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Methane hydrate decomposition and sediment deformation in unconfined sediment with different types of concentrated hydrate accumulations by innovative experimental system
Applied Energy 226 (2018) 916–923
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Methane hydrate decomposition and sediment deformation in unconfined sediment with different types of concentrated hydrate accumulations by innovative experimental system
T
⁎
Yi Wanga,b,c,1, Jing-Chun Fengd,e,1, Xiao-Sen Lia,b,c, , Yu Zhanga,b,c, Han Hana,b,c a
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR China c Guangdong Province Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d School of Enringing, Sun Yat-sen University, Guangzhou 510275, PR China e Guangdong Research Center for Climate Change, Guangzhou 510275, PR China b
H I GH L IG H T S
dissociation in different types of gas hydrate accumulations are studied. • Hydrate of hydrate morphology and distribution on gas production is not obviously. • Influence collapse of sediment is observed in the grain-displacing hydrate dissociation. • Structure • Radial Shrinkage Effect is found and analyzed during pore-filling hydrate dissociation.
A R T I C LE I N FO
A B S T R A C T
Keywords: Sediment deformation Hydrate Decomposition Unconfined sediment Hydrate accumulations Experiment
Methane hydrates are regarded as a potential source of energy supply. Geological features of different types of concentrated gas hydrate accumulations show great variations. In this study, methane hydrate decomposition in unconfined sediment with different types of concentrated hydrate accumulations are firstly investigated by experiments, and the influence of hydrate decomposition on sediment deformation is analyzed. Two types of concentrated hydrate accumulations are selected, which are grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. An innovative high pressure set-up with a quick-opening top cover is applied to investigate hydrate decomposition under the geological conditions of the hydrate reservoir in the South China Sea. Experimental results indicate that the influence of hydrate morphology and hydrate distribution on gas production is not obviously. The average heat transfer rates during grain-displacing hydrate dissociation and porefilling hydrate dissociation are also similar. However, the sediment deformation characteristics for different types of concentrated hydrate accumulations are totally different. Structure collapse of porous media is firstly observed in the experiments within the grain-displacing hydrate, which indicates that the sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. Meanwhile, the radial shrinkage effect of sediment is found during pore-filling hydrate dissociation, due to the cementation effect of hydrate.
1. Introduction Methane hydrate is a kind of naturally-occurring clathrate, in which a host cage of water traps guest molecules of methane. The guest molecules do not participate in building chemically bounds to the water molecules, but are enclosed in crystalline cage [1]. Methane hydrate is similar to ice, but the physical and chemical properties of hydrate are different with those of ice. When pressure and temperature exceed ⁎
1
those for hydrate stable, the solid crystalline cages decompose to liquid water, and the methane gas are released from host cages [2]. If decomposed at standard pressure and temperature, about 160 volumes of methane gas can be released from one volume of methane hydrate [3]. Therefore, methane hydrate is also called as “flammable ice”. Methane hydrate has been discovered in both onshore and offshore environments all over the world [4]. Onshore hydrate reservoirs have been found in the permafrost and polar regions. Offshore hydrate
Corresponding author at: Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail address: [email protected] (X.-S. Li). These authors contributed equally to this work.
https://doi.org/10.1016/j.apenergy.2018.06.062 Received 30 December 2017; Received in revised form 14 May 2018; Accepted 9 June 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
vm Vpore NH MW/I MH ρW/I ρH sH0 sW0. nm01
Abbreviation HBS DS CPDS IMH
Hydrate Bearing Sediments Depressurizing Stage Constant-Pressure Depressurizing Stage Intermediate of Methane Hydrate
Symbols SG SW Si SH nm,0 nm, G nm,W
saturations of gas saturations of water saturations of ice saturations of hydrate mole quantity of the total injected methane gas [mol] mole quantity of free methane gas [mol] mole quantity of methane dissolved in water [mol]
vmp k Vp mW nH
gas molar volume [mL/mol] total pore volume of the sediments [mL] the hydration number of methane hydrate molar mass of water [g/mol] molar mass of methane hydrate [g/mol] densities of water/ice [g/cm3] density of hydrate [g/cm3] initial saturation of hydrate before hydrate decomposition initial saturation of water initial amount of the methane gas before hydrate dissociation [mol] gas molar volume under the conditions during hydrate decomposition [mL/mol] dissociation ratio of hydrate accumulative volume of gas production [L] accumulative amount of the water production [g] molar quantity of the remaining hydrate [mol]
properties and the deformation mechanism of the hydrate bearing sediments (HBS) during hydrate decomposition. Sultan et al. [26,27] reported that hydrate destabilization could increase the pore pressure and reduce the sediment strength. Kono et al. [28] carried out hydrate decomposition experiments by depressurization, and found that the rate of hydrate decomposition was related to sediments properties. Haligva et al. [29] investigated the influence of sediment volume on the gas production from hydrate dissociation. Serials of gas hydrate decomposition experiments were performed by Jung et al. [30]. In these experiment, the sediments are made of different particle sizes. The fine particle migration with multi-phase flow in pore and the fracture structure formed in sediment during hydrate decomposition are found and reported. A theory of critical fines fraction is proposed to explain these phenomena. On the other hand, to investigate the change of the sediment mechanical properties during hydrate decomposition, a serial of mechanical properties were tested by triaxial shear experiment [31] and centrifuge experiment [32]. Meanwhile, models for the change of mechanical properties in HBS also have been presented [33]. However, pervious researches mainly focused on the influence of sediment on the mechanical properties and deformation mechanism. Few literatures were reported about the influence of hydrate distribution on the sediment deformation mechanism of the HBS and the production behaviors during hydrate decomposition until now. According to the different types of concentrated gas hydrate in sediment, hydrate reservoirs can be divided into three categories, which are grain-displacing hydrate (veins, nodules, and fracture-fills), porefilling hydrate in sediment, and seafloor hydrate mounds [34]. Seafloor hydrate mounds are solid masses of gas hydrate which can be directly observed on the seafloor. Although seafloor hydrate mounds are easy for exploitation, the value for commercial exploitation can be ignore due to the small reserves and widely distribution. Therefore, gas production from this kind of occurrence is not being actively considered until now, which also will not be investigated in this work. Grain-displacing hydrate can be defined that the solid hydrate participates in constituting the structure of the HBS in the form of veins, nodules, and fracture-fills. Hydrate concentrations of grain-displacing hydrate are normally ranged from 10% to 30%. Because sediment for the graindisplacing hydrate generally is mud, which lacks of matrix permeability and mechanical stability, sand production and sediment deformation issues may be occurred during gas production from the grain-displacing hydrate. Recently, the pore-filling hydrate in sandy sediment is considered as the most potential category of gas hydrate deposit for commercial production in future. This potential has been reported both in field test and in numerical simulation. The typically pore-filling hydrates within sand reservoirs are found to have hydrate saturation in
