Methane Hydrate Reservoir Research Articles

Numerical modelling of CO2-in-water emulsion injection into a water-saturated core

Depressurization method shows promise for methane hydrate (MH) reservoir production, but the reformation of hydrates due to a lack of geothermal heat presents a significant challenge for commercial production. This study proposes a novel approach for MH gas recovery by injecting a CO2-in-water (C/W) emulsion into an MH reservoir aquifer. The exothermic nature of CO2 hydrate formation and liquid CO2 dissolution in water in this method provides heat to the MH layer, effectively preventing hydrate reformation. Subsequently, 1-D numerical models were developed and implemented using the MATLAB Reservoir Simulation Toolbox (MRST). These models were rigorously validated against experimental data under homogeneous conditions and varying injection schemes. The investigation indicated that the average volume fraction of CO2 hydrate within the hydrate particles was found to be 55% of the volume of liquid CO2 droplets. The alternating C/W emulsion injection increased the driving force for CO2 dissolution in water, thereby promoting subsequent hydrate particle decomposition and CO2 hydrate dissolution in water, which effectively mitigated blockage within the porous medium. In conclusion, water alternating C/W emulsion injection, along with the continuous injection of a low volume fraction of liquid CO2 within the C/W emulsion, exhibits promising potential for sustained injection and provides adequate heat to the MH layer, preventing hydrate reformation. Furthermore, the developed numerical model reasonably predicts the behavior of various injection schemes. This developed model can be used for devising long term sub-seabed carbon capture and storage development plan.

A Study on the Heterogeneity and Anisotropy of the Porous Grout Body Created in the Stabilization of a Methane Hydrate Reservoir through Grouting

To solve the sand problem during the depressurization of methane hydrate (MH), we proposed a method to build a porous grout body with sufficient permeability and strength around the wellbore through inhibitor pre-injection and grouting, and verified its effectiveness and potential in our previous research using artificial cores created with silica sand and alternative hydrates such as TBAB- hydrate and iso-butane hydrate. However, all of the artificial cores mentioned above were created with high homogeneity, injected, cured, and had their physical properties measured in the vertical direction, which differs from real reservoir conditions. To investigate the effects of grouting in a more realistic fluid flow, we conducted further experiments using horizontal 1D cores, 1D cubic models, and a 2D cross-sectional model mimicking the near wellbore. These experiments revealed that (1) the generated gas somewhat suppressed the effects of grouting as in the case of previous experiments, and (2) grouted reservoirs would be heterogenous and anisotropic due to the fluid densities and the distribution of grout particles and turbidite sediments, but sufficient permeability and satisfactory strength could still be attained. The above series of experiments demonstrated that our method has the potential to effectively produce actual MH, preventing sand problems even in heterogeneous and anisotropic grouted reservoirs.

Thermal-hydro coupling model of methane hydrate reformation in porous media

Methane hydrates are crystalline compounds found in marine sediments and permafrost regions. Methane hydrates remain stable under both low-temperature and high-pressure conditions. When a methane hydrate reservoir is heated or depressurized, methane hydrates become unstable and decompose into water and methane gases. The heat absorption process during the decomposition of methane hydrates influences the temperature field. Methane hydrate reformation occurs during the extraction process, significantly reducing the hydraulic conductivity of the reservoir and hindering the long-term stable extraction of methane hydrates. In this paper, a numerical model is established by coupling seepage and heat processes. The temperature variation owing to the heat absorption process of methane hydrate decomposition is quantified based on the proposed model. The effect of spatial lithology on methane hydrate conversion is also analyzed, and the results for hydrate, water, gas, and permeability in the model are summarized. The numerical model also reflects the heterogeneity of marine sediments. Finally, the sensitivities of different physical parameters (permeability and porosity) and pressure gradients to the reformation rate of methane hydrate reforming are discussed. The results of this study provide scientific data supporting the influence of secondary hydrate formation on long-term extraction efficiency in the actual engineering hydrate extraction process.

