Role of different types of water in bentonite clay on hydrate formation and decomposition

Yu Zhang ,Lei Zhang ,Chang Chen ,Hao-Peng Zeng ,Xiao-Sen Li, *,Bo Yang

1 Key Laboratory of Gas Hydrate,Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou,Guangdong 510640,China

2 Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,Guangzhou,Guangdong 510640,China

3 University of Chinese Academy of Sciences,Beijing 10083,China

4 Guangzhou Special Pressure Equipment Inspection and Research Institute,Guangzhou 510663,China

Keywords:Methane hydrate Bentonite Formation and decomposition Bound water Capillary water

ABSTRACT The equilibrium and kinetic of hydrate in sediments can be affected by the presence of external components like bentonite with a relatively large surface area.To investigate the hydrate formation and decomposition behaviors in bentonite clay,the experiments of methane hydrate formation and decomposition using the multi-step decomposition method in bentonite with different water contents of 20%,40% and 60% (mass) were carried out.The contents of bound,capillary and gravity water in bentonite clay and their roles during hydrate formation and decomposition were analyzed.In bentonite with water content of 20%(mass),the hydrate formation rate keeps fast during the whole formation process,and the final gas consumption under different initial formation pressures is similar.In bentonite with the water contents of 40%and 60%(mass),the hydrate formation rate declines significantly at the later stage of the hydrate formation.The final gas consumption of bentonite with the water contents of 40%and 60%(mass)is significantly higher than that with the water content of 20% (mass).During the decomposition process,the stable pressure increases with the decrease of the water content.Hydrate mainly forms in free water in bentonite clay.In bentonite clay with the water contents of 20% and 40% (mass),the hydrate forms in capillary water.In bentonite clay with the water content of 60% (mass),the hydrate forms both in capillary water and gravity water.The bound water of dry bentonite clay is about 3.93%(mass)and the content of capillary water ranges from 42.37% to 48.21% (mass) of the dry bentonite clay.

1.Introduction

Natural gas hydrate (NGH) is widely distributed in seabed sediments and terrestrial permafrost [1-3],and commonly found in sand or clay type sediments under thermodynamically stable conditions where the pressure is greater than the hydrate equilibrium pressure at a given temperature [2,3].For the purpose to exploit natural gas hydrate resource,it is important to know the equilibrium conditions of natural gas hydratesin-situin order to determine the location of the bottom of gas hydrate stability zone(BGHS)and estimate the total amount of gas in gas-hydrate fields.Therefore,the hydrate stability in sediments and its behaviors during hydrate formation and decomposition are crucial in determining the feasibility and method of extracting natural gas from gashydrate reservoirs [4,5].