reservoirs mainly have been found in the continental margins [5]. It is due to the fact that the pressure and temperature conditions in these regions are suitable to form and sustain hydrate, and methane and water are present in these natural settings. The volume of methane store in methane hydrate all over the world is huge. The common perception of the total carbon content in gas hydrate is more than that of all of the conventional fossil fuels [6]. With researchers obtain new information about the location and concentration of methane hydrate, the estimation of natural gas hydrate is continually refined and improved. Therefore, methane hydrate is considered as a potential source of methane for energy supply [7,8]. In order to observe hydrate dissociation in the real environment and assess the feasibility of exploitation technologies for commercial production, depressurization [9,10], thermal stimulation [11,12], inhibitor injection [13], carbon dioxide replacement [14], and the combined application [15] have been applied and investigated for hydrate destabilization [16]. The models of hydrate dissociation using different methods have been reported [17,18]. Over past few decades, a series of field tests on gas production from hydrate deposits were carried out. The field test in the Mallik region of Canada in 2008 proved that gas production from hydrate accumulations was technically feasible from a sand-dominated reservoir, which promotes the potential of hydrates to be considered as an important recoverable energy resource [19]. During the field test in the Prudhoe Bay area (On the north slope of Alaska/ USA) in 2012, the methane exchange by CO2 injection into a methane hydrate reservoir has been tested for the first time [20]. Not only hydrate field test on hydrate deposits in the permafrost regions, but also the marine hydrate field tests were successfully conducted in the Nankai Trough by Japan in 2013 [21] and in the Shenhu area of South China Sea by China in 2017 [22]. In both of these marine hydrate field tests, the depressurization is applied for gas recovery. On the other hand, gas hydrate has been related to a serial of issues on geohazards. The hazards generally are due to the destabilizing effects of methane hydrate dissociation and the rapidly fluids (gas/water) releasing into unconsolidated geological systems. Gas hydrate geohazards may occur due to natural processes or industrial activities. Primary researches mainly focus on designing different field programs to evaluate natural geohazard process that may occur in both marine and arctic deposits [23,24]. Furthermore, the geohazards during gas production from hydrate reservoir will become a bottleneck for the exploitation techniques of gas hydrate. Because methane hydrate generally occurs in unconsolidated sediments, the hydrate decomposition leads to the decrease of sediment strength and significant sediment deformation [25]. Recently, some researches have been reported to investigate the change of mechanical 917
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measured at a temperature range of 223.15 K to 473.15 K and a currency of ± 0.1 K. The top cover of the reactor exhibited nine wellheads corresponding to the nine measuring points on each layer within the vessel. The present study chose wellhead 5 as chosen as the production well. The other wellheads were sealed. Two pressure transducers (TRAFAG NAT 8251.84.2517 type) were located at the inlet and the outlet, and measured at the range of 0–25 MPa and a currency of ± 0.02 MPa. A thermostatic water bath was placed outside the kettle to ensure stable environmental temperature conditions. A desander was employed throughout the process to intercept particle outlet flow at a bearing pressure range of 0–40 MPa. A back pressure valve (TESCOM, Emerson Electric Co., USA) was employed to manually maintain the pressure at a range of 0–30 MPa at a currency of ± 0.2 MPa. The outlet flow reached the gas-liquid separator and was separated to gas and water, wherein the liquid flowed down to the container through an electronic scale (Santorius Co.) with the concrete vision Santorius BS 2202S (0–2200 g, ± 0.01 g) and the gas flowed to the flowmeter (Seven Star Co.) with vision D07-11CM (0–10 L/min, ± 2%). Furthermore, methane gas (99.9% purity) was produced by the Guangzhou Gas Group Co. Ltd., Conghua branch. Quartz sand made by Bandao Silica Sands Co. was used to simulate the sediments. During the experimental procedure, the temperature, pressure, and volume of the cumulative gas production, are recorded by the data collection unit in time.
the range from 60% to 90% in the pore space of sediment [34]. Due to the different properties of the different types of concentrated gas hydrate in sediment, the characteristics of sediment deformation during hydrate dissociation in the different types of concentrated gas hydrate should be different, and the production behaviors and heat transfer characteristics also may be different. However, the experimental investigations on these problems were not reported in the literatures. In this work, a novel cubic reactor with a quick-opening cover was firstly applied to investigate methane hydrate formation and decomposition in unconfined sediment under the geological conditions of the hydrate reservoir in the South China Sea. The experiments of methane hydrate decomposition with different types of concentrated hydrate accumulations are compared. And the influence of hydrate decomposition on the sediment deformation is investigated. Therefore, the innovation of this research can be concluded as: (1) a novel set-up for hydrate research was built, the change of sediment morphology can be observed by using the quick-opening cover; (2) a new method of hydrate sample formation for different types of concentrated gas hydrate in sediment are carried out; (3) the experimental results of the sediment deformation and production behaviors during hydrate dissociation in the different types of concentrated gas hydrate are firstly reported.
2. Experimental section 2.1. Apparatus
2.2. Experimental procedure
Fig. 1 shows the experimental apparatus schematic, temperature measurement points and wellheads distribution. As seen in Fig. 1a, the experimental apparatus consists of a cubic high-pressure reactor equipped with a quick-opening component, a temperature-controlling water bath, an output unit, a back-pressure regulator, a gas-liquid-solid three-phase separation system, a data acquisition system, and some measurement units. The cubic high-pressure reactor which is made of stainless steel 316 served as the core component. The inner of the reactor has a cubic cross-section with the inner length of 90 mm and the inner volume of 729 ml, which can withstand pressure up to 30 MPa. An innovative designing on the cubic reactor was the quick-opening top cover, which consisted of a pair of stainless clamps and a top cover with a rubber O-ring. The stainless clamps were applied to fix the top cover on the reactor, and the rubber O-ring was used for sealing. By using this quick-opening top cover, the top cover can be disassembled in 1 min. As seen in Fig. 1b, 27 Pt100 thermocouples were evenly distributed inside the reactor on three layers, specifically the top (A), middle (B), and bottom (C). Each layer was placed on 9 thermocouples that were
2.2.1. Hydrate formation In this work, in order to investigate the methane hydrate decomposition with different types of concentrated hydrate accumulations, two types of concentrated hydrate accumulations were selected, which were grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. Formation methods for the grain-displacing hydrate and pore-filling hydrate were different. During the grain-displacing hydrate formation (Run1), the 729 ml reactor was fulfilled with the sediments and ice particles in the form of nodules. The sediment used in this work is quartz sand with the grain sizes of 300–450 µm. The porosity of the sediment was approximately 48%, and the permeability was approximately 5.0 × 10−11 m2(=50.0 Darcies). The temperature of the water bath was set below 265.15 K. Afterwards, methane gas was injected into the reactor to pressurize the vessel up to 20 MPa. The ice particles directly turned into hydrate. Thus, the ice distribution turned into hydrate distribution. Therefore, the grain-displacing hydrate (nodules) was synthesized.
Fig. 1. (a) Schematic of the experimental apparatus; (b) Temperature measurement points and wellheads distribution. 918
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Fig. 2. Changes of pressures and volumes of gas production during hydrate dissociation during Runs 1–2.
Fig. 3. Changes of temperatures of T5B and T7B during hydrate dissociation for Runs 1–2.
On the other hand, during pore-filling hydrate formation (Run2), quartz sand was filled into the reactor. And then, deionized water (amount of water was equally with the amount of ice in grain-displacing hydrate formation) and methane were injected to pressurize the vessel up to 20 MPa. The water bath temperature kept as 281.85 °C during the whole hydrate formation process. And then, pressure gradually decreased during the hydrate formation. After hydrate formation, the amount of the hydrate formation for the grain-displacing hydrate and the pore-filling hydrate could be similar.
quantity of the total injected methane gas, free methane gas, and methane dissolved in water, respectively; Vpore stands for the total pore volume of the sediments assuming they are incompressible and are considered constant; NH is the hydration number of methane hydrate; MW/I and MH represent the molar mass of water and methane hydrate, respectively; and ρW/I and ρH are the densities of water/ice and hydrate, respectively. 2.3.2. Hydrate dissociation During hydrate decomposition, the saturation changes of different phases can be calculated as:
2.2.2. Hydrate dissociation The initial pressures is adjusted to 13.5 MPa, and the temperature of water bath was increase to 281.15 K. The initial pressure and temperature conditions were selected as geological conditions of the hydrate reservoir in the South China Sea. The experiments of hydrate decomposition were carried out using depressurization. The production pressure was set to 4.70 MPa by the back-pressure regulator, which was close to the production pressure of the field test in Nankai Trough. Subsequently, the hydrate gradually decomposed and methane gas released from outlet. After hydrate was completely dissociated in the sediment, residual methane gas in the sediment was released gradually, and the system pressure declined to atmosphere. During the hydrate dissociation, the data were recorded in real time. Finally, the quickopening top cover is opened to observe sediment deformation.