Numerical simulation of enhanced gas recovery from methane hydrate using the heat of CO2 hydrate formation under specific geological conditions

Methane hydrate (MH) is found under the seabed in Japan's exclusive economic zone and is attracting attention as a domestic natural gas resource. One of the methods for extracting methane gas (MG) from sub-seabed MH reservoir is depressurisation. However, the production decelerates after a certain time partly due to the cooling of the reservoir caused by the heat absorption during MH dissociation. Therefore, several enhanced gas recovery (EGR) methods have been proposed, such as injecting CO2 into a sub-seabed sand layer to form CO2 hydrate (CDH) and, using the heat of CDH formation. This method is two functions: EGR and the reduction of greenhouse gas emission. In this study, a numerical method was developed to investigate the performance of the EGR from MH, using the heat of CDH formation. In our previous study, CDH formation model had been installed into a programme code for two-phase flow of CO2 and water in porous media. Based on this code, we newly implemented the state equation, enthalpy model, viscosity model, and heat conduction model of MG and the dissociation model of MH. Then, simulations were performed to form CDH in specific sand-mud alternate layers and to dissociate MH. As a result, it was suggested that increasing the CO2 injection amount potentially increases MH dissociation and the vertical distance between the mud layer capping the injected liquid CO2 and the bottom-simulating reflector of CDH is important to ensure a large CDH region to generate a large amount of heat. It was also shown that even after stopping CO2 injection, MH dissociation can continue for a certain period.

An Improved Continuum Approach for Unconsolidated Formations on the Field Scale

Summary With the development of unconventional resources, such as oil sands and methane hydrate reservoirs, the importance of the mechanical performance model for underground unconsolidated rocks has increased significantly. The commonly used numerical approach for unconsolidated rocks is the discrete-element method (DEM). However, the extensive calculations required by the DEM make it inadequate for simulating unconsolidated rock behavior on a field scale. An alternative is the continuum approach, used to simulate the behavior of unconsolidated rocks on the field scale. In previous continuum approaches, unconsolidated rocks have been modeled as a visco-plastic fluid (i.e., Bingham fluid). The continuum approach based on visco-plastic fluid uses pressure (scalar) to describe the stress state of the particles. However, this approach does not account for the difference between the maximum and minimum principal stresses of the in-situ stress field when simulating the mechanical performance of unconsolidated rocks. Here, we developed an improved continuum approach for unconsolidated rocks and used the finite-element method as a numerical approach. Our improved model can consider the difference between the maximum and minimum principal stresses of the in-situ stress field and the pore pressure of the unconsolidated formation. We validated our numerical model with the angle of repose test, a benchmark problem for unconsolidated rocks. The validation results confirm the accuracy of our unconsolidated model. For the coupled model between the unconsolidated model and the flow model, we used an analytical solution to verify its reliability. Unconsolidated rock performances in an unconsolidated reservoir with fluid injection have been investigated based on our coupled model. The simulation results show that injection can activate the movement of unconsolidated rock particles, leading to changes in the distribution of effective stress and permeability. Our model has the potential to address large-scale unconsolidated rock issues and contribute to energy extraction.

Strength analysis of hydrate-bearing sandy sediments in excess gas and excess water based on drained triaxial compression tests

The safe and efficient production of methane hydrate (MH) has attracted much attention, and the engineering geological risks in MH production are highly valued. Safety evaluation of the MH reservoirs is necessary, and the strength characteristics are the key. In this study, the mechanical behaviors of MH-bearing sandy sediments in excess gas and excess water were studied by conducting drained triaxial compression tests, so as to provide reference for the mechanical evaluation of MH reservoirs in natural gas-rich and water-rich environments. The results show that: (1) the effects of MH saturation and effective confining stress on the mechanical properties of sediments are coupled. The hydrate cementation effect is restricted under high effective confining stress; (2) strain softening behavior is prone to occur under higher MH saturation and is hindered by the high effective confining stress; (3) the MH-bearing samples exhibit higher stiffness, failure strength and strain softening tendency in excess gas than in excess water; (4) in excess gas, the MH contribution to the failure strength of MH-bearing samples starts from the cementation component, while in excess water, it starts from the friction component; (5) theoretical analysis based on the Lade-Duncan criterion suggests that the sandy sediments containing MH in excess water can be considered cohesionless until the MH saturation exceeds 40%. This study provides a theoretical reference for geological safety evaluation during MH development.