In recent years,a lot of researches have been carried out to study BGHS in sediments.Clay minerals are common constituents(main components) of natural sediments.From the results of field and laboratory investigations,as well as theoretical studies,it has been found that clay minerals are likely to play an important role in controlling the occurrence of NGH.The capillary inhibition of hydrate stability in narrow pores in sediments has previously been found to enhance the stable pressure of hydrate at a given temperature through the experiments and theories [6,7].The role of clay particles on hydrate formation and decomposition in sediments is highly variable due to their low gas permeability and the varying water activity in clay pores that depend on the water content[8].It has been found that the hydrate stability in sediments can be affected by the presence of external components like bentonite with relatively large surface area [9].Several reports have focused on the effect of the clay on the hydrate formation and dissociation.Chaet al.[10]firstly found the thermodynamic promoting effect on the hydrate formation in the solution that contains bentonite.Park and Sposito [11] studied the thermodynamic promoting effect of montmorillonite surfaces on CH4hydrates by using molecular simulations.Similar phenomena were also verified through the experiments in bentonite suspension but with a much smaller temperature rise.It was found that hydrate might form from CH4gas and water that are absorbed between the molecular plates of montmorillonite.Guggenheim and van Groos [12] measured the equilibrium conditions of the interlayer hydrate,and found the equilibrium temperature shifting near 1 °C compared to bulk hydrate.Uchidaet al.[13]measured the decomposition conditions of methane hydrate in bentonite with different water contents.They found that when the water content is lower than 40% (vol),the equilibrium hydrate temperature is higher than bulk hydrate at a given pressure,and the ΔT(difference between the measured equilibrium hydrate temperature and that of bulk hydrate)increases as the water content decreases.They speculated that the inhibition effect on hydrate formation is due to hydrate formed in the interlayers of swelled bentonite mineral.For the water content between 40% and 80% (vol),little effect was observed on the hydrate decomposition.When the water content exceeds 80%,the sample is a solution containing bentonite particles,and has a slight thermodynamic promoting effect on the hydrate formation.As was described by Kotkoskieet al.[14],the equilibrium conditions of gas hydrates in bentonite-rich clay are determined by the competition between the promoting effect of the bentonite minerals and the inhibiting effect of the electrolytes dissolved into solutions from the minerals.The gradual increase of ΔTwith increasing water content,or equivalently,decreasing of bentonite concentration.To sum up,the experimental and theoretical researches have revealed that the equilibrium hydrate conditions in bentonite clay are affected by the water content,which may be influenced by electrolyte inhibiting,interlayer hydrate formation and minerals promoting.Sawet al.[15] studied the effects of bentonite clay,silica sand and the grain size of silica sand on methane hydrate formation and decomposition.They found that bentonite clay greatly affects the hydrate formation and decomposition temperature.More heat is required to dissociate hydrate in the presence of sand compared with the hydrate in the presence of clay.Kumaret al.[16] investigated the effects of different proportions of the clay/sand and water saturation on hydrate formation.The study found that the hydrate formation rate increases with the increase of the pore volume.The increase of clay reduces the water conversion (percentage of water converted to hydrate)and hydrate formation rate,and the maximum water conversion reaches at the water saturation of 75% (vol) in pure sand.

Water in clay is generally classified as bound water and free water,and bound water is generally defined as the water in the electric double layer,while the water outside the electric double layer is defined as the free water,which can be divided into capillary and gravity water [17-19].Previous investigations of bound water on the surface of clay particles defined the bound water into weakly and strongly bound water based on critical temperatures.Although,numerous investigations of effects of different clay types on hydrate formation and decomposition have been carried out[20-22],there is still a lack of definitive and systematic methods to determine the types of water in clay and their corresponding effects on the hydrate formation and dissociation.In this study,considering the bentonite is a fine clay mined from the natural geological environment which can provide a large surface area for adsorption of water and methane gas,the isothermal adsorption experiments on bentonite were conducted,and the formation and decomposition experiments of methane hydrate in bentonite clay with different water contents were carried out.The contents of bound,capillary and gravity water in bentonite clay were analyzed,and their roles in hydrate formation and decomposition were discussed.

2.Experimental

2.1.Experimental apparatus

Fig.1 shows the schematic of the experimental apparatus.Detailed descriptions of the apparatus can be found in our previous study [23].The core component of the apparatus was a highpressure reactor (20 MPa) with the total internal volume of 56.91 cm3.The reactor and supply vessel were immersed in a water bath.The pressures in the reactor and supply vessel were measured by two pressure transducers(±0.025 MPa).The temperatures inside the reactor and water bath were measured by Pt100 temperature sensors (±0.1 °C).The pressure of the outlet of the reactor was controlled by a back-pressure regulator.The pressure and temperature were monitored and recorded by a data acquisition system.

2.2.Materials

The methane used in the experiment was provided by Foshan Huate Gas Co.,Ltd.,and its mole fraction was ≥99.9%;the conductivity of deionized water was 18.25 mΩ·cm-1,which was prepared by ultrapure water equipment of Nanjing Chaochun Water Co.,Ltd.;Na-Bentonite was used in the experiments and supplied by Aladdin Co.,Ltd.