sH = sH0 (1−k )
sG =
(1)
nH =
Each saturations of different phases can be expressed as Eqs. (2)–(4) [35–38].
vm nm, G Vpore
sW / I =
(7)
ρH sH VPore MH
(8)
3.1. Production behaviors
mW / I , inj−NH (nm0−nm, G ) MW / I
(nm0−nm, G ) MH sH = ρH Vpore
NH MW ρH SH 0 k mW − MH ρW VPore
(6)
3. Results and discussions
(2)
ρW / I Vpore
Vpore
where the initial saturation hydrate and water before hydrate decomposition can be expressed as sH0 and sW0. The hydration number is NH, which is 6 in this work. The initial amount of the methane gas before hydrate dissociation is nm01 (mol). The gas molar volume under the conditions during hydrate decomposition is vmp (mL/mol). k is the dissociation ratio of hydrate in reactor. The accumulative volume of gas production is Vp (L), and the accumulative amount of the water production is mW (g). In addition, the molar quantity of the remaining hydrate, nH, in the sediment during the dissociation experiments can be calculated as follows:
2.3.1. Hydrate formation During hydrate formation, there are 4 phases in the reactor. The relationship of 4 phases can be calculated as follows:
sG =
(nm01 + sH 0 Vpore kρH MH −nm, w−VP / vmp) vm
sW = sW 0 +
2.3. Calculation
sG + sW / I + sH = 1
(5)
(3)
The curves of system pressure and cumulative volume of gas releasing during hydrate decomposition for Runs 1–2 are given in Fig. 2. As shown in Fig. 2, the system pressures decrease from initial pressures (13.5 MPa) to the production pressures (4.70 MPa) within 11 min (Point B) and 12 min (Point B′) for Run1 and Run2, respectively. Meanwhile, volume of methane gas production in the depressurization stages are 48.96 L and 43.81 L for Runs 1–2, respectively. And then, the
(4)
in which SG, SW, Si, and SH represent the saturations of gas, water, ice, and hydrate. The gas molar volume, vm, (mL/mol), can be calculated by fugacity model of Li et al. [39]. nm,0, nm, G, and nm,W represent the mole 919
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Fig. 4. Changes of temperature distributions during hydrate dissociation for Run 1.
Fig. 5. Changes of temperature distributions during hydrate dissociation for Run 2.
Fig. 6. Temperature-pressure curves of 3B, 5B, and 7B during hydrate dissociation for Run 1.
Fig. 7. Temperature-pressure curves of 3B, 5B, and 7B during hydrate dissociation for Run 2.
production pressures, which are controlled by back-pressure valve, maintain at 4.70 MPa in the remaining period of the experiments. Finally, gas production rates decrease to 0, and the hydrate
decomposition experiments are finished. The cumulative volumes of produced gas for Run 1 and Run 2 are 72.2 L and 70.2 L, respectively. During grain-displacing hydrate dissociation in Run1, the gas 920
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Fig. 8. Sediment surface deformation before and after hydrate dissociation for Run1.
Fig. 9. Sediment surface deformation before and after hydrate dissociation for Run2.
Fig. 10. Diagram illustrating radial shrinkage of sediment during hydrate decomposition.
(CPDS). As also shown in Fig. 2, during pore-filling hydrate dissociation in Run2, gas production profile also can be divided into 3 stages. Points A′ and B′ in Fig. 2 are given to partition 3 stages. The gas production characteristics of pore-filling hydrate in Stages 1–3 is similar to those of grain-displacing hydrate. In addition, rate of gas production in Stage 2 of Run2 is slower than that of Run1. The reason for the result may be that the lower heat transfer rate in pore-filling hydrate leads to the lower rate of hydrate dissociation of Run 2. In general, gas production behaviors in Run 1 and Run 2 are similar, which indicates that the influence of hydrate morphology and hydrate distribution on gas production is not obviously. Because hydrate decomposition in sediment in reactor is mainly depended on heat transfer, the similar production behavior may indicate that the rate of heat transfer during hydrate decomposition in different types of concentrated hydrate accumulations are similar.
production process consists of three stages. During Stage 1 (0–10 min), system pressure drops from the initial pressure (13.5 MPa) to the hydrate destabilization pressure (6.15 MPa) at initial temperature (281.85 K), which can be calculated by the fugacity model of Li et al. [39]. Because the pressure is above hydrate decomposition pressure, no hydrate in sediment can be dissociated in Stage 1. Released gas in Stage 1 is free gas in pore. During Stage 2 (10–11 min, A-B), the pressure continuously decreases from hydrate destabilization pressure (6.15 MPa) to production pressure (4.70 MPa). System pressure in Stage 2 is lower than the hydrate destabilization pressure, which leads to hydrate dissociation. Therefore, the rate of gas production increases obviously. The produced gas in Stage 2 is the mixture of free gas from pore and gas releasing from hydrate dissociation. Because the pressures in Stages 1–2 are decreased, the stages 1–2 are called as the Depressurizing Stage (DS). Afterwards, during Stage 3 (11 min – end, B-C), the pressure keeps constant (4.70 MPa) until the end of the experiment, which can be described as the Constant-Pressure Depressurizing Stage 921
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drop from X to Y, causing temperature drop. As seen in Fig. 7, hydrate also starts to decomposed quickly at Y, Y′, and Y″. When pressure drops from X (X′, X″) to Y (Y′, Y″), the decreases of temperatures are smaller those in Run 1, because there is no IMH during pore-filling hydrate formation. The temperature drop is due to the throttling effect of depressurization. During depressurization from Y (Y′ and Y″) to Z (Z′ and Z″), the P-T curves overlap with the curve of hydrate phase equilibrium, which indicates that hydrates at 3B, 5B, 7B are decomposed simultaneously in this process.
3.2. Heat transfer characteristics Fig. 3 shows the changes of temperatures of T5B and T7B during hydrate decomposition for Runs 1–2. As seen in Fig. 1b, the 5B is located at the center of the reactor, and the 7B is located at a corner of the cubic reactor. As shown in Fig. 3, during the DS, the temperatures all rapidly decrease from 282 K to 278 K due to sensible heat consumption by hydrate dissociation. However, the lowest value of the T5B is lower than that of the T7B in Run 1, but the lowest temperatures of the T5B and T7B are similar in Run 2. The reason for this phenomenon is that the hydrate in Run1 concentrates in the center of the reactor, and the hydrate in Run2 is evenly distributed in the reactor. In Run 1, temperature in center decreases with hydrate dissociation, and temperatures in corners also decrease due to heat transfer. In Run 2, temperatures in the entire reactor decrease in a same rate. During the CPDS, temperatures gradually recover to 281 K due to heat transfer from surrounding. However, the increase rate of the temperature at T5B of Run 1 is obviously slower than that that of Run 2. It is also due to the different hydrate distributions in Runs 1–2. However, the durations for temperature recovery for Run1 and Run 2 are similar, which indicates that the average heat transfer rates in reactor during hydrate dissociation for Runs 1–2 are similar. In order to acquire the characteristics of heat transfer during hydrate dissociation, Figs. 4 and 5 are given the change of temperature distributions at different time points in Run1 and Run2, respectively. During the DS, temperature distributions at 0 min, 5 min, and 10 min are drawn in Figs. 4 and 5. During the CPDS, the temperature distributions at 40 min, 70 min, 130 min, and 200 min are selected to investigate the heat transfer during hydrate decomposition. In the DS, the temperatures in Run 1 and Run2 obviously drop from 0 min to 10 min, due to the sensible heat consuming by hydrate decomposition. Whereas, in Run 1, there is a low-temperature region around the center of reactor. On the other hand, the temperatures are evenly distributed in Run2. It is due to the fact that the hydrate in Run 1 is distributed at the center of the Layer A, and the hydrate in Run 2 is evenly distributed. As also shown in Figs. 4 and 5, during CPDS (40–200 min), the temperatures in sediment gradually recover in both experiment. the temperatures increase from the boundary to the center, which indicates that the heat consumption for hydrate decomposition in the CPDS is the heat transfer from surrounding. In addition, by comparing the changes of temperature distributions in the CPDS of Run 1 to those of Run 2, temperatures recovery rate around the boundary of Run1 is higher than that of Run2 in the CPDS. It is also because that the hydrate is evenly distributed in Run 2, and the hydrate dissociation around the boundary prevents the temperature recovery rate. And the hydrate is not located around the boundary in Run1. However, the temperature distributions at 200 min for both Run1 and Run2 are similar, which indicates that the average heat transfer rates of both experiments are similar. The results lead to the similar gas production behaviors of Runs 1–2, which have been discussed in last section.