Characteristics of mud flocs in the subseabed methane hydrate reservoir and behaviours in flowing water

Recently, it was found that mud flocculates, or flocs, coexist with methane hydrate within turbidite layers off the Japanese coast. There is a concern that mud flocs may be broken down by the flow of pure water generated from the dissociation of methane hydrate, causing sand troubles in the production wells. To clarify the characteristics of the flocs, in this study, the samples of mud flocs collected from natural cores having contained methane hydrate were observed before and after sonication in a pure-water ultrasonic bath. Then, these flocs were placed in distilled water flow to investigate their mechanical stability. As a result, it was confirmed that the flocs mainly consist of silt together with a little clay and fine sand. In addition, these flocs were found to be stable for flow rates on the order of magnitude of 10−3 m/s, which is expected inside the strata during gas production from methane hydrate. However, it was also found that, after the distilled water flow experiments, the flocs were decomposed into silt and clay particles when pressed with a handheld needle in the air, suggesting that the floc might be a source of the fine particles as underground stress conditions change, although not many flocs are likely to be immersed in pure water alone.

Large-Scale Nonequilibrium Molecular Studies of Thermal Hydrate Dissociation.

The energy content of methane hydrate reservoirs (MHRs) is at least twice that of conventional fossil fuels. So, there is considerable interest in their commercial development by heating, among other dissociation mechanisms. However, a few researchers have highlighted the potentially uncontrollable release of methane from MHRs, which could occur because of global warming. Therefore, it is crucial to understand the kinetics of thermal hydrate dissociation to safely develop these resources and prevent the release of this greenhouse gas into the environment. Although there have been several molecular studies of thermal dissociation, most of these use small simulation domains that cannot capture the transient nature of the process. To address this limitation, we performed coarse-grained molecular dynamics (CGMD) simulations on a significantly larger domain with a hundred times more hydrate unit cells than those used in previous studies. We monitored the kinetics of dissociation using an image-processing algorithm and observed the dynamics of the process while maintaining a thermal gradient at the dissociation front. For the first time, we report the formation of an unstable secondary dissociation path that triggers gas bubbles within the solid hydrate. The kinetics of thermal dissociation appears to occur in three stages. In the first stage, the energy of the system increases until it exceeds the activation energy, and dissociation is initiated. Consistent dissociation occurs in the second stage, whereas the third stage involves the dissociation of the remaining hydrates across a nonplanar and heterogeneous interface.

Intermediate water warming caused methane hydrate instability in South China Sea during past interglacials

Methane hydrates are widely distributed along continental margins, representing a potential source of methane to the ocean and atmosphere, possibly influencing Earth’s climate. Yet, little is known about the response of methane hydrates to global climate change, especially at the timescale of glacial-interglacial cycles. Here we present a chronology of methane seepage from seep carbonates derived from a series of tens to hundreds of meters long hydrate-bearing sediment records from the South China Sea, drilled at water depths of 664−871 m. We find that six out of seven episodes of intense methane seepage during the last 440,000 years were related to hydrate dissociation, all coinciding with major interglacials, the so-called Marine Isotope Stages 1, 5e, 7c, 9c, and 11c. Using numerical modeling, we show that these events of methane hydrate instability were possibly triggered by the rapid warming of intermediate waters by ∼2.5−3.5 °C in the South China Sea. This finding provides direct evidence for the sensitivity of the deep marine methane hydrate reservoir to glacial-interglacial climatic and oceanographic cyclicity.

A 3D modeling study of effects of heterogeneity on system responses in methane hydrate reservoirs with horizontal well depressurization

Production tests in marine methane-bearing sediments have been carried out worldwide. Pilot horizontal well tests have exhibited the potential of using horizontal well depressurization for methane hydrate production. Many studies have focused on the 2D thermal-hydraulic-mechanical-chemical (THMC) responses induced by horizontal wells, which targeted certain vertical planes penetrating the well, while 3D numerical modeling of the associated heat and mass transfer behaviors and induced geomechanical responses can provide behaviors not captured by 2D modeling. Additionally, it can give elaborated results in heterogeneous cases. This study investigates the effects of heterogeneity caused by interlayers on the coupled THMC behaviors in a 3D domain depressurized by a horizontal wellbore. Full geomechanics is considered in the model. The model is verified against a widely used benchmark problem. Numerical results show that, due to the nature of 3D fluid flow induced by depressurization, flow patterns in vertical and horizontal directions are different, which leads to the phenomenon that the system response patterns captured by 3D THMC modeling are different from those in 2D THMC modeling. 2D THMC modeling may simplify the coupled THMC system response with the presence of horizontal wells. Also, the system response is mainly governed by the thickness of hydrate-free interlayers in heterogeneous cases. The effect of heterogeneity on pressure is observed in all investigated locations while its effect on temperature and displacement is only observed in planes penetrating the wellbore. The negative impact of heterogeneity on methane production is magnified by thicker hydrate-free interlayers.

