Increasing Hydrate Saturation Research Articles

Analysis of permeability anisotropy of marine hydrate-bearing sediments using fractal theory combined with X-CT

Permeability anisotropy plays a crucial role in accurately predicting fluid productions during hydrate exploitation. However, the mechanisms governing permeability anisotropy have yet to be fully elucidated. This study conducted experiments on methane hydrate formation/dissociation in both unsaturated and saturated quartz sands, followed by X-ray computed tomography scanning and pore structure characterizations. Fractal parameters, encompassing fractal dimension, tortuosity fractal dimension, and maximum pore diameter, were employed to characterize the evolution of pore structure and changes in permeability anisotropy. Results show that the fractal dimension and maximum pore diameter decreased, while the tortuosity fractal dimension increased with the increase of hydrate saturation. Larger maximum pore diameter and smaller tortuosity fractal dimension in the horizontal direction become more pronounced with increasing hydrate saturation, resulting in heightened permeability anisotropy (kh/kv). The permeability anisotropy increases from 1.77 to 3.20 with hydrate saturation increasing from 0 to 0.38 in unsaturated sands, and from 1.47 to 4.70 with hydrate saturation increasing from 0 to 0.49 in saturated sands. Gas and water relative permeability anisotropy also amplifies with increasing hydrate saturation. Extrapolating to field tests, the findings suggest that horizontal wells present a more favorable option for gas production, especially in reservoirs with high hydrate saturation.

Experimental study on the impact of effective stress and cyclic loading on permeability of methane hydrate clayey sediments

Permeability evolution directly determine gas production and its efficiency of gas hydrates in the South China Sea. Due to existence of submarine overlying rocks and change of soil internal mechanics during the hydrate exploitation, permeability evolution is significantly affected. Therefore, in this paper, influence of effective stress on the porosity and permeability of sediment with different hydrate saturation was studied, and functional relationship between them was revealed. Effect of cyclic loading and unloading stress on the permeability recovery and damage degree was also discussed. The results show that when hydrate saturation increases, the relation of permeability and effective stress are power function rules, while porosity and effective stress are exponential function rules in clayey sediments. An empirical prediction equation for gas permeability of clay sediments with effective stress was presented, which takes into account hydrate saturation and porosity. In addition, during cyclic loading experiments, the results showed a linear increase in permeability during unloading. The permeability recovery rate and permeability damage rate are completely opposite with change of hydrate saturation. Permeability damage rate of kaolin is different from that of other two kinds of sediment. With the increase of cycle times, the proportion of plastic deformation of sediment decreases gradually.

Distribution of gas hydrate in fractured reservoirs: implications from anisotropic elastic and electrical numerical simulations

SUMMARY Fractured hydrate-bearing reservoirs are extensively discovered worldwide and show notable anisotropic geophysical properties. Hydrate distribution in fractures significantly affects the anisotropic properties, and hence plays an important role in the accurate assessment of hydrate resources. However, the knowledge about how the hydrate distribution impacts the anisotropic geophysical properties of fractured reservoirs, which is the premise for the identification and quantification of hydrate in fractured reservoirs, is still poorly understood. To obtain such knowledge, we forward study the effects of various hydrate distribution (including floating, bridging and evolving hydrate distribution) in aligned fractures on the anisotropic elastic, electrical and joint elastic–electrical properties of a digital core using validated numerical methods. We show that for all the hydrate distribution, the anisotropic velocities increase, while the conductivities decrease with increasing hydrate saturation, with the effects of the floating and bridging distribution being the least and greatest, respectively. We also show that the anisotropic velocities and conductivities for the floating and bridging distribution vary approximately linearly with hydrate saturation, leading to linear correlations between the elastic and electrical properties. Further investigation illustrates that the difference in the slopes of the linear joint correlations between the two distribution is significantly greater than that of the individual properties as a function of hydrate saturation. The results have revealed the distinct effects of hydrate distribution on the anisotropic elastic and electrical properties of fractured reservoirs, and have confirmed the superiority of the joint elastic–electrical properties for the distinguishment of hydrate distribution in fractures over individual physical properties.

