• Energy Research

Abstract The permeability of methane hydrate (MH)-bearing sediments is closely related to the exploitation efficiency of MH deposits. During the exploitation of MHs, the stress state of hydrate-bearing reservoirs is complex, and stratum deformation may occur simultaneously. Both the change in the stress state and stratum deformation may influence the permeability of hydrate-bearing sediments. In this paper, a series of permeability measurement tests was conducted to study the influence of the effective confining pressure and triaxial compression process on the permeability of MH-bearing samples. The test results indicated that the permeability of hydrate-bearing samples was influenced by the effective confining pressure. With increasing effective confining pressure, the permeability of the pure sand sample gradually decreased. For the hydrate-bearing samples, the permeability was also influenced by the effective confining pressure. When the effective confining pressure reached a specific value, a sharp drop in the permeability of the hydrate-bearing samples occurred. The change in permeability might be caused by the increase in fine hydrate particles in the pore space. Based on the test results, the permeability of the hydrate-bearing samples was influenced by the triaxial compression process. During the shearing process, the permeability greatly increased when the axial strain reached a specific value. This phenomenon might be caused by the formation of a shear band. During the production of hydrates, controlling the change in stress applied to the hydrate particles could possibly promote production. Furthermore, the appearance of shear bands or fissures might also increase production by certain methods, such as the hydraulic fracturing process.

The mechanical behaviors of hydrate-bearing marine sediments (HBMS) drilled from the seafloor need to be understood in order to safely exploit natural gas from marine hydrate reservoirs. In this study, hydrates were prepared using ice powder and CH4 gas, and HBMS from the Shenhu area in the South China Sea were remolded using a mixed sample preparation method. A series of triaxial tests were conducted on the remolded HBMS to investigate the effects of soil particle gradation and the existence of hydrate on the mechanical properties of hydrate reservoirs. The results show that the stiffness and failure strength of HBMS decrease along with the decrease of mean particle size and soil aggregate morphology change at different drilling depths, and the reduction of failure strength is more than 20% when the drilling depth drops by 30 m. A better particle gradation of marine sediments may boost the stiffness and failure strength of HBMS. In addition, the existence of hydrate plays an important role in the strength behaviors of HBMS. The reduction of failure strength of HBMS with 30% initial hydrate saturation is more than 35% after complete hydrate dissociation.

Abstract Geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments, representing a potentially vast natural gas source. Numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous, interdependent (coupled) processes associated with potential production strategies for these gas hydrate reservoirs. Confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators (GHRSs) grows with successful comparisons against laboratory and field test results, but such results are rare, particularly in natural settings. The hydrate community recognized another approach to building confidence in the GHRS: comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate-bearing systems. The United States Department of Energy, National Energy Technology Laboratory, (DOE/NETL), sponsored the first international gas hydrate code comparison study, IGHCCS1, in the early 2000s. IGHCCS1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation. Subsequently, GHRSs have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling. This paper contributes to the validation of these recent GHRS developments by providing results from a second GHRS code comparison study, IGHCCS2, also sponsored by DOE/NETL. IGHCCS2 includes participants from an international collection of universities, research institutes, industry, national laboratories, and national geologic surveys. Study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components. The five problems range from simple geometries with analytical solutions to a representation of the world's first offshore production test of methane hydrates, which was conducted with the depressurization method off the coast of Japan. To identify strengths and limitations in the various GHRSs, study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences. The GHRSs evolved over the course of IGHCCS2 as researchers modified their simulators to reflect new insights, lessons learned, and suggested performance enhancements. The five benchmark problems, final sample solutions, and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators.

The CH4-CO2 replacement method has attracted global attention as a new promising method for methane hydrate exploitation. In the replacement process, the mechanical stabilities of CH4 and CO2 hydrate-bearing sediments have become problems requiring attention. In this paper, considering the hydrate characteristics and burial conditions of hydrate-bearing cores, sediments matrices were formed by a mixture of kaolin clay and quartz sand, and an experimental study was focused on the failure strength of CH4 and CO2 hydrate-bearing sediments under different conditions to verify the mechanical reliability of CH4-CO2 replacement in permafrost-associated natural gas deposits. A series of triaxial shear tests were conducted on the CH4 and CO2 hydrate-bearing sediments under temperatures of −20, −10, and −5 °C, confining pressures of 2.5, 3.75, 5, 7.5, and 10 MPa, and a strain rate of 1.0 mm/min. The results indicated that the failure strength of the CO2 hydrate-bearing sediments was higher than that of the CH4 hydrate-bearing sediments under different confining pressures and temperatures; the failure strength of the CH4 and CO2 hydrate-bearing sediments increased with an increase in confining pressure at a low confining pressure state. Besides that, the failure strength of all hydrate-bearing sediments decreased with an increase in temperature; all the failure strengths of the CO2 hydrate-bearing sediments were higher than those of the CH4 hydrate-bearing sediments in different sediment matrices. The experiments proved that the hydrate-bearing sediments would be more stable than that before CH4-CO2 replacement.

