Natural Gas Production from a Marine Clayey Hydrate Reservoir

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Natural Gas Production from a Marine Clayey Hydrate Reservoir Formed in Seawater using Depressurization at Constant Pressure, Depressurization by Constant Rate Gas Release, Thermal Stimulation and their Implications for Real Field Applications Vishnu Chandrasekharan Nair, PAWAN GUPTA, and Jitendra S. Sangwai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00187 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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Research Article

Natural Gas Production from a Marine Clayey Hydrate Reservoir Formed in Seawater using Depressurization at Constant Pressure, Depressurization by Constant Rate Gas Release, Thermal Stimulation and their Implications for Real Field Applications

Vishnu Chandrasekharan Nair, Pawan Gupta, Jitendra S. Sangwai* Gas hydrate and Flow assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

Corresponding Author: Jitendra Sangwai: [email protected] Phone: +91-44-2257-4825 (Office) Fax: +91-44-2257-4802

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ABSTRACT Depressurization approach of methane production from natural gas hydrate reservoir has been identified as the most energy efficient production approach. However, some of the field scale studies involving constant pressure depressurization did not yield significant success. To address this, the constant rate gas release depressurization approach can overcome the drawbacks of constant pressure depressurization approach. The experimental investigations of these methods with and without thermal stimulation have not yet been investigated in detail for marine clayey hydrate reservoirs formed in seawater to understand their comparative effectiveness for methane gas recovery. Although common production approaches have been experimented by quite a lot of researchers on hydrate bearing sand sediments, energy recovery from hydrate rich clayey sediments have not yet been investigated in detail, which form the major dominant hydrate reservoirs of hydrate resource pyramid across the globe. This work investigates in detail the potency of five different natural gas production techniques such as constant rate gas release and constant pressure depressurization, thermal stimulation and their combination to produce natural gas out of the marine clayey hydrate system. To simulate marine conditions, mud samples with 3 wt% of bentonite clay in seawater have been used for methane hydrate formation at an initial pressure of 8±0.2 MPa and temperature of 278.15±1 K. The thermodynamic phase equilibrium study of methane hydrate in marine clayey system has also been conducted to understand the phase stability of hydrate. Subsequently, the study on five different methane recovery approaches to recover natural gas from marine clayey hydrate systems have been carried out to understand their efficacy. For constant rate gas release depressurization, two rates, viz., 10mL/min and 20mL/min have been used, while for constant pressure depressurization, two set pressures of 3.5 and 2.3 MPa have been used. Thermal stimulation was carried out by increasing the hydrate reservoir temperature from 278.15 K to 298.15 K. Field implications of these five production schemes have also been discussed in detail for their real field applications.

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Keywords: Combined Thermal Stimulation and Depressurization; Depressurization at Constant Pressure; Depressurization by Constant Rate Gas Release; Marine Clayey Hydrate Reservoir; Thermal Stimulation.

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1. INTRODUCTION Natural gas hydrates are crystalline ice like compounds that are formed when water makes a cage-like structure around small methane molecules. The cage-like structures are formed by means of hydrogen bonds and the guest gas molecules are stabilised by van der Waals forces.1,2 Formation of hydrates is possible in any natural or artificial environment if water is present with guest gas molecules provided that these two constituents must come in contact at low temperature and high pressure conditions. They serve as an abundant reservoir of natural gas. It is estimated that, under STP, a unit volume of gas hydrate can store up to 180 unit volumes of methane gas.3,4 While the estimates of total conventional natural gas resources contribute to about 13,000 trillion cubic feet (TCF), the global reserves of natural gas hydrates stands at a range of 100,000 to 300,000,000 TCF.5 Owing to the less pollution that occurs when gas is burnt as compared to oil and coal, gas hydrates have significant importance in the clean energy requirements of the world. As the available oil and gas reserves are depleting, it is important to explore new energy resources available in the form of natural gas hydrates which are present in deep ocean environments under high pressure conditions and those in permafrost regions under low temperature conditions.6 Hydrate bearing sediments across the world range from fine grained to coarse grained, and of varying sizes. Based on the sediment type in which hydrate is formed, gas hydrate can be classified into three main categories such as: (1) clay dominated, such as in Gumusut-Kakap offshore deposit, Malaysia,7 Blake plateau in the western Atlantic Ocean and Krishna-Godavari (KG) basin, India;7–9 (2) Sand dominated hydrate reservoirs such as the Eileen-Tarn in Northern Alaska,9 Shenhu area, South China Sea;10 and (3) Complex gas hydrate reservoirs (clay with partial sand/silt) in offshore Indian Peninsula.11

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Even though the sand dominated hydrate reservoirs are favourable for methane recovery, about 90% of global hydrate reserves are stored in fine grained clayey sediments.7,12,13 Minimal research has been done for hydrate bearing clayey sediments across the world due to several possible impediments during gas production.11,14,15 In the recent past, investigators have given more emphasis on hydrate studies involving coarse grained sediments which lies on the top of the hydrate resource pyramid and hence very less literature is available on clayey dominated reservoirs.13 It is to be noted that the methane concentration in gas hydrate (1:~7) exceed methane solubility in water (1:~700 at SPT). Therefore, in water saturated sediments hydrate formation is transport limited. Since advection is slow in case of poor conductive clayey sediments, diffusive transport will prevail.12 In case of finer sediments, the capillarity dominates over the skeleton force (of clay particles) and hence gas hydrates will tend to be more segregated in them due to lower stress associated with, thus leading to the formation of interconnected networks as seen in the Krishna Godavari Basin and the Ulleung Basin.12,16,17 Marine clays can be of various types like bentonite, kaolinite and illite.18 Among these, bentonite is the most common clay found in hydrate reservoirs including offshore KG basin, India.19 The clay percentage of present in hydrate bearing sediments ranges from 0 to 100%. The minimum clay content was found to be ~2.79% in KG basin, offshore India.20 Since modelling studies on clayey hydrate reservoirs are very rare, field tests have not yet been performed till date. The unconsolidated nature and lack of matrix permeability of the mud makes the application of common production approaches challenging.21 According to Jung et al.,22 the in situ hydrate saturation is limited in case of fine clay particles. Hydrate saturation in pore filling types reserves can be greater than 70% in clean sands, whereas in clayey sediments, this can be as low as 30%.22