Analysis of competition and interference mechanisms of multiple hydraulic fractures in deep-sea hydrate reservoirs in the South China Sea

Improving the fundamental theory of hydraulic fracturing in deep-sea hydrate reservoirs is of great significance in enhancing the efficiency of gas hydrate mining. In this paper, a multi-cluster fracturing model for a horizontal well was established with a three-dimensional extended finite element method and the measured geo-mechanical data from hydrate reservoirs in the South China Sea. The performance, fracture initiation, and expansion mechanisms of multi-hydraulic fractures are investigated under different fracturing sequences, hydrate saturation and cluster deployment patterns. Some interesting results have been discovered: In the case of three-cluster, the fractures produced by sequential fracturing are notably more affected by pre-existing fractures. Furthermore, the effect of fracture interference between simultaneous and sequential fracturing is reflected in the middle fracture is compressed by the fractures on its left and right sides. The increase in hydrate saturation enhances the cementing ability of the sediments and results in an increased difficulty of fracture initiation for the five-cluster simultaneous fracturing. It also results in a decrease in the geo-stress field, where the interaction between fractures plays a dominant role in the process of fracture expansion, leading to non-uniform expansion.

A bounding surface model of methane hydrate-bearing carbonate sand-silt mixtures

The geological environment of marine methane hydrate-bearing sediments (MHBS) is highly complex. Mastering the mechanical behavior of MHBS is crucial for analyzing geomechanical hazards and evaluating its stability. The marine sediment in the South China Sea (SCS) contains a significant amount of porous foraminifera shells, which serve as the primary storage space for natural gas hydrates and differ significantly from other sea areas. In the laboratory, porous carbonate sand (CS) and cohesion-less silt were utilized to prepare host sand of methane hydrate. In this paper, based on the analysis of experimental data, a bounding surface model was proposed to describe the mechanical behavior of artificial MHBS. The effect of methane hydrates on the elastic strain, normal consolidation and critical state of host sand is discussed, and corresponding equations for the model parameters are established, taking into consideration the cementing mechanism of hydrates. These equations are defined as functions of hydrate saturation, carbonate sand content (CSC), and initial void ratio. The increase in hydrate saturation significantly reduces the compressibility of the host sand and improves its strength. The proposed model is applied to predict the results of isotropic consolidation and drained triaxial tests on MHBS, demonstrating satisfactory performance.

Study of gas-liquid two-phase flow characteristics in hydrate-bearing sediments

The characteristics of gas-liquid two-phase flow in hydrate-bearing sediments (HBS) are critical for natural gas hydrate production. This study experimentally investigates the flow patterns in the presence of hydrates and employs the phase field method for accurate simulations. Additionally, flow pressure differences in a capillary model were analyzed, and the impacts of occurrence patterns, saturation, and wettability of hydrates were investigated by numerical simulation. The results show that flow patterns are predominantly influenced by flow velocity. At high flow rates (Reynolds number >10), different gas-liquid ratios result in a variety of flow patterns, including annular, Taylor, and bubble flow. However, at lower flow rates (Reynolds number <10), the flow pattern consistently manifests as Taylor flow, regardless of gas-liquid ratio variations. Notably, the Jamin effect (a significant pressure differential caused by hydrates within the flow) was observed, which intensifies with increasing hydrate saturation and decreasing contact angle. Pore-filling hydrate patterns, in contrast to grain-coating, exhibit a greater number of gas-liquid interfaces, leading to larger pressure differences and a more evident Jamin effect. These findings aid in understanding the fluid migration in HBS and help estimate the natural gas production capacity.

Numerical Investigation on Mesoscale Evolution of Hydraulic Fractures in Hydrate-Bearing Sediments

Hydraulic fracturing is widely recognized as a potential stimulation technology for the development of challenging natural gas hydrate. However, the fracturing behavior of non-diagenetic hydrate reservoirs has peculiar characteristics that are different from those of conventional oil and gas reservoirs. Herein, a fully coupled fluid-mechanical model for simulating hydraulic fracturing in hydrate-bearing sediments (HBS) was established based on the discrete element method, and the influence of hydrate saturation, in situ stress, and injection rate on the meso-fracture evolution was investigated. The results indicate that with the increase in hydrate saturation, the fracture morphology transitions from bi-wing to multi-branch, thereby enhancing fracture complexity. Both tensile and shear failure modes exist, and the tensile failure between the weakly cemented sediment particles is dominant. The tensile strength of HBS is an exponential function of hydrate saturation, with the breakdown pressure being governed by hydrate saturation and in situ stress, with the form being consistent with the classical Kirsch equation. Additionally, lower in situ stress and higher injection rates are conducive to the generation of microcracks, whereas an excessive injection rate reduces the fracture length. These findings contribute to understanding the meso-evolution mechanism of hydraulic fractures and guide the design of on-site hydraulic fracturing plans of natural gas hydrate reservoirs.