Abstract Natural gas hydrate is a new alternative energy that has attracted global attention in recent years. Depressurization is considered a fundamental method of producing natural gas from gas hydrate-bearing sediments (GHBSs). However, soil compaction during depressurization is a significant problem for production efficiency and safety. The compressibility of soil affects the hydrate dissociation in the coupled process of heat transfer, fluid flow, and soil compaction. In this study, a fully coupled Thermo-hydro-chemo-mechanical (THCM) model is applied to simulate Masuda's core-scale gas production experiments. The effects of compressibility on the changes in gas production rate, pore pressure, temperature, hydrate saturation, permeability, and heat conductivity are investigated by varying the parameters governing compressibility including the bulk modulus of host sediments and hydrate-enhanced bulk modulus. The results show that the higher compressibility corresponds to a larger reduction in porosity further impacting the variation in effective permeability, heat conductivity, and heat convection during depressurization. In Masuda's test, the pressure changes indicate that the soil compaction might occurs during depressurization. Because the real field production is implemented under confining condition, Masuda's test should be developed to consider the compressibility of GHBSs.

Nature gas hydrates (NGHs) are considered as an ideal substitute for traditional fossil fuel. The evaluation of the physical properties of hydrate-bearing sediment (HBS) is important to develop reasonable exploitation strategies and clarify the corresponding main controlling factors. In this study, to determine the influence of grain size distribution on the physical properties of HBSs, 24 hydrate-bearing models were reconstructed based on a novel pore-scale 3D morphological modeling algorithm. A series of pore system evaluations and permeability simulations were carried out by pore network modeling (PNM). Moreover, thermal and electrical simulations were carried out using the finite volume method (FVM). The results are as follows: (1) The smaller the grain size is, the smaller the radius of the pores and throats of HBSs will be, which makes it easier for the connectivity between pores and throats to deteriorate as the hydrate saturation increases. (2) With increasing hydrate saturation, the permeability of HBSs gradually decreases. According to the decline rate, it can be divided into two stages: in the first stage, it will decrease sharply, while in the second stage, it will decrease slowly. In general, the smaller the grain size is, the lower the permeability of HBSs will be. (3) The apparent thermal conductivity of HBSs will increase as the hydrate saturation increases. The smaller the grain size is, the higher the apparent thermal conductivity will be. (4) The smaller the grain size is, the lower the apparent electrical conductivity and the higher the resistivity index under the same hydrate saturation will be.

Abstract The commercial exploitation of natural gas hydrates (NGHs) has been a growing research focus due to its features of enormous reserves and clean fuel. To guarantee the safe and efficient production of NGHs, we have proposed a novel strategy of water flow erosion to promote methane hydrate (MH) decomposition based on the tremendous seawater resource and the fundamental process of water-gas flow during NGHs exploitation. In this study, the synthetic effects of traditional-thermodynamic-factors (temperature, salinity, pressure) and fluid flow on MH decomposition characteristics, which is known little about yet, are comprehensively analyzed via in-situ magnetic resonance imaging (MRI). The temporal-spatial behaviors of MH decomposition are visually investigated. The results indicate that the pressure, salinity, temperature and water flow synergistically increased MH decomposition efficiency. Additionally, the propagation of the decomposition front along the interface between MH and ambient phase shows that the water flow rate and heat transfer are two crucial factors for accelerating MH decomposition. The higher water flow rate also efficiently complements the insufficient decomposition driving force due to the heat loss during MH decomposition process. The highest average decomposition rate (1.1%/min) and the relatively less water injection volume (320 mL) can be archived in this study. Furthermore, the decomposition rate has a significant dependence on temperature under lower water flow rate.

Abstract Natural gas hydrates were widely distributed in marine sediments and permafrost areas, which have attracted global attentions as potential energy resources. Permeability characteristics of sediments determine the technical and economic feasibility of natural gas energy production and energy production efficiency from the hydrate reservoirs. But clay possesses significant water sensitivity and swelling characteristics in hydrate reservoirs, which may significantly affect permeability changes. Therefore, the mechanism significantly affects the gas energy production of hydrate reservoir. This study presented here focuses on the phenomenon of gas phase permeability changes due to hydrate decomposition. In this paper, the experimental study of the methane hydrate decomposition was carried out by depressurization, and the gas phase permeability characteristics changes of three kinds of clay before and after hydrate decomposition were investigated. The results show that the gas phase permeability of clay decreases gradually with the hydrate decomposition. The possible explanations for this phenomenon are that the formation of the bound water and swelling of clay, which block the pore channel of gas flow. In addition, after the hydrate complete decomposition, the value of the gas phase permeability damage (Ratio of permeability after hydrate decomposition to that before hydrate decomposition) of the clay firstly decreases and then increases with the increase of initial hydrate saturation. When hydrate saturation is 20%, Hydrate decomposition has the most influence on gas phase permeability damage of clay. And after the hydrate decomposition, the swelling of kaolin is less than illite and the swelling of illite is less than montmorillonite. The predicted porosity of clay after hydrate decomposition is calculated by Ives and Pienvichitr model and Tien’s model. This work could be valuable to research on the gas energy production from the hydrate reservoir.

Creep behaviors of methane hydrate-bearing frozen specimens are important to predict the long-term stability of the hydrate-bearing layers in Arctic and permafrost regions. In this study, a series of creep tests were conducted, and the results indicated that: (1) higher deviator stress (external load) results in larger initial strain, axial strain, and strain rate at a specific elapsed time. Under low deviator stress levels, the axial strain is not large and does not get into the tertiary creep stage in comparison with that under high deviator stress, which can be even up to 35% and can cause failure; (2) both axial strain and strain rate of methane hydrate-bearing frozen specimens increase with the enhancement of deviator stress, the decrease of confining pressure, and the decrease of temperature; (3) the specimens will be damaged rather than in stable creep stage during creeping when the deviator stress exceeds the quasi-static strength of the specimens.