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Since gas hydrates are comparatively immobile and impermeable, they need to be dissociated into their constituent gas and water before the methane recovery from hydrate reservoirs is possible. This requires latent heat of hydrate dissociation. Decomposition of gas hydrates is based on three principles; (1) Thermal stimulation method in which an external means such as hot steam or brine injection is used to heat the reservoir above the hydrate dissociation temperature,23,24 (2) Depressurisation in which reservoir pressure is reduced below the equilibrium hydrate dissociation pressure,25,26 (3) Chemical injection method using inhibitors such as methanol, glycol or polymers to reduce the hydrate dissociation temperature at reservoir pressure,27,28 (4) CO2 sequestration in which CO2 gas is injected into natural gas hydrate reservoirs to exchange methane from hydrate cages by CO2 gas, in which leads to the production of natural gas.29,30 Each of the above techniques has its own advantages and disadvantages. Thermal stimulation is least energy efficient since huge heat loss is incurred, and is challenging to implement.14 Even if depressurization is relatively an energy efficient process, it may lead to hydrate reformation during natural gas production and will choke the production tubing and wellbore.14,31,32 Inhibitor injection is not a viable option since the matrix permeability is of the order of 2.4 x 10-22 to 1.6 x 10-19 m2,33 which is too low as in case of clayey reservoir.34 The main hurdles associated with CO2 sequestration are the availability of large quantity of CO2 gas nearby and the liquefaction of CO2 at the injection pressure condition.35,36 In order to overcome these problems, researchers are modifying the existing production methods and also analysing the effectiveness of different combinations of these methods.10,32,37 From the studies conducted so far, it has been observed that depressurization technique is the least energyintensive, where the heat for dissociation of hydrate is provided by surrounding formation.38,39 Field tests at Nankai Trough, Japan and Mackenzie Delta, North Slope, Alaska also revealed 6


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that depressurization is a promising natural gas production method from hydrate reservoir.4,26,40–42 Depressurisation can be achieved by two methods; viz., depressurisation at constant pressure (by suddenly reducing the reservoir pressure to a predetermined value below the hydrate equilibrium pressure) and depressurisation by constant rate gas release.21,43 In laboratory studies and real field applications, the former approach is conventionally used.42,44,45 However, maintaining a constant pressure near the well-bore during hydrate dissociation in field conditions is hard to achieve. This is because hydrate dissociation normally occurs at variable rate and, also, the produced gas along with water is recovered at variable rate in order to maintain the constant pressure near well-bore, which is quite challenging to implement. Also, due to the sudden expansion of the natural gas and the pressure reduction in case of depressurisation at constant pressure, the temperature of the reservoir typically drops drastically due to Joule-Thomson effect and endothermic nature of the hydrate dissociation.2,22,38,46–48 Also, the sensible heat of the hydrate reservoir gets exhausted all of a sudden since heat energy from nearby strata does not get adequate time to propagate towards the dissociation site.49–51 This leads to hydrate reformation and ice formation within the vicinity of production well (or near well-bore) and as a result the forward propagation of dissociation front is further hindered significantly within the reservoir.7,38,49,52,53 As the matrix permeability of clayey hydrate reservoirs is very less, this effect will be more pronounced. However, the depressurization by constant rate gas release is characterised by controllable flow rate, hence it is easily manageable for hydrate production studies in real field applications. Also, other production issues such as hydrate reformation and ice formation are expected to be minimum in this approach since Joule-Thomson effect is weak due to the slow and gradual decrease in the pressure near well-bore. This method can be applied to a clayey hydrate reservoir and can be appropriate for any type of hydrate reservoir with an overlying/underlying gas cap.

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The studies on the production of methane from clayey hydrate reservoirs are rare in open literature. No field trails have been conducted so far to recover methane from clayey hydrate reservoir. Even though combination of production approaches have gained interest among the researchers,32,37,54 very little efforts have been made to analyse the synergic effect of different combinations of production methods in a clayey hydrate reservoir. Also the studies involving depressurization using constant rate gas release from the hydrate reservoir is rare in the open literature, and so its combination with other production methods. Depressurization at constant rate gas release was found to be an efficient recovery method for methane recovery from hydrate reservoirs.21 Since, conventional depressurization method represents more convincing way of methane recovery from hydrate reservoir,34,42 it is essential to conduct a study to analyse the relative advantages of depressurization using constant rate gas release over conventional depressurization at constant pressure. In addition, to simulate more realistic marine hydrate reservoir, actual seawater is a better choice than using the pure water, as the dissociation of hydrate in the presence of seawater differs from hydrate formed in pure water.55,56 In this study, a simulated marine clayey hydrate reservoir has been prepared using bentonite clay (3 wt%) and seawater obtained from Elliot's beach, Chennai, India. Hydrate in clay sediments and seawater are formed at an initial formation pressure of 8±0.2 MPa and formation temperature of 278.15 K. After the end of hydrate formation experiments, initial hydrate reservoir pressure and temperature for all the methane recovery studies used are 6.1±0.2 MPa and 278.15 K. Subsequently, five methane recovery approaches (with total of 16 experiments) have been carried out, in which three are standalone methods and two are combinations. These include: (1) Depressurization using constant rate gas release (CRD); (2) Depressurization at constant pressure (CPD); (3) Thermal stimulation (TS); (4) Combination 8


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of depressurization using constant rate gas release with thermal stimulation (CRD+TS); (5) Depressurization at constant pressure with thermal stimulation (CPD+TS). Two different rates of gas release, viz., 10mL/min and 20mL/min were used for CRD cases and three dissociation pressures viz., 3.5, 2.3 and 2 MPa were used in case of CPD experiments. The rationale behind using these three pressures for CPD cases has been discussed in detail in the results and discussion section. In case of thermal stimulation, the temperature of the hydrate reactor was increased from T=278.15 K to T+ ΔT = 298.15 K in 2.5 h, ΔT = 20K, at a rate of 8 K/h. 2. EXPERIMENTAL SECTION 2.1 Materials For hydrate formation and dissociation experiments, we have used methane (CH4) gas, bentonite clay and seawater. The details of suppliers and description of materials are provided in Table 1. 2.2 Experimental set-up The schematic diagram of the experimental set-up is shown in Fig. 1. It consists of a three dimensional jacketed high-pressure reactor made of SS-316 stainless steel and having a 1.4 L capacity, which is used for hydrate formation and gas recovery studies. The maximum working pressure of the reactor is 10 MPa. To measure the pressure and temperature inside the reactor, an HD20V4T (Delta Ohm, Italy) piezo-resistive pressure transducer and a Pt-100 classA temperature sensor with a working range up to 637 K are attached to the reactor. In order to maintain the system at the desired experimental temperature, ethylene glycol-water mixture supplied from a water bath (FP 50, Julabo, Germany) was circulated inside the jacket of the reactor. A magnetic stirrer was attached to the reactor for agitating the bentonite mud solution and methane gas continuously for faster kinetics of hydrate formation. Among the different recovery methods employed in the current study, the set-up shown in Fig. 1(a) was used for 9