Undrained Triaxial Shear Tests on Hydrate-Bearing Fine-Grained Sediments from the Shenhu Area of South China Sea

Changes in undrained shear strength are important to the stability analysis of hydrate reservoirs during natural gas hydrate production. This study proposes a prediction model of undrained shear strength of hydrate-bearing fine-grained sediments based on the critical state theory. Several consolidated undrained triaxial shear tests are conducted on hydrate-bearing fine-grained samples from the Shenhu area of the South China Sea. The effects of effective consolidation stresses and hydrate saturations on the undrained shear strength are investigated. The results show that the undrained shear strength increases linearly with increasing effective consolidation stress. When the hydrate saturation is greater than the effective hydrate saturation, the undrained shear strength significantly increases with increasing hydrate saturation. The undrained shear strength of hydrate-bearing fine-grained sediments is a two-parameter function of effective hydrate saturation and a void ratio. The instability risk of the hydrate reservoir under undrained conditions is greater than that of under-drained or partially drained conditions. Furthermore, low-porosity reservoirs face more shear strength loss from hydrate decomposition yet lower risk than high-porosity ones. These results can improve the understanding of mechanical properties of hydrate-bearing fine-grained sediments under undrained conditions. This study also has implications for the design of marine structures in areas with hydrate-bearing sediment.

Pore-scale analysis of hydrate saturation on the physical parameters of hydrate-bearing sediment with different particle shapes

Methane hydrate is the most potential novel energy source with huge reserves to be developed, and is the priority of countries' future energy development strategies. Evaluating the basic physical characteristics of hydrate reservoirs, especially permeability, thermal, and electrical conductivity, is essential for clarifying the main controlling factors of physical characteristics and formulating a reasonable development strategy. Accordingly, 16 cementing hydrate-bearing models with different particle shapes were constructed account of a new three dimensions morphologic modeling algorithm for hydrate in this study. Based on the above models, the evolution of pore space distribution and basic physical properties of hydrate-bearing sediments (HBS) have been analyzed via pore network modeling (PNM) and finite volume method (FVM). The results show that (1) the quantity of pores and throats of HBS with round particles is higher, and the average radius is smaller than those of HBS with irregular particles, (2) increasing hydrate saturation leads to a decrease in absolute permeability, with a more rapid decrease in permeability for HBS with irregular particles, (3) the thermal conductivity of HBS with round particles exceeds those of HBS with irregular particles, and the their curves remain parallel with increasing hydrate saturation, (4) the resistivity index of HBS with round particles increases faster as hydrate forms, and the conductivity is always smaller than those of HBS with irregular particles.

Isotropic compression behavior for methane hydrate-bearing soil with varying temperature and pore pressure

Hydrate exploitation may trigger large-scale seafloor subsidence, especially when the depressurization method is used. The isotropic compression characteristics for methane hydrate-bearing soil (MHBS) need to be investigated to evaluate this deformation failure. Change in temperature and pore pressure (TP) condition can generate a considerable influence on the mechanical deformation and strength of MHBS. However, few studies focus on the isotropic compression deformation of MHBS. This paper carried out a series of isotropic compression tests on MHBS with different hydrate saturations and TP conditions. Experimental results show that the MHBS specimens exhibit a completely different isotropic compression curve at different hydrate saturations and TP conditions. The compressibility of MHBS is reduced remarkably with increased hydrate saturation, raised pore pressure, and decreased temperature. Hydrate saturation does not affect the compression index, but it explicitly reduces the swelling index. The yield stress is also enhanced with increasing hydrate saturation. The compression index of MHBS is independent of TP condition, while the swelling index is weakened and the yield stress is enhanced as temperature decreases and/or pore pressure increases. Through employing the phase state parameter as an indicator for TP condition, the influence of TP condition on the isotropic compression characteristics of MHBS is analyzed. Besides, the isotropic compression mechanism of MHBS at different hydrate saturation and TP condition is illustrated.