Gas-saturated carbon dioxide hydrates above sub-seabed carbon sequestration site and the formation of self-sealing cap

Storing CO2 in sub-seabed sediments is a promising CO2 sequestration method to reduce atmospheric CO2 concentrations and mitigate climate change. Leaked CO2 from the sequestration site can form CO2 hydrates in the sediment and reduce the effective permeability of the sediment layer, developing a self-sealing cap above the storage site. Previous studies usually treat the CO2 hydrate layer similarly to a methane hydrate reservoir. However, unlike methane, CO2 has a larger solubility that increases significantly with depth, leading to high gas saturation levels in the sediment. Near the base of the CO2 hydrate stability zone, CO2 free gas, CO2 hydrate and dissolved CO2 coexist, and elevated surface energies of the curved surfaces of bubbles and hydrate crystals in pores shift the phase equilibria. The bubbles entrapped in the three-phase zone help to reduce the effective permeability besides accumulate CO2 hydrates. We simulate the three-phase zone in a shallow seabed using Monte Carlo methods in packed synthetic mono-dispersed spherical sediment grains, taking into account of the surface curvatures of the gas–liquid and hydrate–liquid interfaces in irregular pore spaces constrained by the sediment grains, and estimate the permeability reduction caused by entrapped CO2 bubbles. Finer sediment grains, larger geothermal gradients and shallower seawater all tend to broaden the zone, and the effective permeability in the three-phase zone can be further reduced by up to an order of magnitude than that with hydrates only. We also analyze the sensitivity of the three-phase zone due to temperature and pressure perturbations, and the results suggest possible thickness and depth variations of the zone due to cyclic temperature and sea level changes. This work demonstrates the difference between the CO2 hydrate-bearing sediment layer and the methane hydrate reservoir, and provides insight into the formation mechanism of the self-sealing cap above the sequestration sites.

Thermal Properties of Pressure Core Samples Recovered From Nankai Trough Wells Before and After Methane Hydrate Dissociation

AbstractThe thermal properties of methane hydrate (MH)‐bearing sediments are necessary for developing gas production technologies (e.g., gas production rate prediction). The direct thermal property measurement of a general MH reservoir is difficult because it demands an advanced coring and measuring technology. Therefore, a good thermal property value estimation model for a system containing sediment grains, water, MH, and gas is necessary. In 2018, the pressure core samples were recovered from the AT‐CW1 and AT‐CW2 wells in Nankai Trough of Japan. Using a single‐sided transient plane source method, we measure herein the thermal property values of those core samples and estimate their thermal conductivity by improving existing models. The measured thermal conductivity (λ), specific heat (ρCp), and thermal diffusivity (α) of the MH‐bearing sediments before and after MH dissociation are obtained. The de Vries model agrees well with the measured thermal conductivity values before and after the MH dissociation. For the random distribution (geometric mean) model, the difference between the estimation values and the data after the MH dissociation increases as the gas saturation Sg′ increases. In the worst case, the distribution model shows an 87.6% error, while the de Vries model yields 5.2% of the same. We propose and use an estimation method of the thermal property values in an MH reservoir, which requires classical thermophysical models and well log data. The in‐situ thermal property values can be estimated when the log data are highly accurate.

Joint-production characteristics of typical marine composite reservoir with water-saturated methane hydrates and higher-pressure gas by depressurization