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depressurization by constant rate gas release (CRD), combination of depressurization by constant rate gas release and thermal stimulation (CRD+TS) and standalone thermal stimulation method (TS). For depressurization at constant pressure (CPD) and its combination with thermal stimulation (CPD+TS), a back pressure (BP) valve and porous filter have also been attached to the set-up as shown in Fig. 1(b). By using a data acquisition system, the real time pressure and temperature values (reaction variables) were recorded as a functions of time in the computer at every 1 min. 2.3 Experimental procedure The experimental procedure comprises the formation of methane hydrate in simulated clayey sediment in seawater at an initial pressure of 8±0.2 MPa and at a constant temperature of 278.15±1 K. The hydrate formation was followed by the methane recovery experiments using various production methods. The total numbers of 16 experiments and their nomenclature with other details on nomenclature, gas release rate, pressure and temperature conditions have been shown in Table 2. 2.3.1 Hydrate formation in seawater-clay system Prior to the start of hydrate formation experiments, mud samples were prepared using seawater and bentonite clay having 3 wt% concentration to simulate marine clay sediments. The reactor was very well cleaned using distilled water after which it was charged with 600mL of the prepared mud sample. The reactor was maintained at a desired experimental temperature of 278.15±1 K by circulation of ethylene glycol-water mixture from the Julabo water bath through the jacket of the reactor. The stirrer was kept running until the stable experimental temperature of the mud sample is attained. The reactor was then pressurized with methane gas up to 0.2 to 0.5 MPa. After stirring for around 5 minutes, methane gas is vented to remove dissolved and free atmospheric air present in the system (purging). After two times of purging, 10


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the reactor was charged with methane gas up to the initial hydrate formation pressure of 8 MPa using a syringe pump (500 D, Teledyne Isco, USA). Again the stirrer was turned on at 400 rpm until hydrate formation in the mud sample is completed. The stirrer was used for quick hydrate formation and uniform hydrate distribution. On the temperature profile, a spike can be observed at the onset of hydrate formation. This is due to the exothermic nature of the hydrate formation reaction. Throughout the hydrate formation, the pressure of the system was found to decrease gradually and after 25-28 hours, the formation is assumed to get completed when no further reduction in pressure of the reactor is observed (less than 0.001 MPa/h) and, finally it reaches a constant value of 6±0.2 MPa in all formation experiments, which represent the initial hydrate reservoir pressure for subsequent methane recovery studies. Subsequently, the stirrer was turned off and the whole system was kept undisturbed for 3 to 4 hours to consolidate the mud hydrate system. 2.3.2 Methane recovery from hydrate reservoir Methane gas was recovered from simulated marine clayey sediments by dissociation of hydrate using following five different production schemes. 2.3.2.1 Hydrate dissociation by CRD Depressurisation at constant rate gas release (CRD) was employed by means of a high precision syringe pump (500 D, Teledyne Isco, USA), Fig. 1a. It is used to release the gas from the reactor at a constant rate and also to store the gas inside its cylinder. The volume inside the syringe pump increases at a constant rate (controlled precisely at 10 or 20 mL/min, see Table 2) as the piston of the syringe pump moves down, thereby decreasing the pressure of the hydrate reservoir gradually. Hydrate dissociation is significant when the pressure of the system reaches near the hydrate equilibrium pressure at the corresponding experimental temperature. The

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temperature of the reactor was kept at 278.15±1K throughout dissociation experiments carried out for 2.5 hours. 2.3.2.2 Hydrate dissociation using CPD As shown in Fig. 1(b), a BP valve was used to maintain the set depressurisation pressure values (3.5 and 2.3 MPa, Table 2) during hydrate dissociation using CPD. A porous filter was placed behind the BP valve to prevent any clay particles from entering into the BP valve. The BP valve was set at the desired experimental pressure and the exit valve of the reactor was opened in order to attain the constant set pressure for CPD. The excess gas inside the reactor was vented out till the constant set pressure is reached. As the set pressure is below the hydrate equilibrium pressure (~4.9 MPa at 278.2 K, as reported in this study henceforth), the hydrate dissociation started momentarily. As the hydrate dissociates, the pressure inside the reactor starts to build up and tries to reach above the CPD set pressure. Even a slight increase in the pressure of the reactor above the set pressure of the BP valve, the BP valve opens to release the gas until the reactor pressure returns to the set pressure. The automatic opening and closing of BP valves continue throughout the methane recovery experiments until the gas is produced due to CPD from the hydrate reservoir. The produced gas is collected in the separate reservoir (which is the cylinder of the syringe pump) continuously. The actual volume of the gas produced is measured from pressure and volume reading of the syringe pump. 2.3.2.3 Hydrate dissociation using thermal stimulation (TS) For methane recovery using thermal stimulation, temperature of the system was increased by step heating [T=278.15 K to T+ ΔT=298.15 K in 2.5 h (ΔT=20 K), at a rate of 8 K/h]. It was achieved by circulation of cold/hot ethylene glycol-water mixture from the jacket of the reactor using Julabo water bath. In order to ensure that that the methane recovery is solely due to thermal stimulation, the reservoir pressure was kept constant (close to 6.0 MPa, 12


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which is the initial hydrate reservoir pressure) by releasing the gas at a rate of 2.6 mL/min. This rate has been determined from gas dissociation profiles of earlier experiments which is sufficient to maintain the constant pressure condition.21 The average pressure of the reservoir during thermal stimulation is found to be 6.1±0.2 MPa. This hydrate reservoir pressure (6 MPa) is much above the equilibrium pressure of the hydrate system at the initial hydrate reservoir temperature of 298.15 K (~4.9 MPa at 278.2 K), thus ensuring only thermal stimulation and no depressurization. 2.3.2.4 Hydrate dissociation by CRD+TS To understand the synergistic effect of depressurization with constant rate gas release and thermal stimulation, a combination of CRD and TS was also explored. As explained in section 2.3.2.1, depressurisation was achieved using a syringe pump by releasing gas at constant rate at 20 mL/min from the reservoir. This is accompanied with simultaneous step heating by circulation of ethylene glycol-water mixture from the Julabo water bath through the jacket of the reactor (section 2.3.2.3). The CRD applied to the marine clay hydrate system was fixed at 20 mL/min along with an increase in temperature (TS) of T=278.15 K to T+ ΔT = 298.15 K in 2.5 h (ΔT = 20 K) at a rate of 8 K/h. 2.3.2.5 Hydrate dissociation using CPD+TS In this study, the other depressurisation scheme, i.e., CPD has also been combined with TS. The dissociation pressure was set at 2 MPa with the help of BP valve (see Table 2 for CPD+TS). The dissociated gas was collected in the syringe pump connected to the reservoir. The ramp profile applied is the same as that of TS and CRD+TS (section 2.3.2.3 and section 2.3.2.4).

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3. RESULTS AND DISCUSSION In this study, various experiments were performed for methane recovery from a simulated hydrate bearing marine sediments. After the formation of hydrate in seawater mud system, different recovery methods such as CPD, CRD, CPD+TS, CRD+TS and TS were carried out for comparing the effectiveness of each method. In this section, detailed analysis on methane hydrate formation and dissociation using above mentioned approaches has been provided. 3.1. Phase equilibrium of the marine clayey hydrate system Six thermodynamic experiments were carried out to understand the phase equilibrium condition of marine clayey hydrate system. The procedure for thermodynamic study is same as the usual procedure of hydrate thermodynamic experiments which is available in open literatures and not repeated here for the sake of brevity.57–59 The phase equilibrium conditions for marine clayey hydrate system has been shown in Fig. 2 along with the methane hydrate equilibrium conditions in pure water,60 seawater55 and 3.3 wt% NaCl+water (obtained from CSMHYD2). It has been found that the seawater clay system acts as a thermodynamic inhibitor similar to seawater and 3.3 wt% NaCl systems. From our earlier study,21 it was found that the bentonite clay is not affecting the stability of methane hydrate significantly as compared to methane hydrate in pure water system. Thus, as expected, in this case the inhibition effect is expected mostly due to the salts present in the seawater, as also observed in various other studies.2,24,55,56,61 This phase equilibrium study is very important in analysing the thermodynamic process path of hydrate dissociation using different production approaches as investigated in this work.