Experimental research on the erosion characteristics of organ-pipe cavitating jet for hydrate-bearing sediments

Cavitating jet is regarded as a promising breaking rate improvement technology in both the marine natural gas hydrate (NGH) fluidization exploitation method and the integrated radial jet drilling and completion method. Experimental studies on the influences of standoff distance, erosion time, inlet flow rate and hydrate saturation on hydrate-bearing sediments (HBS) breakage by organ-pipe cavitating jet (OPCJ) were conducted. The methane HBS synthesized in laboratory were employed as the experimental samples. The breaking effect were evaluated by four quantifiable indicators: erosion pit volume and maximum depth, and jet specific energy and specific cutting energy. Results show that under the same inlet flow rate, the standoff distance and erosion time have significant influences on the erosion pit shape. For the erosion pit volume and depth, the optimal standoff distances are respectively 4 mm and 5 mm (the diameter of nozzle is 1 mm), in which condition the OPCJ efficiencies of erosion and penetration on HBS are greatly improved. The ultimate erosion depth is 215 mm at an inlet flow rate of 0.085 L/s. The critical breaking velocities of OPCJ and conical jet (CJ) on HBS with hydrate saturation of 50% are respectively estimated to be 17.8 m/s and 24.2 m/s. Increasing hydrate saturation significantly decreases the erosion efficiency of OPCJ on HBS. The inlet flow rate is the most significant factor for the erosion pit volume and depth. This study provides a reference on the potential field applications for the NGH fluidization exploitation method and the integrated radial jet drilling and completion method.

Pore-scale modeling of seepage behaviors and permeability evolution in heterogeneous hydrate-bearing sediments and its implications for the permeability model

Permeability is one of the key factors determining the recovery of gas resources from natural gas hydrate (NGH) reservoirs. The heterogeneous distribution of hydrates induced by random nucleation and growth in the pore space of hydrate-bearing sediments (HBS) inevitably results in pore space structure change, thereby significantly affecting fluid flow and permeability. In this paper, the seepage behaviors in a series of two-dimensional (2D) idealized HBS models were simulated based on the pore-scale computational fluid dynamics (CFD) modeling to obtain permeability and pore-scale seepage field. The 2D hydrate heterogeneity degree coefficient (DH) was proposed to quantitatively characterize the hydrate distribution heterogeneity. The pore-throat evolution characteristics were quantified by average pore-throat size and pore-throat size distribution. The results show that the occurrence and growth of hydrates within pores lead to the reconstruction and evolution of pore space structure and the formation of new fluid migration channels. The main reason for the reduction of HBS permeability is that with the increase of hydrate saturation, pores and throats with small sizes gradually become dominant, pore space connectivity is weakened, and the tortuosity for fluid flow increases. Additionally, the evolution of flow path and flow channel morphology caused by the growth of hydrates with different pore habits leads to the difference in pore-scale seepage behaviors and permeability variations in the homogeneous model. However, the effect of hydrate pore habits on permeability is no longer obvious due to the spatial heterogeneity of sediment grains and gas hydrates in the heterogeneous model. The effective flow path for pore fluid controlled by overall hydrate distribution and pore-throat characteristics would be critical. The comparison of calculated permeability results reveals that the accuracy of reported permeability reduction models is limited due to the ignorance of the HBS heterogeneity, the actual hydrate pore habits and morphology, and the pore structure geometry. It is hoped that the findings of this study can provide some insights into the pore-scale seepage characteristics of HBS, providing important implications for the development of the permeability model that can be easily implemented in the numerical simulators for field-scale gas production optimization of hydrate reservoir.

Investigation on permeability anisotropy in unconsolidated hydrate-bearing sediments based on pore-scale numerical simulation: Effect of mineral particle shape and pore-filling

Resulting from the preferential alignment of sediment particles, permeability anisotropy of unconsolidated strata is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the effect of mineral particle shape and pore-filling NGH on the permeability anisotropy are considered in this study. A simulation with combining methods of particle flow code and computational fluid dynamics is conducted to construct a series of numerical samples and calculate the anisotropic permeabilities. Different particles shapes and hydrate saturations are preinstalled in these samples. It is firstly proved that the elongated particles have little effect on permeability anisotropy. Meanwhile, the preferential alignment of plate-shaped particles is the main reason for permeability anisotropy. In principle, the difference between intermediate orthogonal axes and short orthogonal axes of plate-shaped particles makes the tortuosity different in directions, thus inducing the permeability anisotropy. The anisotropy ratio decreases with the increase of hydrate saturation, which is caused by the nonlinear decline of main channel flow resulting from the thrombus-like hydrate particles. The results provide an insight into the formation mechanism of permeability anisotropy and display potential application value in NGH exploitation, groundwater environment protection and geological disaster prevention.