Fig.1.Schematic diagram of the experimental device.

In this study,the bentonite clay was dried by heating the sample at 105°C until there was no further weight loss before the measurements and experiments of hydrate formation and decomposition [24].The measured density of the dry bentonite clay was 1.5179 g·ml-1using the true density meter (VPY-30,Quantachrome).Specific surface area and pore size analyzer(ASIQMO002-2,Quantachrome) were used to measure the distributions of pore volume and the specific surface area of the samples.The measured specific surface area of the bentonite clay was 6.826 m2·g-1,the average pore diameter was 3.0019 nm,and the pore volume was 0.058 ml·g-1.

2.3.Experimental procedures

In the adsorption isotherm experiments,three grams of dried bentonite sample were filled in a watch glass reactor,then placed in humidification chambers equilibrated with the relative humidity (RH) values of 98%,which was controlled by saturated potassium nitrate solutions [25].The containers were placed in a temperature-controlled thermotank at (25 ± 0.2) °C.

In the hydrate formation experiment,the bentonite clay was dried in an oven at 105°C for 24 h and cooled under the room temperature.Desired bentonite and deionized water were then mixed well in a beaker.The reactor was cleaned by using deionized water for three times and then dried.The prepared bentonite was compacted into the reactor.The reactor was rinsed with methane for 3 times to remove the air in the reactor.After the device was airtight,the experiment was carried out.The water bath was set to 20 °C.When the temperature in the reactor was stabilized,methane was injected into the reactor through the supply vessel to a predetermined pressure.The temperature of the water bath was then set to a desired temperature for the hydrate formation.As the temperature gradually decreased and the hydrate formed,the pressure in the reactor gradually decreased.When the pressure in the reactor kept constant for 24 h,it was considered that the hydrate formation process was ended.After the hydrate formation,the temperature of the vessel was raised at a rate of 0.1°C·h-1until all hydrate in the reactor decomposed completely.The changes of temperature and pressure in the reactor were recorded to determine the decomposition condition of the hydrate.

3.Results and Discussion

3.1.Results of isothermal adsorption

According to the literature [26,27],the weight loss for the clay that was heated at 105 °C for 24 h was free water and parts of weakly bound water.In this study,in order to maintain the stability of bentonite composition and structure,the bentonite fully dried at 105 °C was used as the medium and different quality deionized water was added to study the influence of water content on the formation and decomposition behaviors of hydrate.Isothermal adsorption experiments on bentonite were conducted to determine the water absorption capacity of the bentonite dried at 105 °C.

The experimental data of the water adsorption are given in Table 1.According to the relevant literature [14],only when the relative humidity is above 98%,the clay can adsorb very little free water in the water.Therefore,it can be considered that the water adsorbed by the bentonite after isothermal adsorption is all bound water.The average value of the maximum adsorption capacity of bentonite samples is about 3.93%(mass).Therefore,the maximum bound water content of bentonite samples is 3.93%(mass)(excluding the original bound water in the dried bentonite).

3.2.Hydrate formation and decomposition processes

To investigate the effects of the different types of water in clay on methane hydrate formation and decomposition,the hydrate formation and decomposition experiments were carried out in bentonite clay with three water contents.The hydrate formation temperature was 5 °C,and the initial formation pressures were 10,12,15 and 18 MPa,respectively.In all experiments,the mass of the dried bentonite clay packed into the reactor was 25 g.The water contents in the bentonite clay were 5 g,10 g and 15 g,respectively,and the corresponding water mass fraction (defined as mass ratio of water to dry bentonite clay) was 20%,40%,and 60% (mass),respectively.Table 2 shows the detailed experimental conditions and results.