Marine natural gas hydrate is water-saturated and usually accompanied with higher pressure underlying gas, and this typical composite reservoir is greatly economical for production. Yet, the experimental simulation on the typical marine composite reservoir is hard, and its joint-production studies are quite rare. In this work, the same 7.5 MPa methane hydrate reservoirs with an approximately 20% saturation underlain with four different pressures (7.1, 8.0, 8.1 and 9.0 MPa) gas were remolded. During the depressurization process from the top of hydrate layer, the water-saturated hydrate layer can allow the overpressure of underlying gas as highest as 6.0 MPa because of the presence of the hydrate seal. The early production stage is independent and lasts for several minutes until the hydrate seal is broken due to the dissociation of some hydrates, and then the massive inflow of underlying gas into hydrate layer induces the dissociation pause or even reformation of hydrates in return. After that, the production of underlying gas become synchronous and in a same depressurization process with the hydrate reservoir. As a result, the joint-production process of water-saturated methane hydrate and higher-pressure gas can be divided into independent and synchronous stages. In addition, the decisive factors of underlying gas and hydrate production processes are gas seepage and hydrate dissociation, respectively, and the production rate of underlying gas is much faster than that of hydrate reservoir. It is therefore suggested that the key of improving joint-production efficiency of the composite reservoir is to accelerate the hydrate dissociation. This work is of great significance for the guidance of spot production of marine natural gas hydrate with higher-pressure underlying gas.

Experimental designs of artificial hydrated cores produced by <i>iso</i>-Butane and elucidation of behaviors of gas phase during the stabilization of methane hydrate reservoir by grout material

Experimental designs of artificial hydrated cores produced by <i>iso</i>-Butane and elucidation of behaviors of gas phase during the stabilization of methane hydrate reservoir by grout material

Numerical Investigation of Depressurization through Horizontal Wells in Methane-Hydrate-Bearing Sediments Considering Sand Production

Sand production has been identified as a key reason limiting sustained and commercial gas production in methane-hydrate-bearing sediments. Production tests in Canada and Japan were terminated partially because of excessive sand production in pilot wells. It is meaningful to carry out numerical investigations and sensitivity analyses to improve the understanding of sand production mechanisms during the exploitation of methane hydrates. This study introduces a numerical model to describe the coupled thermal–hydraulic–mechanical–chemical responses and sand production patterns during horizontal well depressurization in methane-hydrate-bearing sediments. The model is benchmarked with a variety of methane hydrate reservoir simulators. Results show that the spatial and temporal evolution patterns of multi-physical fields are different and the hydromechanical evolutions are the fastest. Gas production and sand production rates are oscillatory in the early stages and long-term rates become stable. Gas production is sensitive to rock physical and operational parameters and insensitive to rock mechanical properties such as cohesion. In contrast, sand production is sensitive to cohesion and insensitive to rock physical and operational parameters. Although cohesion does not directly affect gas productivity, gas productivity can be impaired if excessive sand production impedes production operations. This study provides insights into the sand production mechanism and quantifies how relevant parameters affect sand production during the depressurization in methane-hydrate-bearing sediments.

Investigations on a controlled microwave heating technique for efficient depressurization in methane hydrate reservoirs

Numerical investigations on a novel technique of controlled microwave heating with depressurization, applied to gas recovery from methane hydrate reservoirs are performed. An in house, multi-phase, multi-component fluid flow solver with microwave radiation is developed and validated with benchmark results in literature. The microwave heating in the reservoir is operated and controlled based on the rise and fall of temperature in the well-bore proximity. Results show that the proposed microwave control technique significantly improves the gas recovery and energy output of combined depressurization and microwave heating. The performance parameters such as total gas recovery and energy efficiency ratio of the controlled microwave heating technique are compared with pure depressurization, depressurization with continuous heating, and pure heating techniques. The proposed control method provides high overall energy efficiency than all other production techniques. Based on the pareto optimization strategy using fuzzy membership function, an optimum well-bore temperature bound for control of microwave operation is determined. The sensitivity study of the process inputs reveals that at the optimum temperature bound, the energy efficiency ratio of the controlled microwave heating becomes independent of the microwave power. The depressurization pressure differential has a linear impact on the energy efficiency ratio of the controlled microwave heating, while a microwave with 915 MHz frequency is seen to provide the maximum energy efficiency ratio.