Gas permeability variation during methane hydrate dissociation by depressurization in marine sediments

Accurate acquisition of permeability change in hydrate-bearing sediments (HBSs) is the key to predicting reservoirs' multiphase fluids distribution and production capacity. Hydrate saturation and effective stress present significant effects on reservoir permeability. Thus, the influence of hydrate saturation and effective stress on gas permeability change of remolded core during decompression was studied. The results showed that effective stress compression and hydrate decomposition determine the flow channel structure within hydrate sediments. Under unconsolidated and low effective stress(σ' = 1.2 MPa) conditions, the gas permeability increased gradually as the decomposition of hydrate, and the magnitude of permeability variation before and after decomposition increased with increasing hydrate saturation, indicating that the effect of hydrate saturation on permeability dynamic change dominated at low effective stresses. When σ′ was increased to 3.2 MPa and 5.2 MPa, the results were contrary, indicating that the effective stress variation on gas permeability dynamic change dominated at higher effective stresses. In addition, it was found that compression led to higher heat transfer efficiency in high-saturation samples. When hydrate saturation was increased from 23.37% to 48.7%, the effect of increasing effective stress on gas production changed from inhibition to facilitation.

Imitating the effects of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments: An experimental study

The drilling fluid invasion into hydrate-bearing sediments will change the geomechanical properties of the reservoir and may lead to uncontrolled geological disasters in the worst case. Native sediments are replaced with artificial samples in this study to investigate the effect of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments. The triaxial test is used primarily to assess the strength behaviors of hydrate-bearing sediments at varying temperatures, pressures, and hydrate saturation levels. The process of water-based drilling fluid penetrating into hydrate-bearing sediments is then experimentally imitated under various experimental conditions, with reservoir temperature of 4°C and pore pressure of 10 MPa. The possible repercussions of drilling fluid invasion and hydrate phase transition are identified by testing the mechanical properties of sediments under various invasion times and temperatures. The findings reveal that when hydrate saturation rises from 5% to 35%, the gas hydrate sediments shift from strain hardening to strain softening, with the critical hydrate saturation value of transition is between 15% and 25%. Peak strength increases with increasing hydrate saturation and pressure, and decreases with increasing temperature, whether drilling fluid invasion is present or not. The peak strength, Young’s modulus, shear modulus, and secant modulus of hydrate sediments all decreased significantly after drilling fluid invasion, although the Poisson’s ratio rose. These mechanical parameters are related to temperature and pressure under the action of drilling fluid. Finally, engineering and research recommendations for reducing the risk of drilling fluid invasion and hydrate dissociation are made based on experimental findings and theoretical analysis. This study innovatively examine the geomechanical mechanical properties of drilling fluid invading hydrate reservoir, which is critical for avoiding production concerns.

Pore-scale modelling of elastic properties in hydrate-bearing sediments using 4-D synchrotron radiation imaging

Gas hydrate contains abundant methane gas, which is considered to be a promising energy resource to mitigate the influence of climate change in the future. Understanding the effect of fluids-solid-hydrate spatial distribution on elastic properties of hydrate-bearing sediments benefits the well-log interpretation. We constructed 4-D hydrate-bearing models covering a wide hydrate saturation range based on high-resolution digital images obtained by Synchrotron Radiation X-Rays Computed Tomography (SRXCT). We analysed the elastic moduli and elastic wave velocities of these models by the static Finite Element Method (FEM). The results demonstrated that the dominant hydrate morphology transits from pore-filling to load-bearing when hydrate saturation increases. At low hydrate saturations, the hydrate formed around some gas bubbles blocks local pore space. Therefore a great enhancement in compressional wave velocity was observed at low hydrate saturations. While the shear wave velocity was less affected due to the presence of thin water films. Based on the simulated results, an empirical model was developed to compute the compressional and shear wave velocities considering the free gas phase. The comparison with some experimental measurements validated the applicability of the model.