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3.2. Hydrate formation in marine clayey sediments The details on hydrate formation experiments are shown in Table 3. Equations used for the calculation of moles of gas consumed during hydrate formation, the rate of gas consumption, gas-to-hydrate conversion, water-to-hydrate conversion have been provided in the Supporting Information (Eqs. S1 to S4). Fig. 3 shows the sample results on methane hydrate formation experiment. Fig. 3(a) shows the sample temperature and pressure curve during hydrate formation in marine clayey sediments. Initially, when the reactor was charged with methane gas, the pressure was maintained at 8±0.2 MPa and the temperature was kept constant at 278.15±1 K throughout the hydrate formation experiment. It took around 25 to 30 hours for the completion of hydrate formation experiment. From Fig. 3(a), it can be observed that the pressure is declined from the initial formation pressure of 8±0.2 MPa to a final pressure of 6±0.2 MPa during the hydrate formation experiment. This pressure (6±0.2 MPa) at the end of the hydrate formation experiment is much above to the equilibrium pressure of pure water and seawater hydrate systems [~4.24 MPa, pure water60 and ~4.9 MPa, seawater55 at 278.15 K], and no further decline in pressure has been observed thereafter indicating the end of hydrate formation. Hydrate induction is clearly visible from the temperature profile, where a small spike in temperature can be detected, at which there is a sudden decrease in the pressure of the system. This is due to the exothermic nature of hydrate formation.2 Also from Fig. 3(a), it can be observed that there is a sudden decline in the pressure from 8.4 to 8 MPa. This due to the dissolution and adsorption of methane gas in the mud at initial stage.

Fig. 3(b) illustrates the cumulative moles of methane consumption and the rate of gas consumption during hydrate formation in simulated marine conditions. The average number of moles of methane consumption was found to be 1.18±0.01 mol in all the hydrate formation experiments. The maximum rate of methane consumption during hydrate formation was 15


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observed to be 0.43±0.06 mol/h, particularly during the initial stage of hydrate formation. The reason for this trend can be correlated to the initial high pressure of the system as the pressure is acting as the driving force during isothermal hydrate formation. As the pressure of the system decreases, the rate of hydrate formation also declines. The average water-to-hydrate conversion and gas-to-hydrate conversion were found to be 21.57 ± 0.26 and 31.38 ± 0.32%, respectively. Table 3 gives the detail of the all hydrate formation experiments. 3.3. Methane recovery from simulated marine clayey hydrate system Five different production schemes, viz., CRD, CPD, CRD+TS, CPD+TS and TS were carried out and their effects on methane recovery from marine hydrate sediments were compared. Before carrying out CPD experiments, it is important to decide the constant pressure conditions for hydrate dissociation to be used for the CPD experiments so that the results can be compared for various production schemes used in this study. 3.3.1. Estimation of dissociation pressure for CPD and CPD+TS Two different hydrate depressurisation approaches using constant pressure and constant rate gas release (CPD and CRD) with or without thermal stimulation (TS) have been performed in the current study to compare their relative effectiveness on the methane recovery from simulated marine clayey hydrate reservoirs. In order to perform a meaningful comparison between CRD and CPD dissociation schemes for methane recovery from marine hydrate reservoirs, the end pressure conditions (at the end of methane recovery experiments) should be the same in both the cases and thus the following procedure has been adapted. Fig. 4 shows dissociation pressures for each CPD and CPD+TS experimental schemes. During CRD experiments, depressurization was achieved by releasing the gas from the hydrate reservoir at constant rate (10 or 20 mL/min). The pressure during the CRD experiments found 16


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to decrease from the initial reservoir pressure (6±0.2 MPa) to a pressure (3.5 MPa for 10 mL/min and 2.3 MPa for 20 mL/min) which is much below the equilibrium pressure of the methane hydrate system (~4.9 MPa at 278.15 K) at the end of two hours of dissociation experiment. The pressures of CRD experiments after 2 hours of dissociation (reference) time were noted down. To perform a meaningful comparison, these pressure values were used as the constant dissociation pressure for the corresponding CPD experiments so that the dissociation time as well as the reservoir conditions at the end of the CPD and CRD experiments are nearly the same. Fig. 4 shows that the constant dissociation pressure for each CPD experiments coincides with the final pressure of their respective CRD experiments. CRD experiments were carried out at two different rates of 10 and 20 mL/min. The corresponding dissociation pressure during CPD are found to be 3.5 and 2.3 MPa for CRD 10 mL/min and CRD 20 mL/min, respectively. Also for the combination of depressurization by constant rate gas release with thermal stimulation (CRD+TS), the pressure at the end of hydrate dissociation was found to be 2 MPa and the same has been used as a constant depressurization pressure during CPD+TS experiment. It can also be observed from the Fig. 4 that the pressure data of CPD (3.5MPa and 2.3MPa) and CPD+TS (2MPa) are almost constant. This ensures the dissociation pressure has been properly maintained throughout the dissociation experiment. Table 2 gives the details on the conditions of pressure, temperature and the rate of gas release used during methane recovery studies. 3.3.2. P-T profile during methane recovery using different production schemes The P-T profile of various production approaches used in this work are plotted together with the phase equilibrium curve of marine clayey hydrate system and is illustrated in Fig. 5. The thermodynamic process path of hydrate dissociation during each methane recovery method can be analysed. The P-T profile of CRDs follows an approximate vertical path with decrease 17


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in pressure, since the temperature of the hydrate reservoir is kept constant at 278.15±1 K irrespective of the pressure decrease, owing to a constant rate gas release from the reservoir. As the hydrate dissociation is endothermic in nature,2 the temperature of the hydrate reservoir fluctuates (decreases slightly) by the order of ~1 K to ~3 K near the hydrate equilibrium condition, where the dissociation is very fast.24 Higher decline in temperature near hydrate equilibrium condition can be observed in the case of CRD (20 mL/min) as compared to CRD (10 mL/min). This is due to the fast dissociation of methane hydrate due to higher dissociation rate leading to considerably faster gas release. The P-T profile of CPDs can be seen as a cluster of points concentrated in a small area below the equilibrium curve of hydrate as there is no considerable change in temperature (278.15 K) or pressure [3.5 MPa for CPD (3.5 MPa), and 2.3 MPa for CPD (2.3 MPa), Table 2] of the system during the hydrate dissociation. In case of TS experiments, the methane recovery has been achieved by increasing the temperature from 278.15 K to 298.15K while keeping the pressure of the system constant (close to 6 MPa as shown in Fig. 5), this is done so as to ensure that the methane recovery is solely due to thermal stimulation and not by depressurization. This is achieved by the controlled gas release (2.6 mL/min) used in case of TS experiments so as not to affect the pressure of the hydrate reservoir. Hence, the thermodynamic process path of thermal stimulation (TS) follows a horizontal line in the P-T diagram. During the combination of depressurization using constant rate gas release and thermal stimulation (CRD+TS), the process path follows a slanted line with simultaneous pressure reduction and temperature increase of the hydrate reservoir. In case of CPD+TS, it follows a horizontal process path as the temperature of the system was increased while keeping the pressure constant (2 MPa, see Table 2) below the equilibrium curve of hydrate system (Fig. 5). 3.3.3. Comparison of pressure and temperature profiles using CRD and CPD experiments 18