Table 1 Results of the isothermal adsorption of dried bentonite clay

Fig.2 shows the comparison of the P-T curves for the experiments in bentonite clay with different water contents of 5 g,10 g and 15 g,respectively.The equilibrium decomposition pressures for bulk methane hydrate are also given in Fig.2.In Fig.2(a),the experiments were performed at the hydrate formation temperature of 5 °C and the initial pressure of 10 MPa.The formation and decomposition process can be displayed from the P-T curve of the experiment with the water content of 5 g.From the beginning of the experiment to point A,the pressure of the system decreases continuously as the temperature decreases.The pressure drop at this stage is due to the temperature drop of the gas.At a specific temperature,the pressure begins to decrease significantly at point A.This is because that methane hydrate begins to nucleate and then rapidly form.As the reaction processes,the pressure of the system reaches stable at point C and the methane hydrate in the reactor reaches a stable state.As the temperature increases gradually,the pressure of the system begins to increase due to the hydrate decomposition and temperature increase.When the slope of the pressure rise curve suddenly changes (E point),the hydrate basically decomposes completely at this time.

From Fig.2(a),it can be seen that under the same initial formation temperature,theP-Tcurves of methane hydrate during hydrate decomposition in bentonite with different water contents are obviously different,indicating that the bentonite affects the hydrate decomposition,and the degree of influence varies with the change of the water content in bentonite.In bentonite with the water content of 5 g,the pressure in the reactor after the hydrate formation is much higher than that in bentonite with the water contents of 10 g and 15 g.It shows that in the amount hydrate formed in bentonite with the water content of 5 g is lower than that in bentonite with the water contents of 10 g and 15 g.For the water contents of 10 g and 15 g,the pressures after the hydrate formation are similar.During the hydrate decomposition process,the pressure is always higher than equilibrium decomposition pressure for bulk hydrate at a given temperature.Moreover,the pressure at the end of hydrate decomposition is also higher than that of bulk methane hydrate.

In bentonite with the water content of 10 g,the pressure curve during the hydrate decomposition process and the pressure at the end of hydrate decomposition are also higher than that of bulk methane hydrate,but lower than that with the water content of 5 g.As shown in Fig.2(a),in bentonite with the water content of 15 g,the pressure increases obviously more slowly than that in bentonite with the water contents of 5 g and 10 g at the beginning stage of the temperature increase,and then the increase rate of the pressure begins to speed up after the temperature increases to close to the equilibrium temperature of bulk hydrate.We suppose that the hydrate does not decompose at the beginning stage of the temperature increase,and the pressure increase is mainly causedby the temperature increase of methane gas.Because the end of the hydrate formation is determined by the pressure decline rate in the reactor,the pressure at the end of the hydrate formation is much higher than the equilibrium decomposition pressure of methane hydrate in bentonite with the water content of 15 g.The reason may be that the gas diffusion rate in bentonite with a higher water content is low,resulting in a slow hydrate formation rate as much hydrate formed in bentonite in the experiment.As the temperature increases gradually,theP-Tcurve becomes gradually close to the equilibrium curve of bulk methane hydrate,and coincides with the equilibrium curve of bulk methane hydrate.After that,the pressure increase rate slows down obviously and the pressure becomes to be lower than the equilibrium decomposition pressure of bulk methane hydrate,indicating that the hydrate decomposition is inhibited,which is‘‘thermodynamic promoting”of the bentonite minerals on hydrate formation at the high water saturation [13,14].There are similar phenomena for experiments with different initial formation pressures,as shown in Fig.2(b) and (c).

The pressure changes during hydrate decomposition indicate that the stability pressure of hydrate in bentonite decreases with the increasing water content,which is also reported by Uchidaet al.[13]They found that the shift of the equilibrium temperature of hydrate in bentonite is more negative for smaller initial water saturation,and is roughly zero in the water saturation range of between 40% and 80% (vol).