3.4. Sediment deformation After hydrate decomposition experiments, the quick-opening top cover of the set-up is opened to observe the sediment deformation directly. Figs. 8 and 9 shows the sediment surface deformation after Run1 and Run2. In Fig. 8, structure collapse of sediment is observed after the hydrate dissociation of Run 1 with the grain-displacing hydrate reservoir in first time, which indicates that the sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. The sediment collapse is due to the fact that the solid hydrate participates in constituting the structure of the HBS in grain-displacing hydrate. When the solid hydrate changes to liquid, the mechanical stability of the HBS is destroyed. As shown in Fig. 9, the fractures near the boundaries and sediment subsidence are observed, which indicates that the sediment particles have trend of gather during pore-filling hydrate dissociation. This result can be described as a radial shrinkage effect of sediment during porefilling hydrate decomposition. In order to avoid the influence of seepage on particle movement, a contrast test without hydrate formation is also carried out. In the contrast test, the pressure changes and production behavior are identical to those of Run 2. However, after the contrast test, the shrinkage phenomenon of sediment is not observed. Therefore, the shrinkage effect should be caused by hydrate decomposition. A cementation effect of hydrate in the HBS were reported. Thus, during the hydrate decomposition, sediment particles may be gathered by shirking hydrate in consolidated sediment, which causes the radial shrinkage effect. The diagram illustrating radial shrinkage of sediment during hydrate decomposition is given in Fig. 10. In general, although production behaviors and heat transfer characteristics during decomposition of grain-displacing hydrate and porefilling hydrate are similar, sediment deformation characteristics for different types of concentrated hydrate accumulations are totally different. In order to prevent geologic hazard during gas production from hydrate reservoirs, careful exploration for concentrated hydrate accumulation type in reservoir and calculation for geological deformation are necessary. 4. Conclusions In this work, a novel cubic reactor with a quick-opening cover was applied to investigate methane hydrate formation and decomposition in unconfined sediment under the geological conditions of the hydrate reservoir in the South China Sea. The experiments of methane hydrate decomposition with different types of concentrated hydrate accumulations are compared. And the influence of hydrate decomposition on the sediment deformation is analyzed. Two types of concentrated hydrate accumulations are selected, which are grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. Formation methods for the grain-displacing hydrate and pore-filling hydrate are different. Dissociation methods for both experiments are identical. The following conclusions are made:
3.3. Hydrate decomposition In order to investigate hydrate decomposition characteristics, temperature-pressure curves during DS of hydrate dissociation for Run 1 and Run 2 are shown in Figs. 6 and 7, respectively. The 3B, 5B, and 7B are selected as the reference points. As seen in Fig. 6, when pressure drops below 6.5 MPa (Y, Y′, Y″), the temperatures decrease rapidly from Y (Y′, Y″) to Z (Z′, Z″), which indicates that the hydrate start to be dissociated rapidly. However, when pressure drops from 13.5 MPa (X, X′, X″) to 6.5 MPa (Y, Y′, Y″), the decrease of temperature is also obviously. It may because that the grain-displacing hydrate is synthesized from ice. The Intermediate of Methane Hydrate (IMH) [40] during hydrate formation from ice may be generated. The IMH is an intermediate between ice and hydrate, which is destabilized during pressure
(1) The gas production behaviors from grain-displacing hydrate dissociation and pore-filling hydrate dissociation are similar, which indicates that the influence of hydrate morphology and hydrate 922
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distribution on gas production is not obviously. (2) The average heat transfer rates during grain-displacing hydrate dissociation and pore-filling hydrate dissociation are similar. The results lead to the gas production rates are similar. However, lowtemperature areas concentrate around the center of Layer A in grain-displacing hydrate dissociation, and the temperatures in porefilling hydrate dissociation are evenly distributed. It is due to the fact that the hydrate in grain-displacing type is distributed at the center of the Layer A, and the hydrate in pore-filling type is evenly distributed. (3) The obvious decrease of temperature before grain-displacing hydrate dissociation may be due to the destabilization of the Intermediate of Methane Hydrate (IMH). (4) Structure collapse of porous media is firstly observed in the experiments within the grain-displacing hydrate, which indicate that sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. (5) The radial shrinkage effect of sediment is found during pore-filling hydrate decomposition, due to the cementation effect of methane hydrate on sediment particles.
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Acknowledgments This work is supported by Key Program of National Natural Science Foundation of China (51736009), National Natural Science Foundation of China (51676190), Pearl River S&T Nova Program of Guangzhou (201610010164), International S&T Cooperation Program of China (2015DFA61790), Science and Technology Apparatus Development Program of the Chinese Academy of Sciences (YZ201619), Frontier Sciences Key Research Program of the Chinese Academy of Sciences (QYZDJ-SSW-JSC033), National Key Research and Development Plan of China (2016YFC0304002, 2017YFC0307306), Youth Science and Technology Innovation Talent of Guangdong (2016TQ03Z862), and Natural Science Foundation of Guangdong (2017A030313313), which are gratefully acknowledged. References [1] Li XS, Xu CG, Zhang Y, Ruan XK, Li G, Wang Y. Investigation into gas production from natural gas hydrate: a review. Appl Energy 2016;172:286–322. [2] Chong ZR, Yang SHB, Babu P, Linga P, Li XS. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl Energy 2016;162:1633–52. [3] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–9. [4] Boswell R. Is gas hydrate energy within reach? Science 2009;325:957–8. [5] 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 Reserv Eval Eng 2009;12:745–71. [6] Moridis GJ, Collett TS, Pooladi-Darvish M, Hancock S, Santamarina C, Boswell R, et al. Challenges, uncertainties, and issues facing gas production from gas-hydrate deposits. Spe Reserv Eval Eng 2011;14:76–112. [7] Boswell R, Shipp C, Reichel T, Shelander D, Saeki T, Frye M, et al. Prospecting for marine gas hydrate resources. Interpretation-J Sub 2016;4:Sa13–24. [8] Song YC, Yang L, Zhao JF, Liu WG, Yang MJ, Li YH, et al. The status of natural gas hydrate research in China: a review. Renew Sust Energ Rev 2014;31:778–91. [9] Yang MJ, Fu Z, Jiang LL, Song YC. Gas recovery from depressurized methane hydrate deposits with different water saturations. Appl Energy 2017;187:180–8. [10] Feng J-C, Wang Y, Li X-S. Large scale experimental evaluation to methane hydrate dissociation below quadruple point by depressurization assisted with heat stimulation. Energy Procedia 2017;142:4117–23. [11] Wang Y, Li XS, Li G, Huang NS, Feng JC. Experimental study on the hydrate dissociation in porous media by five-spot thermal huff and puff method. Fuel 2014;117:688–96. [12] Fitzgerald GC, Castaldi MJ, Zhou Y. Large scale reactor details and results for the formation and decomposition of methane hydrates via thermal stimulation dissociation. J Petrol Sci Eng 2012;94–95:19–27.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Methane hydrate decomposition and sediment deformation in unconfined sediment with different types of concentrated hydrate accumulations by innovative experimental system
T
⁎
Yi Wanga,b,c,1, Jing-Chun Fengd,e,1, Xiao-Sen Lia,b,c, , Yu Zhanga,b,c, Han Hana,b,c a
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR China c Guangdong Province Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d School of Enringing, Sun Yat-sen University, Guangzhou 510275, PR China e Guangdong Research Center for Climate Change, Guangzhou 510275, PR China b
H I GH L IG H T S
dissociation in different types of gas hydrate accumulations are studied. • Hydrate of hydrate morphology and distribution on gas production is not obviously. • Influence collapse of sediment is observed in the grain-displacing hydrate dissociation. • Structure • Radial Shrinkage Effect is found and analyzed during pore-filling hydrate dissociation.