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Fig. 6 depicts the pressure and temperature profiles during methane recovery experiments as a function of time. In Fig. 6, the pressure profiles of CRD [Fig. 6(a and b)] and CRD+TS (Fig. 6c) experiments can be distinguished into three phases. The phase boundaries were defined based on the slop change in the pressure curve. However these boundaries are approximates as the hydrate dissociation starts and completes gradually. During phase 1, a rapid

pressure decline has been observed which is mainly due to the release of the gas from the free zone of the hydrate reservoir to the accumulator due to gas expansion whilst gas release from hydrate dissociation is significantly low since the pressure is much above the hydrate equilibrium condition. At the onset of hydrate dissociation, the slope of pressure profile is relatively reduced as soon as the pressure reaches near hydrate equilibrium pressure (~4.9 MPa at experimental temperature of 278.15 K). This is because gas release by hydrate dissociation tries to supports the maintenance of reservoir pressure. This region is marked as phase 2. Later on, the pressure decline became rapid as further gas is produced from hydrate in sediments due to dissociation, which hardly supports the pressure maintenance of the reservoir indicating the end of hydrate dissociation as in phase 3. In Fig. 6, the pressure profile of CPD [Fig. 6(a and b)] and CPD+TS (Fig. 6c) experiments follows a horizontal line as the dissociation pressure is almost constant throughout the hydrate dissociation [which indicate the constant pressure dissociation (CPD) scheme]. When the temperature profiles are compared, the temperature decline due to the endothermic nature of hydrate dissociation can be clearly observed in case of CRD and CRD+TS than CPD and CPD+TS (particularly during phase 2 in all the cases as in Fig. 6a-c). It is supposedly due to the constant rate condition adopted in case of CRD and CRD+TS conditions giving enough time for hydrate dissociation from the entire reservoir. In case of CPD and CPD+TS, the hydrate dissociation is expected to happen at a very high rate at the beginning and gradually progressing from the top of the reservoir to the bottom. But, in the 19


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current study, since the temperature sensor is placed near the bottom of the reservoir, it was not possible to capture the temperature decline due to hydrate dissociation at the top layers of the reservoir.

3.3.4. Comparison of moles of gas release and rate of gas release for different production schemes The moles of gas released and rate of gas release during hydrate dissociation have been calculated using Eq. S6 and S7 in Supporting Information, respectively. During the initial analysis of the results on various production scheme, it has been observed that the cumulative number of moles of gas released during CPD and CPD+TS experiments were considerably less when compared to the number of moles of gas consumed in the hydrate phase during hydrate formation, than in case of CRD and CRD+TS schemes. For ruling out the chance of any experimental errors, the experiments were repeated thrice, but obtained the same trend as mentioned. This difference may be possible due to the fact that some amount of gas (due to hydrate dissociation) during CPD and CPD+TS could have been escaped from the hydrate reservoir during the initial venting phase of free gas to set the set point pressure of CPD (3.5 MPa and 2.3 MPa) and CPD+TS (2 MPa). Haligva et al. also reported this kind of observation in case of CPD and stated that they could not capture the whole gas released during the initial pressure reduction using CPD scheme.45 To confirm our belief, and to improve the accuracy of the experimental results by incorporating any gas escaped from hydrate phase during venting period in CPD and CPD+TS schemes (which if not accounted for, may results in experimental errors), an additional supplementary experiment has been performed as shown in Fig. 7. This experiment was performed after the hydrate formation inside the reactor as mentioned in Section 3.2. In this case, the amount of free gas has been measured at the end of hydrate 20


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formation. Subsequently, the pressure of the system has been decreased in three steps to match the set point pressure of CPD and CPD+TS schemes (e.g., 3.5 MPa, 2.3 MPa and 2 MPa) using the BP valve attached to the hydrate reactor. During this, the released gas at each step has been collected in the cylinder of the syringe pump and an accumulator (separate reservoir) connected to the syringe pump and measured. The number of moles of gas released at each step has been calculated and the initial number of moles of free gas was subtracted from it. This gave the amount of gas escaped from the hydrate phase during the initial venting phase of the CPD and CPD+TS experiments. Fig. 7 shows the number of moles of gas released from the hydrate during the initial venting phase for pressure settings of each dissociation pressures during CPD and CPD+TS. This value has been added to the number of moles gas released for corresponding CPD and CPD+TS experiments. This has improved the accuracy of the experiments performed during the whole study and provided an important information while making comparisons of different production schemes studied in this work. Fig. 8 shows the comparison of number of moles of gas recovered during different depressurization approaches (CPD, CRD, CPD+TS and CRD+TS). Table 4 provides the details on various hydrate dissociation experiments. As discussed above during CPD and CPD+TS experiments a small fraction of methane gas dissociated from hydrate was also found to be escaping during the venting phase for setting up the dissociation pressure. This amount of gas escaped from hydrate dissociation has been measured by a separate experiment and incorporated in the initial point of corresponding experimental data. This is the reason that the moles of gas recovered (CPD and CPD+TS) are not starting from the origin. In Fig. 8(a), number of moles of gas recovered during CRD (10 mL/min) is compared with the corresponding CPD (3.5 MPa), see details discussed as in section 3.3.1. It can been seen that, during hydrate dissociation, CPD yielded more number of moles of methane gas as compared 21


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to CRD. But in case of CRD (20 mL/min) and corresponding CPD (2.3 MPa), see Fig. 8(b), number of moles of gas released is found to be higher in CRD, though initially CPD has shown faster dissociation. In case of CRD+TS (20 mL/min) and CPD+TS (2 MPa), both methods yield almost equal number of moles at the end hydrate dissociation as shown in Fig. 8(c). All the CRD experiments show a gradual increase in the recovery while the CPD experiments show a comparatively sudden increase in the recovery during the initial stage of dissociation experiments and thereafter reaches a plateau. It is because in all the CPD experiments, the set pressure and temperature condition fall well within unstable region of hydrate equilibrium curve, thus releasing the gas from hydrate phase quickly. However, in case of CRD experiments, pressure of the hydrate reservoir gradually decreases and reaches the hydrate equilibrium value after some time from the start of dissociation experiment. Thus, the moles of gas released during CRD and CRD+TS scheme increases gradually during the hydrate dissociation time. Further implications of CPD and CRD schemes on real filed application has been discussed subsequently. Fig. 9(a-c) compares the rate of gas released during CPD and CRD experiments. During all the CPD experiments, it was found that the dissociation starts with a very high rate of gas release followed by an exponential decline thereafter. However, during CRD experiments, the rate of gas release gradually increases and reaches a maximum value (when the reservoir conditions are near the hydrate equilibrium condition of ~4.9 MPa at 278.15 K) and then decreases thereafter. It is because, during CPD experiments, the thermodynamic state of the system falls in the hydrate unstable condition (the set pressure and temperature) at the very beginning of the dissociation experiment. However, during CRD experiments, the thermodynamic process path starts from hydrate stable condition and reaches the hydrate unstable condition gradually (see Fig. 5). In all the cases, the maximum rate of gas release 22