3.3.Gas consumption

Fig.3 shows the curves of gas consumption during hydrate formation in different experiments.It can be seen from Fig.3 that in the bentonite with the water content of 5 g,the hydrate basically maintains a high formation rate during the whole formation process and reaches the maximum gas consumption in a short time.In different experiments,the final gas consumption is similar.The final gas consumption of the hydrate formation in bentonite with the water contents of 10 g and 15 g is significantly higher than that with the water content of 5 g.Moreover,after maintaining a relatively fast process,the hydrate formation rate decreases significantly and enters a long-term slow formation process.At a low water content,the contact between water and gas is abundant,and the formation of hydrate does not significantly hinder the mass transfer process,resulting in the rapid hydrate formation during the whole formation process.At a high-water content,gas first reacts with water in direct contact to form hydrate.After that,the formation of hydrate will hinder the contact between unreacted water and methane gas and reduce the formation rate of hydrate.In addition,due to the distribution of water in bentonite and the randomness of hydrate formation,the influence of mass transfer process on the formation rate of hydrate has a certain uncertainty.Therefore,in the experiments in the bentonite clay with the water contents of 10 g and 15 g,the gas consumption rate at the later stage has no obvious relationship with the initial formation pressure and water content.

As shown in Table 2 and Fig.3,it can be found that the final gas consumption of the hydrate formation under different initial formation pressures in bentonite with the water content of 5 g is similar,and significantly lower than that with the water contents of 10 g and 15 g.It also can be found that there is no obvious relationship between the final gas consumption and initial formation pressure,and the gas consumption under 18 MPa is slightly lower than that under other initial formation pressures.The increasing initial formation pressure means the higher driving force for hydrate formation,generally results in the higher gas consumption.However,the final gas consumption is affected not only by the driving force for hydrate formation,but also by the heat and mass transfer in sediments,which is affected by the inhomogeneous gas-water contact and hydrate distribution in sediments.In lab experiments,the limited hydrate formation time usually does not allow the stable pressure of the hydrate to reach during hydrate formation due to that the pressure change is not allowed to be measured with a very slow diffusion of methane gas.At a higher gas hydrate formation pressure,there may be a higher hydrate formation rate at the gas-water interface,resulting in a greater gas diffusion resistance,which may be the reason why the final gas consumption at a higher pressure is lower.

Fig.3.Gas consumption during methane hydrate formation in bentonite clay with different water contents.

Fig.4.Final gas consumption per mole of water during methane hydrate formation in bentonite clay.

Fig.4 shows the comparison of final gas uptake (gas consumption per mole of water)in bentonite with different water contents under different experimental conditions.From Fig.4,it can be found that the final gas consumption per mole of water increases with the decrease of the water content in bentonite clay,which is different from the final gas consumption.With the decreasing water content,the same gas consumption leads to a higher percentage of the water conversion.Therefore,even the final gas consumption in bentonite with the water contents of 10 g and 15 g is higher,their final gas consumption per mole of water is lower than that with the water content of 5 g.However,it should be pointed out that the hydrate formation in this study is not at a constantpressure condition,the pressure during hydrate formation and the final pressure varies in different experiments.The higher final gas consumption with the high water content results in a lower final pressure,which prevents further conversion of water and results in lower final gas consumption per mole of water.

3.4.Analysis of hydrate formed by different types of water

The measured bound water content is 3.93%(mass)in dry bentonite clay.Therefore,there is approximately 0.983 g bound water in 25 g dry bentonite clay.It can be seen from Table 1 that after the formation of hydrate,the quantity of unreacted water in each experiment is significantly higher than that of water adsorbed in dry bentonite samples.Generally,due to the adsorption of bound water on the surface of clay and its higher hydrate formation pressure,hydrate forms more easily in free water including capillary water and gravity water.In order to further analyze the formation of hydrate in different types of water,we calculated the change of the unreacted water in the sample during hydrate decomposition by multi-steps heating method.Due to the different initial formation pressures in different experiments,the temperature difference(ΔT) between the hydrate stability temperature (equilibrium decomposition temperature) in pure water and the actual system is used as the parameter to compare the change of unreacted water content during hydrate decomposition in different experiments,as shown in Fig.5.Due to the slow heating rate (0.1 °C·h-1) used in the hydrate decomposition process,which is the general heating rate used to measure the hydrate phase equilibrium conditions in porous media,it can be considered that the hydrate temperature and pressure conditions in the system can reach a relatively stable state in each step of the heating process,and ΔTin this state can be considered as the difference between the stable temperature of the hydrate in this state and that in pure water.In porous media,ΔTis mainly caused by the capillary pressure of porous media.