A R T I C LE I N FO
A B S T R A C T
Keywords: Sediment deformation Hydrate Decomposition Unconfined sediment Hydrate accumulations Experiment
Methane hydrates are regarded as a potential source of energy supply. Geological features of different types of concentrated gas hydrate accumulations show great variations. In this study, methane hydrate decomposition in unconfined sediment with different types of concentrated hydrate accumulations are firstly investigated by experiments, and the influence of hydrate decomposition on sediment deformation is analyzed. Two types of concentrated hydrate accumulations are selected, which are grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. An innovative high pressure set-up with a quick-opening top cover is applied to investigate hydrate decomposition under the geological conditions of the hydrate reservoir in the South China Sea. Experimental results indicate that the influence of hydrate morphology and hydrate distribution on gas production is not obviously. The average heat transfer rates during grain-displacing hydrate dissociation and porefilling hydrate dissociation are also similar. However, the sediment deformation characteristics for different types of concentrated hydrate accumulations are totally different. Structure collapse of porous media is firstly observed in the experiments within the grain-displacing hydrate, which indicates that the sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. Meanwhile, the radial shrinkage effect of sediment is found during pore-filling hydrate dissociation, due to the cementation effect of hydrate.
1. Introduction Methane hydrate is a kind of naturally-occurring clathrate, in which a host cage of water traps guest molecules of methane. The guest molecules do not participate in building chemically bounds to the water molecules, but are enclosed in crystalline cage [1]. Methane hydrate is similar to ice, but the physical and chemical properties of hydrate are different with those of ice. When pressure and temperature exceed ⁎
1
those for hydrate stable, the solid crystalline cages decompose to liquid water, and the methane gas are released from host cages [2]. If decomposed at standard pressure and temperature, about 160 volumes of methane gas can be released from one volume of methane hydrate [3]. Therefore, methane hydrate is also called as “flammable ice”. Methane hydrate has been discovered in both onshore and offshore environments all over the world [4]. Onshore hydrate reservoirs have been found in the permafrost and polar regions. Offshore hydrate
Corresponding author at: Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail address: [email protected] (X.-S. Li). These authors contributed equally to this work.
https://doi.org/10.1016/j.apenergy.2018.06.062 Received 30 December 2017; Received in revised form 14 May 2018; Accepted 9 June 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
vm Vpore NH MW/I MH ρW/I ρH sH0 sW0. nm01
Abbreviation HBS DS CPDS IMH
Hydrate Bearing Sediments Depressurizing Stage Constant-Pressure Depressurizing Stage Intermediate of Methane Hydrate
Symbols SG SW Si SH nm,0 nm, G nm,W
saturations of gas saturations of water saturations of ice saturations of hydrate mole quantity of the total injected methane gas [mol] mole quantity of free methane gas [mol] mole quantity of methane dissolved in water [mol]
vmp k Vp mW nH
gas molar volume [mL/mol] total pore volume of the sediments [mL] the hydration number of methane hydrate molar mass of water [g/mol] molar mass of methane hydrate [g/mol] densities of water/ice [g/cm3] density of hydrate [g/cm3] initial saturation of hydrate before hydrate decomposition initial saturation of water initial amount of the methane gas before hydrate dissociation [mol] gas molar volume under the conditions during hydrate decomposition [mL/mol] dissociation ratio of hydrate accumulative volume of gas production [L] accumulative amount of the water production [g] molar quantity of the remaining hydrate [mol]
properties and the deformation mechanism of the hydrate bearing sediments (HBS) during hydrate decomposition. Sultan et al. [26,27] reported that hydrate destabilization could increase the pore pressure and reduce the sediment strength. Kono et al. [28] carried out hydrate decomposition experiments by depressurization, and found that the rate of hydrate decomposition was related to sediments properties. Haligva et al. [29] investigated the influence of sediment volume on the gas production from hydrate dissociation. Serials of gas hydrate decomposition experiments were performed by Jung et al. [30]. In these experiment, the sediments are made of different particle sizes. The fine particle migration with multi-phase flow in pore and the fracture structure formed in sediment during hydrate decomposition are found and reported. A theory of critical fines fraction is proposed to explain these phenomena. On the other hand, to investigate the change of the sediment mechanical properties during hydrate decomposition, a serial of mechanical properties were tested by triaxial shear experiment [31] and centrifuge experiment [32]. Meanwhile, models for the change of mechanical properties in HBS also have been presented [33]. However, pervious researches mainly focused on the influence of sediment on the mechanical properties and deformation mechanism. Few literatures were reported about the influence of hydrate distribution on the sediment deformation mechanism of the HBS and the production behaviors during hydrate decomposition until now. According to the different types of concentrated gas hydrate in sediment, hydrate reservoirs can be divided into three categories, which are grain-displacing hydrate (veins, nodules, and fracture-fills), porefilling hydrate in sediment, and seafloor hydrate mounds [34]. Seafloor hydrate mounds are solid masses of gas hydrate which can be directly observed on the seafloor. Although seafloor hydrate mounds are easy for exploitation, the value for commercial exploitation can be ignore due to the small reserves and widely distribution. Therefore, gas production from this kind of occurrence is not being actively considered until now, which also will not be investigated in this work. Grain-displacing hydrate can be defined that the solid hydrate participates in constituting the structure of the HBS in the form of veins, nodules, and fracture-fills. Hydrate concentrations of grain-displacing hydrate are normally ranged from 10% to 30%. Because sediment for the graindisplacing hydrate generally is mud, which lacks of matrix permeability and mechanical stability, sand production and sediment deformation issues may be occurred during gas production from the grain-displacing hydrate. Recently, the pore-filling hydrate in sandy sediment is considered as the most potential category of gas hydrate deposit for commercial production in future. This potential has been reported both in field test and in numerical simulation. The typically pore-filling hydrates within sand reservoirs are found to have hydrate saturation in
reservoirs mainly have been found in the continental margins [5]. It is due to the fact that the pressure and temperature conditions in these regions are suitable to form and sustain hydrate, and methane and water are present in these natural settings. The volume of methane store in methane hydrate all over the world is huge. The common perception of the total carbon content in gas hydrate is more than that of all of the conventional fossil fuels [6]. With researchers obtain new information about the location and concentration of methane hydrate, the estimation of natural gas hydrate is continually refined and improved. Therefore, methane hydrate is considered as a potential source of methane for energy supply [7,8]. In order to observe hydrate dissociation in the real environment and assess the feasibility of exploitation technologies for commercial production, depressurization [9,10], thermal stimulation [11,12], inhibitor injection [13], carbon dioxide replacement [14], and the combined application [15] have been applied and investigated for hydrate destabilization [16]. The models of hydrate dissociation using different methods have been reported [17,18]. Over past few decades, a series of field tests on gas production from hydrate deposits were carried out. The field test in the Mallik region of Canada in 2008 proved that gas production from hydrate accumulations was technically feasible from a sand-dominated reservoir, which promotes the potential of hydrates to be considered as an important recoverable energy resource [19]. During the field test in the Prudhoe Bay area (On the north slope of Alaska/ USA) in 2012, the methane exchange by CO2 injection into a methane hydrate reservoir has been tested for the first time [20]. Not only hydrate field test on hydrate deposits in the permafrost regions, but also the marine hydrate field tests were successfully conducted in the Nankai Trough by Japan in 2013 [21] and in the Shenhu area of South China Sea by China in 2017 [22]. In both of these marine hydrate field tests, the depressurization is applied for gas recovery. On the other hand, gas hydrate has been related to a serial of issues on geohazards. The hazards generally are due to the destabilizing effects of methane hydrate dissociation and the rapidly fluids (gas/water) releasing into unconsolidated geological systems. Gas hydrate geohazards may occur due to natural processes or industrial activities. Primary researches mainly focus on designing different field programs to evaluate natural geohazard process that may occur in both marine and arctic deposits [23,24]. Furthermore, the geohazards during gas production from hydrate reservoir will become a bottleneck for the exploitation techniques of gas hydrate. Because methane hydrate generally occurs in unconsolidated sediments, the hydrate decomposition leads to the decrease of sediment strength and significant sediment deformation [25]. Recently, some researches have been reported to investigate the change of mechanical 917