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(which is particularly during the beginning of CPD schemes) is higher for CPD experiments as compared to corresponding CRD experiments (also reported in Table 4). Figs. 10 (a and b) show the comparison of cumulative number of moles of gas recovered during different CRD and CPD experiments along with TS. From Fig. 10(a), it can be observed that the cumulative gas recovery at the end of 2 hours (reference) experimental time is in the order of CRD+TS(20 mL/min) > CRD(20 mL/min) ≥ TS > CRD(10 mL/min). It follows the order of decreasing driving force for the hydrate dissociation. The TS (8 K/h) and CRD (20 mL/min) has almost the same methane recovery potential in the current experimental setup. Almost in all the cases, the dissociation is completed in an about 2.5 hours. Fig. 10(b) shows the comparison of number of moles of gas recovered (cumulative) during CPD experiments and TS experiment. It has been observed that, in case of TS and CPD+TS, the cumulative recovery reaches to completion in an about 2 hours from the start of methane recovery experiment. During CPD (2.3 MPa) and CPD (3.5 MPa), the final moles of methane recovered was found to be approximately equal but less than CPD+TS and TS cases. The reason for this trend is discussed subsequently. Fig. 11(a) and Fig. 11(b) shows the average rate of gas release during CRD and CPD experiments (with and without TS), respectively. In Fig. 11(a), the maximum rate of gas release (0.89 mol/h, Table 4) was observed in case of CRD+TS due to its higher driving force as compared to other experiments. Similarly in Fig. 11(b), the maximum gas release rate (1.31 mol/h, Table 4) among the CPD experiments was observed in case of CPD+TS. Contrary to the expected initial higher rate of gas release in case of CPD (2.3 MPa) due to its lower set pressure and thus higher driving force for hydrate dissociation than CPD (3.5 MPa), it was found to be lower. For confirmation, CPD (2.3 MPa) experiment was repeated thrice but obtained the same results. Even though, for CPD (3.5 MPa) with a difference in the driving force of ΔP= 1.2 MPa (3.5 MPa-2.3 MPa) than the corresponding equilibrium pressure 23


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of methane hydrate in clay+seawater system (~4.9 MPa at 278.15 K), the rate of gas release in the initial phase during CPD (2.3 MPa) was found to be lower than CPD (3.5 MPa). Though considerably high recovery of methane gas was also expected in case of CPD (2.3 MPa) as compared to CPD (3.5 MPa) [also see Fig. 10(b)], but it was not as anticipated. It is expected that the hydrate dissociation from the upper layer of the hydrate reservoir might have occurred due to the sudden release of gas from the overlying gas cap of the reservoir in case of CPD (2.3 MPa). As a result of this, temperature of the upper hydrate layer was likely to decrease due the endothermic nature of hydrate dissociation. This could lead to the formation of ice and/or reformation/self-preservation of hydrate at the upper layer which in turn hinder further propagation of the dissociation front within the reservoir, which is in combination with the induced Joule-Thomson effect. This is assumed to be the reason behind the initial low rate of gas release in case of CPD (2.3 MPa) than CPD (3.5 MPa). In case of CPD+TS, the heat provided externally helps to negate the Joule-Thomson effect and also dissociate the hydrate more effectively than CPD (3.5 and 2.3 MPa), thus giving higher recovery and the rate of gas release (also as observed in Fig. 10). Similar observations have been reported in open literatures indicating lower gas recovery due to possible ice formation and/or reformation or selfpreservation of hydrate.38,49,52,53,62,63 In case of only TS experiment (Figs. 10 and 11), the cumulative moles of gas release and the rate of gas release is found to be lower initially, but increases gradually. The rate of gas release reaches at maximum value at 1.2 hrs (approx.) when the temperature of the reservoir reaches close to near-equilibrium conditions of the hydrate dissociation (~280.1 K at 6 MPa for TS case). 3.3.6. Methane recovery using different production methods Fig. 12 and Table 4 show the average percentage of gas recovered (cumulative) using different methane recovery methods from simulated marine clayey hydrate reservoir. As 24


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observed in this study, CRD+TS gave a maximum percentage of recovery among all the recovery methods in all the cases. Comparing the respective CRD and CPD experiments, CRD (20 mL/min) experiment yields more recovery as compared to CPD (2.3 MPa), while CRD (10 mL/min) gave less recovery as compared to corresponding CPD (3.5 MPa). Hence, it can be interpreted that there must be a certain gas release rate for CRD to outperform the corresponding CPD production schemes, and the precise threshold (crossover) to be determined may depend upon the reservoir conditions and may need further investigations. The combined approaches, such as CRD+TS and CPD+TS, show better recovery as compared to the corresponding standalone depressurization methods (only CPD or CRD cases), with CPD+TS yielding slightly lower recovery as compared to CRD+TS. This is supposedly due to the fact that the TS has provided enough heat energy to overcome the hurdles of standalone CPD, such as possible ice formation and/or reformation or self-preservation of hydrate. The standalone TS method gave more recovery as compared to the standalone CPD experiments. But the CRD (20 mL/min) was found to be better in terms of recovery as compared to TS experiment. In addition, it was observed that there is not much difference in the recovery percentage of CPD (2.3 MPa) and CPD (3.5 MPa). It can, thus, be inferred that providing a greater driving force could not always ensure high methane recovery during CPD experiments due to the reasons discussed in the previous section. As compared to pure water hydrate system,21 slightly higher methane recovery using CRD schemes has been observed in case of seawater-clayey hydrate system. The thermodynamics inhibition of saltwater is expected to be the main reason for the higher recovery in the presence of seawater system as compared to pure water system. 4. FIELD IMPLICATIONS

