeed higher for the lower BHP, which is similar to the finding from the aforementioned studies [33,56]. However as can be seen, water was preferentially produced nearing the end of the depressurization tests at higher BHPs

Fig. 9. Comparison of the water production profile during the first 30 h of production stage between different extents of depressurization.

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instead of gas in this study. The reason being is that for a higher BHPs (i.e. lower dissociation drive), hydrates dissociation was slower, causing the production of small gas bubbles which could not coalesce to form a continuous phase. It should also be noted that at the end of hydrate dissociation, some water remained in the HBS [12]. Therefore, a fine tune of gas and water production profile can be done to enhance the feasibility of a particular recovery approach. To quantitatively compare the gas and water production profiles, we computed the t90,A and t90,G in the following section.

4.2.4. Comparison of gas and water production profiles As discussed in the previous subsections, it can be seen that despite a higher initial rate being observed for both aqueous phase (mainly water) and gaseous phase (mainly methane) at low BHP, the sustained production profile differed drastically between the two phases. To quantitatively distinguish the difference between the production behavior of the two phases, we analysed the t90 for both phases in Fig. 10 and Table 2, which is defined as the time taken for 90% of the final amount of gaseous/aqueous phase products to be produced. As can be seen, both t90,A and t90,G are shorter at a lower BHP, and the difference between the two parameters become more pronounced at higher BHP. In general, t90,A is higher than t90,G, signifying that aqueous phase was continuously produced nearing the end of hydrate dissociation. This implies that for slower hydrate dissociation, gas production is hampered by water due to the absence of continuous free gas phase or a gas migration pathway [12]. It should be noted that in the hydrate field, water fluxes continuously towards the HBS, which may render gas production even more difficult than in a closed system. The incorporation of wellbore have been considered in several works to resolve the issue of heat and mass transfer limitation to the inner core [43,54,63]. As a continuation of this work which found that gas flow is essential in sustaining gas production, we will employ different well configurations to devise strategy to facilitate gas flow by inducing a change in the flow behavior during hydrate dissociation. Another aspect not included in the current study is the issue of sand production, which has been observed in a number of field tests [64–66]. In our current work, filters were installed in the crystallizer compartment as the sand prevention technology. To enable energy recovery from hydrate deposits, effective sand control technology and accurate sand production modelling is required [67].

Fig. 10. Average t90 for aqueous phase and gaseous phase production under different BHP along with the standard deviation.

5. Conclusion The production profiles from a water saturated HBS of 40% hydrate, 50% aqueous solution and 10% gas saturation were investigated employing different bottom hole pressures (BHP), ranging from 2.1 MPa to 5.0 MPa. Apart from the intuitive results that lower BHP can result in a faster gas production rate, we found that the cumulative gas production was higher and cumulative water production was lower for the case of low BHP. Production below ice point was found to be advantageous for hydrate dissociation, as evident from the rapid initial gas production rate, higher cumulative gas production and lower cumulative water production. In addition, it was found that the cumulative gas production is directly proportional to the extent of BHP. Analyzing t90 of both gaseous and aqueous phase products at different BHP, we found that water was preferentially produced nearing the end of hydrate dissociation, especially for the higher BHP (i.e. lower pressure driving force for hydrate dissociation). It is essential to devise production strategy and develop innovative well bore design configurations which minimizes water production and facilitate gas flow in future laboratory studies. Acknowledgment The financial support from the National University of Singapore (R-279-000-420-750 and R-261-508-001-646) is greatly appreciated. Zheng Rong Chong thanks NUS for the President’s Graduate Fellowship. Zhenyuan Yin would like to thank the industrial postgraduate programme (IPP) for the scholarship. References [1] Sloan ED, Koh CA. Clathrate hydrates of natural gases. CRC Press Llc.; 2008. [2] Englezos P. Clathrate hydrates. Ind Eng Chem Res 1993;32:1251–74. [3] Davidson DW. Clathrate hydrates. In: Franks F, editor. Water in crystalline hydrates aqueous solutions of simple nonelectrolytes. Boston, MA: Springer, US; 1973. p. 115–234. [4] Boswell R, Collett TS. Current perspectives on gas hydrate resources. Energy Environ Sci 2011;4:1206–15. [5] Chong ZR, Yang SHB, Babu P, Linga P, Li X-S. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl Energy 2016;162:1633–52. [6] Collett T, Bahk JJ, Frye M, Goldberg DS, Husebø J, Koh CA, et al. Historical methane hydrate project review. Report, Consortium for Ocean Leadership; 2013. [7] Li X-S, Xu C-G, Zhang Y, Ruan X-K, Li G, Wang Y. Investigation into gas production from natural gas hydrate: a review. Appl Energy 2016;172:286–322. [8] Collett T, Bahk J-J, Baker R, Boswell R, Divins D, Frye M, et al. Methane hydrates in nature—current knowledge and challenges. J Chem Eng Data 2015;60:319–29. [9] Konno Y, Jin Y, Shinjou K, Nagao J. Experimental evaluation of the gas recovery factor of methane hydrate in sandy sediment. RSC Adv 2014;4:51666–75. [10] Yin Z, Chong ZR, Tan HK, Linga P. Review of gas hydrate dissociation kinetic models for energy recovery. J Nat Gas Sci Eng 2016;35:1362–87. [11] Moridis GJ, Collett TS, Boswell R, Kurihara M, Reagan MT, Koh C, et al. Toward production from gas hydrates: current status, assessment of resources, and simulation-based evaluation of technology and potential. SPE Reservoir Eval Eng 2009;12:745–71. [12] Heeschen KU, Abendroth S, Priegnitz M, Spangenberg E, Thaler J, Schicks JM. Gas production from methane hydrate: a laboratory simulation of the multistage depressurization test in Mallik, Northwest Territories, Canada. Energy Fuels 2016;30:6210–9. [13] Zhou Y, Castaldi MJ, Yegulalp TM. Experimental investigation of methane gas production from methane hydrate. Ind Eng Chem Res 2009;48:3142–9. [14] Myshakin EM, Anderson BJ, Rose K, Boswell R. Simulations of variable bottomhole pressure regimes to improve production from the double-unit Mount Elbert, Milne Point Unit, North Slope Alaska hydrate deposit. Energy Fuels 2011;25:1077–91. [15] Merey S, Sinayuc C. Investigation of gas hydrate potential of the Black Sea and modelling of gas production from a hypothetical Class 1 methane hydrate reservoir in the Black Sea conditions. J Nat Gas Sci Eng 2016;29:66–79. [16] Konno Y, Masuda Y, Akamine K, Naiki M, Nagao J. Sustainable gas production from methane hydrate reservoirs by the cyclic depressurization method. Energy Convers Manage 2016;108:439–45. [17] Kurihara M, Sato A, Funatsu K, Ouchi H, Yamamoto K, Numasawa M, et al. Analysis of production data for2007/2008 Mallik gas hydrate production tests

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Please cite this article in press as: Chong ZR et al. Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.04.031