As shown in Fig.5,in bentonite clay with the water content of 5 g,the amount of the unreacted water is relatively close in different experiments under similar ΔT.When the hydrate is completely decomposed,ΔTis in a range of 0.84-1.00°C,which can be considered to be basically the same under the temperature measurement accuracy of±0.1°C in this study.With the increase of temperature during hydrate decomposition,ΔTand the hydrate amount decrease gradually,whereas the amount of free water increases.It shows that the hydrate in bentonite clay is at or near stable conditions,and the increase of temperature breaks the equilibrium state of hydrate and causes the hydrate decomposition.In the experiments with different initial formation pressures,there is a similar relationship between the amount of unreacted water and ΔT.Additionally,it can be seen that ΔTincreases rapidly with the decrease of the amount of the unreacted water,indicating that the equilibrium temperature of the hydrate in bentonite clay shifts to an extremely low temperature when the free water is almost consumed.

Fig.5.The changes of the unreacted water during hydrate decomposition in different experiments.

As can be seen from Fig.2,during a long period of temperature rise from the beginning of heating up in bentonite clay with the water content of 15 g,the pressure has a slight increase,which may be mainly caused by the gas expansion as a result of the temperature increase.It can be seen from Fig.5 that with the decreasing ΔTthe unreacted water remains basically unchanged,also indicating that the hydrate is basically not decomposed in a long period from the beginning of heating up.This shows that the hydrate formed in bentonite keeps stable in a long period with the increase of temperature,indicating that the temperature is still lower than the stable temperature of the hydrate during the early period of heating.As mentioned above,the hydrate formation process is affected by the mass transfer process at a high-water content.Therefore,with a large amount of hydrate formed,the hydrate formation speed becomes extremely slow,even there is still a large amount of unreacted water and enough driving force for hydrate formation.As shown in Fig.5,in the experiment under the initial formation pressure of 15 MPa,the unreacted water content begins to increase with the decrease of ΔTfrom point A,indicating that the hydrate begins to decompose.Currently,the stable temperature of hydrate is still higher than that of hydrate in pure water.It shows that the hydrate formed in capillary water begins to decompose at this time.As the hydrate continues to decompose and the water continues to increase,ΔTgradually decreases close to 0.In this study,since the measurement accuracy of temperature is 0.1 °C,it is assumed that the decomposition conditions of hydrate are basically the same as that of hydrate in pure water when ΔTis less than 0.1°C.Therefore,from point B,the stable condition of the hydrate is basically the same as that of hydrate in pure water system,indicating that the hydrate stable condition is not affected by the capillary pressure,and the hydrate forms in gravity water.For the experiments with different initial formation temperatures,the unreacted water is about 11.576-13.035 g when the stable condition of the hydrate is basically the same as that of bulk hydrate.Therefore,taking the amount of the bound water into account,it was calculated that the capillary water in the bentonite ranges from 10.593 g to 12.052 g,being 42.37%to 48.21%(mass)of the dry bentonite used in this study.The difference of the calculated capillary water may be due to that some water may not be in full contacted with clay due to the uneven distribution of water,resulting in more local gravity water and different contents of the capillary water.This also shows that in bentonite with the water content of 15 g,some hydrates form in capillary water,while others form in gravity water.Compared with the experiments with various initial pressures,the amounts of the unreacted water at the beginning of hydrate decomposition are quite different,indicating that the amounts of hydrate formed in capillary water and gravity water are also different.This shows that although the equilibrium pressure of hydrate formation of capillary water is higher than that of gravity water,hydrate can form in capillary water and gravity water at the same time under enough hydrate formation driving force.Their proportions will not be the same under different experimental conditions.