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Methane recovery from natural gas hydrate is a complex process, and it incorporates heat and mass transfer, kinetics of hydrate dissociation, multiphase flow, thermodynamics and formation stability.64,65 Different production methods have been tested in the lab as well as in the field to identify the most suitable method for methane recovery for different types of hydrate reservoirs.32,43,63 Due to the ease in methane production, most of the studies have been conducted mainly in high permeable sandy hydrate reservoir at field and lab scales. However, maximum hydrate deposits are associated with clayey sediments as nodules or veins with very low matrix permeability.7,34 Therefore, it is the need of an hour to study and develop an efficient methane production method that can be applicable to vast majority of hydrate reservoir (both in clay and sand dominated hydrate reservoirs). Depressurization approach of methane production from hydrate reservoir has been identified as the most energy efficient production approach.26 The production of methane hydrate by depressurization are subjective to several factors, such as reservoir properties, production rate, production pressure, ambient temperature, temperature of the surrounding strata, combination of other dissociation methods, sand production, etc. In order to ensure continuous gas production with minimum water production, improved formation stability and wellbore stability, researches in the lab and field scales have modified the conventional standalone single step depressurization production method. Multistep depressurization and combination of depressurization method with other recovery method such as thermal stimulation, inhibitor injection have been investigated.21,37,64,65 For example, multistep depressurization approach was implemented in the Mallik 5L-38, Canada (2008) Nankai Trough, Japan (2017). Initially in Nankai Trough, single step depressurization approach was used (in 2013), however the production of the gas couldn’t last for more than a week.42 Subsequently, in Nankai Trough, Japan (in 2017) and also in Mallik, Canada (in 2008), the pressure of the reservoir was decreased in steps of 13.5, 7, 5, 3 MPa (for Nankai Trough) and 11, 7.4, 5, 4.2 MPa (for Mallik), respectively.64,66 Zhou et al.65 carried out simulation 26


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studies using the single and multistep depressurization schemes to understand methane recovery from the hydrate reservoirs by CMHGS simulator. From these studies, it has been found that multistep depressurization approach is more efficient than the single step depressurization approach. Here, in our study, we have adopted hydrate depressurization using constant rate gas release (CRD) and also constant pressure depressurization (CPD) approach. In case of CRD, pressure has been decreased gradually as the gas is released from the hydrate reservoir thereby achieving countless multistep depressurization. Class 1 type of hydrate reservoir naturally has an underlying free gas cap unlike other classes of hydrate reservoirs. However, after initial dissociation of hydrate using any production methods and pumping out the free water, free gas layer can be formed in other classes of hydrate reservoirs as well, which can be produced using CRD scheme more effectively than CPD scheme. Also, occurrence of other production issues such as hydrate reformation/self-preservation and ice formation is expected to be minimum in case of CRD approach since Joule-Thomson effect is weak due to the slow and gradual decrease in the pressure due to slow and controlled rate of hydrate dissociation and also near well-bore skin damage due to hydrate reformation could be minimal. Apart from that, the sensible heat of the reservoir will not get exhausted all of a sudden since heat energy from nearby strata gets adequate time to propagate towards the dissociation site in CRD schemes than CPD schemes. At the outset of this study, it has been found that CRD and CRD+TS could be more beneficial production schemes than another methods for most of the hydrate reservoirs. 5. CONCLUSION To develop an efficient approach to recover methane from clayey marine natural gas hydrate sediments, five different production methods have been investigated, viz., depressurisation by constant rate gas release, depressurisation at constant pressure, thermal 27


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stimulation, and their combinations. For depressurisation by constant rate gas release experiments two different gas release rates, viz., 10 mL/min and 20 mL/min were used, while two corresponding constant pressure conditions of 3.5 and 2.3 MPa were used for depressurisation at constant pressure experiments. Combination of depressurisation at constant pressure and thermal stimulation experiment have also been conducted at 2 MPa. Thermal stimulation was carried out from 278.15 to 298.15 K. The key findings of the current study have been given below: 1. Phase equilibrium studies reveals that the seawater-clay system acts a thermodynamic inhibitor due to the presence of salts in the seawater. The thermodynamic inhibition effect of seawater-clay system was found to be similar to seawater and 3.3 wt% NaCl. 2. The pressure profiles of depressurisation by constant rate gas release and its combination with thermal stimulation experiments can be distinguished into three phases. During phase 1, a rapid pressure decline has been observed which is mainly due to the release of the gas from the free zone. During phase 2, gas release by hydrate dissociation supports the maintenance of reservoir pressure. During phase 3, the pressure decline became rapid as further gas produced from hydrate in sediments due to dissociation hardly supports pressure maintenance of the reservoir. The pressure profile of depressurisation at constant pressure and its combination with thermal stimulation experiments follows a horizontal line as the dissociation pressure is almost constant throughout the hydrate dissociation. 3. Throughout the experimental run, the methane recovery is gradual for all the depressurisation by constant rate gas release experiments, but for depressurisation at constant pressure experiments, recovery is high during the initial stage of dissociation and thereafter reaches a plateau.

28


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4. From the depressurisation by constant rate gas release experiments (comparing two gas release rate, viz., 20 and 10 mL/min), it was observed that the higher rate of gas release will lead to faster hydrate dissociation. So it is important to determine the optimum release rate depending on the reservoir conditions for the efficient production of natural gas. 5. Higher methane recovery was observed in case of depressurisation by constant rate gas release (20 mL/min) and depressurisation by constant rate gas release + thermal stimulation (20 mL/min) experiments as compared to the corresponding depressurisation at constant pressure (2.3 MPa) and depressurisation at constant pressure + thermal stimulation experiments (2 MPa). This is perhaps due to the formation of ice and/or reformation/self-preservation of hydrate at the upper layer which in turn hinders further propagation of dissociation front within the reservoir. 6. While comparing the standalone depressurisation at constant pressure experiments, it was found that the maximum rate of recovery was observed to be higher in case of 3.5 MPa as compared to 2.3 MPa. Also, there was not much difference in the recovery percentage of depressurisation at constant pressure (2.3 MPa and 3.5 MPa). Therefore, greater driving force could not always ensure high methane recovery during depressurisation at constant pressure experiments due to the possible ice formation and/or reformation or self-preservation of hydrate. 7. The combined methods (depressurization + thermal stimulation) was found to have higher dissociation potential as compared to standalone methods due to higher driving force. 8. It has been found that multistep depressurization approach is more efficient than the single step depressurization approach. Here, in our study, we have adopted hydrate depressurization using constant rate gas release in addition to constant pressure 29


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depressurization approach. In case of depressurization by constant rate gas release, pressure has been decreased gradually as the gas is released from the hydrate reservoir thereby achieving countless multistep depressurization, which is not the case in constant pressure depressurization approach.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Details on the various equations used for the calculation of moles of gas consumed, rate of gas consumption, gas-to-hydrate conversion and water-to-hydrate conversion during hydrate formation also moles of gas released and rate of gas release during hydrate dissociation are provided.