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measured at a temperature range of 223.15 K to 473.15 K and a currency of ± 0.1 K. The top cover of the reactor exhibited nine wellheads corresponding to the nine measuring points on each layer within the vessel. The present study chose wellhead 5 as chosen as the production well. The other wellheads were sealed. Two pressure transducers (TRAFAG NAT 8251.84.2517 type) were located at the inlet and the outlet, and measured at the range of 0–25 MPa and a currency of ± 0.02 MPa. A thermostatic water bath was placed outside the kettle to ensure stable environmental temperature conditions. A desander was employed throughout the process to intercept particle outlet flow at a bearing pressure range of 0–40 MPa. A back pressure valve (TESCOM, Emerson Electric Co., USA) was employed to manually maintain the pressure at a range of 0–30 MPa at a currency of ± 0.2 MPa. The outlet flow reached the gas-liquid separator and was separated to gas and water, wherein the liquid flowed down to the container through an electronic scale (Santorius Co.) with the concrete vision Santorius BS 2202S (0–2200 g, ± 0.01 g) and the gas flowed to the flowmeter (Seven Star Co.) with vision D07-11CM (0–10 L/min, ± 2%). Furthermore, methane gas (99.9% purity) was produced by the Guangzhou Gas Group Co. Ltd., Conghua branch. Quartz sand made by Bandao Silica Sands Co. was used to simulate the sediments. During the experimental procedure, the temperature, pressure, and volume of the cumulative gas production, are recorded by the data collection unit in time.
the range from 60% to 90% in the pore space of sediment [34]. Due to the different properties of the different types of concentrated gas hydrate in sediment, the characteristics of sediment deformation during hydrate dissociation in the different types of concentrated gas hydrate should be different, and the production behaviors and heat transfer characteristics also may be different. However, the experimental investigations on these problems were not reported in the literatures. In this work, a novel cubic reactor with a quick-opening cover was firstly applied to investigate methane hydrate formation and decomposition in unconfined sediment under the geological conditions of the hydrate reservoir in the South China Sea. The experiments of methane hydrate decomposition with different types of concentrated hydrate accumulations are compared. And the influence of hydrate decomposition on the sediment deformation is investigated. Therefore, the innovation of this research can be concluded as: (1) a novel set-up for hydrate research was built, the change of sediment morphology can be observed by using the quick-opening cover; (2) a new method of hydrate sample formation for different types of concentrated gas hydrate in sediment are carried out; (3) the experimental results of the sediment deformation and production behaviors during hydrate dissociation in the different types of concentrated gas hydrate are firstly reported.
2. Experimental section 2.1. Apparatus
2.2. Experimental procedure
Fig. 1 shows the experimental apparatus schematic, temperature measurement points and wellheads distribution. As seen in Fig. 1a, the experimental apparatus consists of a cubic high-pressure reactor equipped with a quick-opening component, a temperature-controlling water bath, an output unit, a back-pressure regulator, a gas-liquid-solid three-phase separation system, a data acquisition system, and some measurement units. The cubic high-pressure reactor which is made of stainless steel 316 served as the core component. The inner of the reactor has a cubic cross-section with the inner length of 90 mm and the inner volume of 729 ml, which can withstand pressure up to 30 MPa. An innovative designing on the cubic reactor was the quick-opening top cover, which consisted of a pair of stainless clamps and a top cover with a rubber O-ring. The stainless clamps were applied to fix the top cover on the reactor, and the rubber O-ring was used for sealing. By using this quick-opening top cover, the top cover can be disassembled in 1 min. As seen in Fig. 1b, 27 Pt100 thermocouples were evenly distributed inside the reactor on three layers, specifically the top (A), middle (B), and bottom (C). Each layer was placed on 9 thermocouples that were
2.2.1. Hydrate formation In this work, in order to investigate the methane hydrate decomposition with different types of concentrated hydrate accumulations, two types of concentrated hydrate accumulations were selected, which were grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. Formation methods for the grain-displacing hydrate and pore-filling hydrate were different. During the grain-displacing hydrate formation (Run1), the 729 ml reactor was fulfilled with the sediments and ice particles in the form of nodules. The sediment used in this work is quartz sand with the grain sizes of 300–450 µm. The porosity of the sediment was approximately 48%, and the permeability was approximately 5.0 × 10−11 m2(=50.0 Darcies). The temperature of the water bath was set below 265.15 K. Afterwards, methane gas was injected into the reactor to pressurize the vessel up to 20 MPa. The ice particles directly turned into hydrate. Thus, the ice distribution turned into hydrate distribution. Therefore, the grain-displacing hydrate (nodules) was synthesized.
Fig. 1. (a) Schematic of the experimental apparatus; (b) Temperature measurement points and wellheads distribution. 918
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Fig. 2. Changes of pressures and volumes of gas production during hydrate dissociation during Runs 1–2.
Fig. 3. Changes of temperatures of T5B and T7B during hydrate dissociation for Runs 1–2.
On the other hand, during pore-filling hydrate formation (Run2), quartz sand was filled into the reactor. And then, deionized water (amount of water was equally with the amount of ice in grain-displacing hydrate formation) and methane were injected to pressurize the vessel up to 20 MPa. The water bath temperature kept as 281.85 °C during the whole hydrate formation process. And then, pressure gradually decreased during the hydrate formation. After hydrate formation, the amount of the hydrate formation for the grain-displacing hydrate and the pore-filling hydrate could be similar.
quantity of the total injected methane gas, free methane gas, and methane dissolved in water, respectively; Vpore stands for the total pore volume of the sediments assuming they are incompressible and are considered constant; NH is the hydration number of methane hydrate; MW/I and MH represent the molar mass of water and methane hydrate, respectively; and ρW/I and ρH are the densities of water/ice and hydrate, respectively. 2.3.2. Hydrate dissociation During hydrate decomposition, the saturation changes of different phases can be calculated as:
2.2.2. Hydrate dissociation The initial pressures is adjusted to 13.5 MPa, and the temperature of water bath was increase to 281.15 K. The initial pressure and temperature conditions were selected as geological conditions of the hydrate reservoir in the South China Sea. The experiments of hydrate decomposition were carried out using depressurization. The production pressure was set to 4.70 MPa by the back-pressure regulator, which was close to the production pressure of the field test in Nankai Trough. Subsequently, the hydrate gradually decomposed and methane gas released from outlet. After hydrate was completely dissociated in the sediment, residual methane gas in the sediment was released gradually, and the system pressure declined to atmosphere. During the hydrate dissociation, the data were recorded in real time. Finally, the quickopening top cover is opened to observe sediment deformation.