In bentonite with the water content of 10 g,the hydrate decomposition behavior is between the experiments with the water contents of 5 g and 15 g.At the time of complete decomposition of hydrate,ΔTis greater than 0.1 °C,indicating that hydrate forms in capillary water in bentonite with the water content of 10 g.According to the analysis from the results of isothermal adsorption and hydrate decomposition,in bentonite clay used in this study,the content of bound water is 3.93% (mass),and no hydrate forms in bound water;the content of capillary water ranges from 42.37%to 48.21% (mass) of the dry bentonite clay.When the unreacted water is less than the total of the bound water and capillary water,the formation conditions of hydrate would be affected by capillary force.With the decrease of unreacted capillary water,the formation of hydrate needs a significantly low formation temperature and a high formation pressure.This indicates that in actual subsea hydrate reservoir with clay-rich sediments,the inhibition effect of sediments on gas hydrates will significantly affect the stability condition of hydrate with a high hydrate saturation.Additionally,the hydrate distribution and the saturation variation of water and hydrate will lead to significant spatial and temporal heterogeneity of hydrate stability in the reservoir.Therefore,in the evaluation the exploitation potential through experiment and simulation,and the development of hydrate exploitation strategies,it is important to finely describe and better understand the distribution of hydrate and water saturations in the reservoir,and the changes of hydrate stability condition during gas production.

4.Conclusions

In this work,the experiments of the hydrate formation and decomposition in bentonite clay with different water contents were carried out,and the effects of the different types of water in bentonite on hydrate formation were analyzed.Some conclusions are as follows:

(1) In bentonite with the water content of 20% (mass),the hydrate formation rate keeps fast during the whole formation process,and the final gas consumption under different initial formation pressures is similar.In bentonite with the water contents of 40% and 60% (mass),the hydrate formation rate declines significantly at the later stage of the hydrate formation,and the final gas consumption has no obvious correlation with the initial formation pressure due to the limited and inhomogeneous mass transfer resistance at the later stage of the hydrate formation in different experiments.The higher final gas consumption and lower gas consumption per mole of water were obtained in bentonite with the water contents of 40% and 60% (mass) compared with those with the water content of 20% (mass).

(2) For the experiments with the water contents of 20%and 40%(mass)in bentonite,theP-Tcurves during methane hydrate decomposition are above the phase equilibrium curve of bulk methane hydrate.For the experiments with the water content of 60% (mass),theP-Tcurve changes close to the hydrate equilibrium curve during almost the whole hydrate decomposition process.However,at the end of the methane hydrate decomposition process,the equilibrium pressure of the hydrate decomposition is lower than that in pure water due to the promoting effect of the surface texture of bentonite and the organic matter.During the decomposition process,the stable pressure increases with the decrease of the water content.

(3) The maximum adsorption capacity of bentonite samples was measured to be about 3.93% (mass) through isothermal adsorption experiment.In different experiments,the unreacted water after hydrate reaction is obviously higher than the bound water.Hydrate mainly forms in free water in bentonite clay.In bentonite clay with the water contents of 20%and 40%(mass),the hydrate forms in capillary water.In bentonite clay with the water content of 60%(mass),the hydrate forms both in capillary water and gravity water.The content of capillary water ranges from 42.37% to 48.21% (mass) of the dry bentonite clay used in the experiments.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (52076208,51736009),the Guangdong Special Support Program (2019BT02L278),the Special project for marine economy development of Guangdong Province (GDME-2020D044),the Science and Technology Program of Guangzhou(20202102080159),and Guangdong Basic and Applied Basic Research Foundation (2022A1515010835),which are gratefully acknowledged.