ACKNOWLEDGEMENT Authors would like to acknowledge the financial support from Earth System Science Organization, Ministry of Earth Sciences, Government of India, through National Institute of Ocean Technology (NIOT), Chennai, India (Grant: NIOT/F&A/PROJ/GHT/01/2K14). Dr. Jitendra S. Sangwai would like to acknowledge the partial financial support from IIT Madras as part of the Institute Research and Development Award (IRDA) − 2017 (ref.: OEC/1718/835/RFIR/JITE). Vishnu Chandrasekharan Nair would like to acknowledge ICSR, IIT Madras for the partial financial support (Proj.No.: ICS/16-17/831/RFIE/MAHS). The support from Dr. S. Ramesh and Dr. G. A. Ramadass (NIOT) has been gratefully acknowledged. 30


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Table 1 Particulars of the materials used in this study. Material

Description

Methane

Purity, 99.97 mol%

Bentonite clay Seawater

Supplier Bhuruka Gas Agency, Banglore, India

200 mesh size; The major mineralsa, SiO2

Vidhya Enterprises,

(64.7%) and Al2O3 (5.9%)

Chennai, India

pHb, 7.5; Conductivityb, 44.5 mS;

Collected from Elliot's

salinityb, 32.20 ppt

beach, Chennai, India

aMeasured

using S1 TITAN, XRF spectrometer (Bruker, United States)

bMeasured

using PC2700, Eutech instruments (Thermo Fisher Scientific, Singapore)

36


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Table 2 Details on the particulars, nomenclature and conditions of pressure, temperature and rate of gas release carried out during methane recovery studies

1, 2

Depressurization using constant rate gas release

Set pressure (MPa) -

3, 4


-

5, 6

Depressurization at constant pressure

7, 8, 9

Depressurization at constant pressure

10, 11


Expt. No

Details

Rate of gas release (mL/min) 10

278.15

CRD (10 mL/min)

No. of experime nts 2

20

278.15

CRD (20 mL/min)

2

3.5

-

278.15

CPD (3.5 MPa)

2

2.3

-

278.15

CPD (2.3 MPa)

3

-

20

278.15-298.15

CRD+TS (20 mL/min)

2

2

-

278.15-298.15

CPD+TS (2 MPa)

3

-

2.6

278.15-298.15

TS

2

Temperature (K)

Nomenclature

+ Thermal stimulation 12, 13, 14

Depressurization at constant pressure + Thermal stimulation

15, 16

Thermal stimulation

Total number of experiments:

37


16

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Table 3 Details on the hydrate formation experiments.

Expt. No.

Nomenclature

Initial

Initial

formation

formation

pressure

temperature

(MPa)

(K)

Gas consumption (mol)

Gas-to-hydrate

Water-to-hydrate

conversion (%)

conversion (%)

1

CRD (10 mL/min)

7.99

278.45

1.03

27.81

18.85

2

CRD (10 mL/min)

8.08

278.25

1.13

30.42

20.68

3

CRD (20 mL/min)

8.20

278.05

1.27

32.27

23.24

4

CRD (20 mL/min)

7.89

278.75

1.13

30.79

20.68

5

CPD (3.5 MPa)

8.06

278.35

1.19

31.88

21.78

6

CPD (3.5 MPa)

8.02

276.95

1.21

32.17

22.14

7

CPD (2.3 MPa)

8.14

277.25

1.19

31.74

21.78

8

CPD (2.3 MPa)

7.94

278.85

1.19

31.73

21.78

9

CPD (2.3 MPa)

8.10

278.85

1.18

31.22

21.60

10

CRD + TS (20 mL/min)

8.19

277.25

1.15

30.42

21.05

11


7.97

278.25

1.18

31.30

21.59

12

CPD + TS (2 MPa)

8.10

277.65

1.23

33.05

22.51

13

CPD + TS (2 MPa)

7.98

278.35

1.21

32.35

22.14

14

CPD + TS (2 MPa)

8.20

278.55

1.19

31.18

21.23

15

TS

7.81

277.55

1.16

31.18

21.23

16

TS

8.17

277.15

1.23

32.37

22.50

38


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Table 4 Details of the hydrate dissociation experiments Expt. No.

Nomenclature

1

CRD (10 mL/min)

Cumulative moles of gas recovered (mol) 0.68

2

CRD (10 mL/min)

0.70

3

CRD (20 mL/min)

1.16

4

CRD (20 mL/min)

0.94

5

CPD (3.5 MPa)

0.81

6

CPD (3.5 MPa)

0.86

7

CPD (2.3 MPa)

0.67

8

CPD (2.3 MPa)

0.72

9

CPD (2.3 MPa)

0.61

10


1.12

11


1.16

12

CPD + TS (2 MPa)

0.72

13

CPD + TS (2 MPa)

0.80

14

CPD + TS (2 MPa)

1.06

15

TS

1.00

TS

1.04

16

Maximum rate of hydrate dissociation (mol/h) 0.46 ± 0.01

0.66 ± 0.02

0.81 ± 0.04

Gas recovery from hydrate dissociation (%) 65.79 62.04 91.55 83.35 71.34 74.48

Average gas recovery from hydrate dissociation (%) 63.92

87.45

72.91

77.59 0.68 ± 0.02

81.75

77.30

72.57 0.89 ± 0.04

95.22 93.49 85.02

1.31 ± 0.13

91.88

94.36

88.81

89.53 0.69 ± 0.01 39


86.16 84.59

85.38

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(a)

(b) Fig. 1. Schematic diagram of experimental set up for methane recovery from marine clayey hydrate systems for: (a) CRD and CRD+TS; (b) CPD and CPD+TS. 40


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Fig. 2. Phase equilibrium conditions of methane hydrate in the presence of pure water, seawater, seawater clay and 3.3 wt% NaCl+water system. Date from literature: Pure water (Nixdorf and Oellrich66); 3.3 wt% NaCl (Sloan and Koh2); Seawater (Dickens61).

(a) 41


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(b) Fig. 3. (a) Pressure and temperature profile during methane hydrate formation in seawater-clay system (Expt. no. 10 in Table 3); (b) Average cumulative methane consumption and rate of gas consumption during hydrate formation experiments.

Fig. 4. Dissociation pressure for each CPD and CPD+TS which matches with the end pressure of corresponding CRD and CRD+TS. 42


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Fig. 5. Pressure–temperature curve of different production methods. Dotted line represents the methane hydrate equilibrium conditions of seawater clay system obtained in this study (also represented in Fig. 2). See colour figure for better clarity.

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(a)

(b)

44


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(c) Fig. 6. Comparison of pressure and temperature profiles during methane recovery using CRD and CPD experiments. a) Expt. nos. 1 and 3; b) Expt. nos. 5 and 7; c) Expt. nos. 10 and 13 in Table 4. T: Temperature; and P: Pressure of the hydrate reservoir.

Fig. 7. Moles of gas released from hydrate during the initial venting phase for different pressure settings of CPD and CPD+TS experiments. 45


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(a)

(b)

46


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(c) Fig. 8. Comparison of the moles of gas release for CRD and CPD experiments.

(a) 47


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(b)

(c) Fig. 9. Average rate of gas release during hydrate dissociation by CRD and CPD experiments.

48


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Energy & Fuels

(a)

(b) Fig. 10. Comparison of cumulative moles of gas released for different production schemes. (a) CRD, TS and CRD+TS; (b) CPD, TS and CPD+TS.

49


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(a)

(b) Fig. 11. Average rate of hydrate of hydrate dissociation for different production schemes. (a) CRD, TS and CRD+TS; (b) CPD, TS and CPD+TS. 50


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Fig. 12. Average recovery of gas (percentage) using different recovery methods in seawater mud system.

*******

51


Natural Gas Production from a Marine Clayey Hydrate Reservoir

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