sH = sH0 (1−k )
sG =
(1)
nH =
Each saturations of different phases can be expressed as Eqs. (2)–(4) [35–38].
vm nm, G Vpore
sW / I =
(7)
ρH sH VPore MH
(8)
3.1. Production behaviors
mW / I , inj−NH (nm0−nm, G ) MW / I
(nm0−nm, G ) MH sH = ρH Vpore
NH MW ρH SH 0 k mW − MH ρW VPore
(6)
3. Results and discussions
(2)
ρW / I Vpore
Vpore
where the initial saturation hydrate and water before hydrate decomposition can be expressed as sH0 and sW0. The hydration number is NH, which is 6 in this work. The initial amount of the methane gas before hydrate dissociation is nm01 (mol). The gas molar volume under the conditions during hydrate decomposition is vmp (mL/mol). k is the dissociation ratio of hydrate in reactor. The accumulative volume of gas production is Vp (L), and the accumulative amount of the water production is mW (g). In addition, the molar quantity of the remaining hydrate, nH, in the sediment during the dissociation experiments can be calculated as follows:
2.3.1. Hydrate formation During hydrate formation, there are 4 phases in the reactor. The relationship of 4 phases can be calculated as follows:
sG =
(nm01 + sH 0 Vpore kρH MH −nm, w−VP / vmp) vm
sW = sW 0 +
2.3. Calculation
sG + sW / I + sH = 1
(5)
(3)
The curves of system pressure and cumulative volume of gas releasing during hydrate decomposition for Runs 1–2 are given in Fig. 2. As shown in Fig. 2, the system pressures decrease from initial pressures (13.5 MPa) to the production pressures (4.70 MPa) within 11 min (Point B) and 12 min (Point B′) for Run1 and Run2, respectively. Meanwhile, volume of methane gas production in the depressurization stages are 48.96 L and 43.81 L for Runs 1–2, respectively. And then, the
(4)
in which SG, SW, Si, and SH represent the saturations of gas, water, ice, and hydrate. The gas molar volume, vm, (mL/mol), can be calculated by fugacity model of Li et al. [39]. nm,0, nm, G, and nm,W represent the mole 919
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Fig. 4. Changes of temperature distributions during hydrate dissociation for Run 1.
Fig. 5. Changes of temperature distributions during hydrate dissociation for Run 2.
Fig. 6. Temperature-pressure curves of 3B, 5B, and 7B during hydrate dissociation for Run 1.
Fig. 7. Temperature-pressure curves of 3B, 5B, and 7B during hydrate dissociation for Run 2.
production pressures, which are controlled by back-pressure valve, maintain at 4.70 MPa in the remaining period of the experiments. Finally, gas production rates decrease to 0, and the hydrate
decomposition experiments are finished. The cumulative volumes of produced gas for Run 1 and Run 2 are 72.2 L and 70.2 L, respectively. During grain-displacing hydrate dissociation in Run1, the gas 920
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Fig. 8. Sediment surface deformation before and after hydrate dissociation for Run1.
Fig. 9. Sediment surface deformation before and after hydrate dissociation for Run2.
Fig. 10. Diagram illustrating radial shrinkage of sediment during hydrate decomposition.
(CPDS). As also shown in Fig. 2, during pore-filling hydrate dissociation in Run2, gas production profile also can be divided into 3 stages. Points A′ and B′ in Fig. 2 are given to partition 3 stages. The gas production characteristics of pore-filling hydrate in Stages 1–3 is similar to those of grain-displacing hydrate. In addition, rate of gas production in Stage 2 of Run2 is slower than that of Run1. The reason for the result may be that the lower heat transfer rate in pore-filling hydrate leads to the lower rate of hydrate dissociation of Run 2. In general, gas production behaviors in Run 1 and Run 2 are similar, which indicates that the influence of hydrate morphology and hydrate distribution on gas production is not obviously. Because hydrate decomposition in sediment in reactor is mainly depended on heat transfer, the similar production behavior may indicate that the rate of heat transfer during hydrate decomposition in different types of concentrated hydrate accumulations are similar.
production process consists of three stages. During Stage 1 (0–10 min), system pressure drops from the initial pressure (13.5 MPa) to the hydrate destabilization pressure (6.15 MPa) at initial temperature (281.85 K), which can be calculated by the fugacity model of Li et al. [39]. Because the pressure is above hydrate decomposition pressure, no hydrate in sediment can be dissociated in Stage 1. Released gas in Stage 1 is free gas in pore. During Stage 2 (10–11 min, A-B), the pressure continuously decreases from hydrate destabilization pressure (6.15 MPa) to production pressure (4.70 MPa). System pressure in Stage 2 is lower than the hydrate destabilization pressure, which leads to hydrate dissociation. Therefore, the rate of gas production increases obviously. The produced gas in Stage 2 is the mixture of free gas from pore and gas releasing from hydrate dissociation. Because the pressures in Stages 1–2 are decreased, the stages 1–2 are called as the Depressurizing Stage (DS). Afterwards, during Stage 3 (11 min – end, B-C), the pressure keeps constant (4.70 MPa) until the end of the experiment, which can be described as the Constant-Pressure Depressurizing Stage 921
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drop from X to Y, causing temperature drop. As seen in Fig. 7, hydrate also starts to decomposed quickly at Y, Y′, and Y″. When pressure drops from X (X′, X″) to Y (Y′, Y″), the decreases of temperatures are smaller those in Run 1, because there is no IMH during pore-filling hydrate formation. The temperature drop is due to the throttling effect of depressurization. During depressurization from Y (Y′ and Y″) to Z (Z′ and Z″), the P-T curves overlap with the curve of hydrate phase equilibrium, which indicates that hydrates at 3B, 5B, 7B are decomposed simultaneously in this process.
3.2. Heat transfer characteristics Fig. 3 shows the changes of temperatures of T5B and T7B during hydrate decomposition for Runs 1–2. As seen in Fig. 1b, the 5B is located at the center of the reactor, and the 7B is located at a corner of the cubic reactor. As shown in Fig. 3, during the DS, the temperatures all rapidly decrease from 282 K to 278 K due to sensible heat consumption by hydrate dissociation. However, the lowest value of the T5B is lower than that of the T7B in Run 1, but the lowest temperatures of the T5B and T7B are similar in Run 2. The reason for this phenomenon is that the hydrate in Run1 concentrates in the center of the reactor, and the hydrate in Run2 is evenly distributed in the reactor. In Run 1, temperature in center decreases with hydrate dissociation, and temperatures in corners also decrease due to heat transfer. In Run 2, temperatures in the entire reactor decrease in a same rate. During the CPDS, temperatures gradually recover to 281 K due to heat transfer from surrounding. However, the increase rate of the temperature at T5B of Run 1 is obviously slower than that that of Run 2. It is also due to the different hydrate distributions in Runs 1–2. However, the durations for temperature recovery for Run1 and Run 2 are similar, which indicates that the average heat transfer rates in reactor during hydrate dissociation for Runs 1–2 are similar. In order to acquire the characteristics of heat transfer during hydrate dissociation, Figs. 4 and 5 are given the change of temperature distributions at different time points in Run1 and Run2, respectively. During the DS, temperature distributions at 0 min, 5 min, and 10 min are drawn in Figs. 4 and 5. During the CPDS, the temperature distributions at 40 min, 70 min, 130 min, and 200 min are selected to investigate the heat transfer during hydrate decomposition. In the DS, the temperatures in Run 1 and Run2 obviously drop from 0 min to 10 min, due to the sensible heat consuming by hydrate decomposition. Whereas, in Run 1, there is a low-temperature region around the center of reactor. On the other hand, the temperatures are evenly distributed in Run2. It is due to the fact that the hydrate in Run 1 is distributed at the center of the Layer A, and the hydrate in Run 2 is evenly distributed. As also shown in Figs. 4 and 5, during CPDS (40–200 min), the temperatures in sediment gradually recover in both experiment. the temperatures increase from the boundary to the center, which indicates that the heat consumption for hydrate decomposition in the CPDS is the heat transfer from surrounding. In addition, by comparing the changes of temperature distributions in the CPDS of Run 1 to those of Run 2, temperatures recovery rate around the boundary of Run1 is higher than that of Run2 in the CPDS. It is also because that the hydrate is evenly distributed in Run 2, and the hydrate dissociation around the boundary prevents the temperature recovery rate. And the hydrate is not located around the boundary in Run1. However, the temperature distributions at 200 min for both Run1 and Run2 are similar, which indicates that the average heat transfer rates of both experiments are similar. The results lead to the similar gas production behaviors of Runs 1–2, which have been discussed in last section.