Comparative Analysis on the Evolution of Seepage Parameters in Methane Hydrate Production under Depressurization of Clayey Silt Reservoir and Sandy Reservoir

Gas hydrates are likely to become an important strategic resource with commercial development prospects. It is therefore of great significance to realize the long-term and efficient production of methane hydrate reservoirs. Previous studies have shown that the lithological characteristics of hydrate reservoirs have a significant impact on reservoir productivity by influencing the evolution of seepage parameters in the process of hydrate production. The porosity (Φ) and initial hydrate saturation (SH) affect the amount of hydrate decomposition and pressure transfer, and also indirectly affect the reservoir temperature field. The permeability (k) directly affects the rate of pressure-drop transmission and methane gas discharge. Due to the differences in seepage parameters caused by different reservoir lithology, a sandy hydrate reservoir (SHR) in Japan and a clayey silt hydrate reservoir (CHR) in China were found to have different gas production rates and the spatial evolution characteristics of the temperature and pressure fields varied in gas hydrate production tests. Therefore, to ensure the long-term and efficient production of the CHR in China, two models were established for a comparative analysis based on a numerical simulation. The two models were depressurizing models of the CHR of the W11 drilling site in the Shenhu Sea area of the South China Sea and the SHR of the AT1 drilling site in the Eastern Nankai Trough of Japan. Both models considered the heterogeneity of seepage parameters, and the TOUGH+HYDARATE (T+H) code was used in subsequent calculations. Four key results were obtained: (a) The order of the significance levels of the lithological parameters on productivity was k > SH > Φ in the CHR and SH > k > Φ in the SHR. (b) The heat conduction and heat convection in the CHR were weaker than in the SHR, which made it difficult to recover the low-temperature area caused by hydrate decomposition. (c) The exploitation of a high k hydrate reservoir should be given priority when the other initial conditions were the same in both the CHR and SHR. (d) The exploitation of both the CHR and SHR should not only rely on the hydrate content or seepage capacity to determine the reservoir exploitation potential, but the combined effect of the two parameters should be fully considered.

Experimental study on hydraulic fracturing behavior of frozen clayey silt and hydrate-bearing clayey silt

There are critical problems in the field trials of natural gas hydrate (NGH) production, such as low production rate, limited recovery range and short period of stabilized production. Reservoir stimulation is a potential means to promote gas recovery from NGHs since previous studies have proved that gas production of other unconventional gas reservoir can be highly enhanced by hydraulic fracturing. However, few studies pay attention to the hydraulic fracturing behavior of clayey silt NGH reservoir. In this work, the effects of sediment properties and fracturing fluid parameters on fracture initiation, propagation and morphology were experimentally investigated by hydraulic fracturing of frozen clayey silt and methane hydrate-bearing clayey silt. The results indicated that the uneven distribution of ice in sediments led to non-uniform fracture propagation phenomenon, such as single-wing fracture and fracture deflection. The fracturing fluid with injection rate of 90 mL/min was conducive to the formation of complex fractures in sediments, which indicated that that increasing injection rate could greatly increase the effect of reservoir reconstruction. When the axial pressure (8 MPa) was 2 MPa higher than the confining pressure, the fracture propagated in the direction perpendicular to the minimum principal stress, which revealed the magnitude of minimum in-situ stress difference controlling the fracture propagation direction. In addition, this work showed the fracability of clayey silt hydrates even with low saturation, which revealed the feasibility of hydraulic fracturing of marine methane hydrate reservoir.

Thermodynamic insights into the production of methane hydrate reservoirs from depressurization of pressure cores

Thermodynamic insights into the production of methane hydrate reservoirs from depressurization of pressure cores

Thermodynamics analysis and ice behavior during the depressurization process of methane hydrate reservoir

As potential alternative energy sources, natural gas hydrate has attracted international attentions. In order to optimize its exploitation process, the thermodynamics characteristics during hydrate dissociation were investigated in this study. Eleven reservoirs with three ratios of gas amounts in gas and hydrate phases (2.1, 2.9 and 4.6) were employed, and four exhaust rates (2.3, 4.7, 7.1 and 8.8 ln/min) were conducted to pressurize the reservoirs to 2.0 MPa. Hydrates dissociated at a constant rate during the depressurization stage, and the dissociation rate increased with higher depressurization rate, while the effect is enhancing with the increase of gas space. The temperature decreased with the dropped pressure in a similar trend, affected by both depressurization rate and hydrate saturation. During constant-pressure stage, the hydrate-bearing reservoir temperature kept at approximately −2.5 °C until ice formation, making the temperature increase to −1.5 °C. Ice formation can nearly quintuple the instantaneous rate of hydrate dissociation, while no obvious changes in hydrate dissociation before and after the end of ice melting. In addition, the induction time and existence duration of ice obey good stepwise spatial and temporal sequences. These results are great significant for pressure controlling and efficiency optimization of later period of methane hydrate exploitation.