3.4. Sediment deformation After hydrate decomposition experiments, the quick-opening top cover of the set-up is opened to observe the sediment deformation directly. Figs. 8 and 9 shows the sediment surface deformation after Run1 and Run2. In Fig. 8, structure collapse of sediment is observed after the hydrate dissociation of Run 1 with the grain-displacing hydrate reservoir in first time, which indicates that the sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. The sediment collapse is due to the fact that the solid hydrate participates in constituting the structure of the HBS in grain-displacing hydrate. When the solid hydrate changes to liquid, the mechanical stability of the HBS is destroyed. As shown in Fig. 9, the fractures near the boundaries and sediment subsidence are observed, which indicates that the sediment particles have trend of gather during pore-filling hydrate dissociation. This result can be described as a radial shrinkage effect of sediment during porefilling hydrate decomposition. In order to avoid the influence of seepage on particle movement, a contrast test without hydrate formation is also carried out. In the contrast test, the pressure changes and production behavior are identical to those of Run 2. However, after the contrast test, the shrinkage phenomenon of sediment is not observed. Therefore, the shrinkage effect should be caused by hydrate decomposition. A cementation effect of hydrate in the HBS were reported. Thus, during the hydrate decomposition, sediment particles may be gathered by shirking hydrate in consolidated sediment, which causes the radial shrinkage effect. The diagram illustrating radial shrinkage of sediment during hydrate decomposition is given in Fig. 10. In general, although production behaviors and heat transfer characteristics during decomposition of grain-displacing hydrate and porefilling hydrate are similar, sediment deformation characteristics for different types of concentrated hydrate accumulations are totally different. In order to prevent geologic hazard during gas production from hydrate reservoirs, careful exploration for concentrated hydrate accumulation type in reservoir and calculation for geological deformation are necessary. 4. Conclusions In this work, a novel cubic reactor with a quick-opening cover was applied to investigate methane hydrate formation and decomposition in unconfined sediment under the geological conditions of the hydrate reservoir in the South China Sea. The experiments of methane hydrate decomposition with different types of concentrated hydrate accumulations are compared. And the influence of hydrate decomposition on the sediment deformation is analyzed. Two types of concentrated hydrate accumulations are selected, which are grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. Formation methods for the grain-displacing hydrate and pore-filling hydrate are different. Dissociation methods for both experiments are identical. The following conclusions are made:
3.3. Hydrate decomposition In order to investigate hydrate decomposition characteristics, temperature-pressure curves during DS of hydrate dissociation for Run 1 and Run 2 are shown in Figs. 6 and 7, respectively. The 3B, 5B, and 7B are selected as the reference points. As seen in Fig. 6, when pressure drops below 6.5 MPa (Y, Y′, Y″), the temperatures decrease rapidly from Y (Y′, Y″) to Z (Z′, Z″), which indicates that the hydrate start to be dissociated rapidly. However, when pressure drops from 13.5 MPa (X, X′, X″) to 6.5 MPa (Y, Y′, Y″), the decrease of temperature is also obviously. It may because that the grain-displacing hydrate is synthesized from ice. The Intermediate of Methane Hydrate (IMH) [40] during hydrate formation from ice may be generated. The IMH is an intermediate between ice and hydrate, which is destabilized during pressure
(1) The gas production behaviors from grain-displacing hydrate dissociation and pore-filling hydrate dissociation are similar, which indicates that the influence of hydrate morphology and hydrate 922
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distribution on gas production is not obviously. (2) The average heat transfer rates during grain-displacing hydrate dissociation and pore-filling hydrate dissociation are similar. The results lead to the gas production rates are similar. However, lowtemperature areas concentrate around the center of Layer A in grain-displacing hydrate dissociation, and the temperatures in porefilling hydrate dissociation are evenly distributed. It is due to the fact that the hydrate in grain-displacing type is distributed at the center of the Layer A, and the hydrate in pore-filling type is evenly distributed. (3) The obvious decrease of temperature before grain-displacing hydrate dissociation may be due to the destabilization of the Intermediate of Methane Hydrate (IMH). (4) Structure collapse of porous media is firstly observed in the experiments within the grain-displacing hydrate, which indicate that sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. (5) The radial shrinkage effect of sediment is found during pore-filling hydrate decomposition, due to the cementation effect of methane hydrate on sediment particles.
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Acknowledgments This work is supported by Key Program of National Natural Science Foundation of China (51736009), National Natural Science Foundation of China (51676190), Pearl River S&T Nova Program of Guangzhou (201610010164), International S&T Cooperation Program of China (2015DFA61790), Science and Technology Apparatus Development Program of the Chinese Academy of Sciences (YZ201619), Frontier Sciences Key Research Program of the Chinese Academy of Sciences (QYZDJ-SSW-JSC033), National Key Research and Development Plan of China (2016YFC0304002, 2017YFC0307306), Youth Science and Technology Innovation Talent of Guangdong (2016TQ03Z862), and Natural Science Foundation of Guangdong (2017A030313313), which are gratefully acknowledged. References [1] Li XS, Xu CG, Zhang Y, Ruan XK, Li G, Wang Y. Investigation into gas production from natural gas hydrate: a review. Appl Energy 2016;172:286–322. [2] Chong ZR, Yang SHB, Babu P, Linga P, Li XS. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl Energy 2016;162:1633–52. [3] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–9. [4] Boswell R. Is gas hydrate energy within reach? Science 2009;325:957–8. [5] 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 Reserv Eval Eng 2009;12:745–71. [6] Moridis GJ, Collett TS, Pooladi-Darvish M, Hancock S, Santamarina C, Boswell R, et al. Challenges, uncertainties, and issues facing gas production from gas-hydrate deposits. Spe Reserv Eval Eng 2011;14:76–112. [7] Boswell R, Shipp C, Reichel T, Shelander D, Saeki T, Frye M, et al. Prospecting for marine gas hydrate resources. Interpretation-J Sub 2016;4:Sa13–24. [8] Song YC, Yang L, Zhao JF, Liu WG, Yang MJ, Li YH, et al. The status of natural gas hydrate research in China: a review. Renew Sust Energ Rev 2014;31:778–91. [9] Yang MJ, Fu Z, Jiang LL, Song YC. Gas recovery from depressurized methane hydrate deposits with different water saturations. Appl Energy 2017;187:180–8. [10] Feng J-C, Wang Y, Li X-S. Large scale experimental evaluation to methane hydrate dissociation below quadruple point by depressurization assisted with heat stimulation. Energy Procedia 2017;142:4117–23. [11] Wang Y, Li XS, Li G, Huang NS, Feng JC. Experimental study on the hydrate dissociation in porous media by five-spot thermal huff and puff method. Fuel 2014;117:688–96. [12] Fitzgerald GC, Castaldi MJ, Zhou Y. Large scale reactor details and results for the formation and decomposition of methane hydrates via thermal stimulation dissociation. J Petrol Sci Eng 2012;94–95:19–27.
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