Experimental study on hydraulic fracturing in clayey-silty hydrate-bearing sediments and fracability evaluation based on multilayer perceptron-analytic hierarchy process

Hydraulic fracturing is considered a promising stimulation technology for low-permeability hydrate reservoirs. To date, only a few studies have focused on hydraulic fracturing in hydrate-bearing sediments. However, the numerous factors that affect fracture initiation and propagation are not clearly understood, and the fracability of non-diagenetic geo-materials has not been systematically evaluated. In this study, a series of true triaxial hydraulic fracturing experiments are conducted on clayey-silty hydrate-bearing sediments to investigate the effects of the key reservoir and engineering parameters on fracture initiation and propagation. Based on the resulting data, a fracability index (FI) that considers multiple factors is developed using a novel method. The results indicate that fracture initiation pressure does not always increase with increasing hydrate saturation. Moreover, a maximum value of 14.92 MPa for the initiation pressure is observed at 40% hydrate saturation. This value is increased by 20.51 MPa when the effective horizontal in-situ stress increases from 1 to 4 MPa, which is in contrast to the tensile crack initiation law of elastic rocks. Additionally, owing to the inhomogeneous hydrate in sediments, fractures expand unevenly, and double fractures are able to form in an isotropic horizontal stress state. The horizontal stress difference is the primary parameter (weight 0.4) that governs the FI, followed by the coefficient of earth pressure at rest (weight 0.31), fracture toughness (weight 0.18), and hydrate saturation (weight 0.05), and vertical in-situ stress (weight 0.05). Increasing the injection rate and fracturing fluid viscosity is an effective method to promote fracture propagation, particularly when Qμ‾> 0.33 (defined by the normalised injection rate and fracturing fluid viscosity) and FI> 0.4. In such conditions, a considerable reconstruction area can be obtained.

The effect of overconsolidation on the mechanical behaviours of hydrate-bearing silty sediments

CO2 sequestration via gas hydrate method shows great potential on promoting global carbon cycling and reducing greenhouse effect through injecting CO2 to hydrate reservoirs. However, the overconsolidation state induced by hydrate formation or interconversion of CO2 and CH4 hydrate would affect the stability of silty reservoirs after CO2 sequestration, but currently, few studies have been presented on this topic. To this end, a sequence of overconsolidation and drained shear tests on CO2 hydrate-bearing silty sediments (HBSSs) were carried out in this study to simulate and elucidate the effect of overconsolidation on the mechanical behaviours of CO2 HBSSs. The results reveal that all HBSSs with different hydrate saturations exhibit strain hardening and shear contraction behaviours with or without overconsolidation, while an increase in the overconsolidation ratio (OCR) enhances the deviator stress and alleviates the contraction effect. In addition, a strengthening effect on the stiffness, strength and resistance of deformation of HBSSs is induced by overconsolidation, while this effect becomes more prominent as the hydrate saturation increases. HBSSs with higher OCRs and hydrate saturations generally have lower stress-dilatancy ratios. Moreover, the shear paths of HBSSs with higher OCRs deviate farther from the normal consolidation line of hydrate-free silty sediments. This study is expected to provide theoretical guidance for the CO2 sequestration via gas hydrate in silty reservoirs.

Hydrate as a by-product in CO2 leakage during the long-term sub-seabed sequestration and its role in preventing further leakage.

Subsurface sequestration of CO2 produced by industrial production is an effective way to control the excessive emission of greenhouse gases and alleviate the potential global warming. However, CO2 leakage is likely to occur in its long-term sub-seabed sequestration, posing a great threat to the marine environment and the marine ecology. Previous investigations mainly focused on the implementation of CO2 sequestration project, ignoring the subsequent potential environmental hazards such as CO2 leakage, let alone the post-treatment of these accidents. In the present work, secondary sequestration mechanism of hydrate-bearing sediment for leaked CO2 was explored, and the effect of two important factors (hydrate saturation and sediment thickness) on it was then analyzed. It is expected to provide reference for exploring engineering measures to secondary sequestrate the leaked CO2 and avoid the catastrophic environmental accidents. The experimental results demonstrate that the role of hydrate in secondary sequestration for leaked CO2 is mainly due to its filling and occupation of the migration channels in sediment. In addition, due to the secondary sequestration of CO2 hydrate, change in seawater pH value caused by dissolution of leaked CO2 in water can be significantly weakened. Besides, with the increasing hydrate saturation and sediment thickness, CO2 hydrate plays a progressively obvious role in secondary sequestration of CO2 and avoiding great change in the marine environment. In this way, the leaked CO2 can be secondary sequestrated by designing/optimizing the characteristics of hydrate-bearing sediment. The investigation demonstrates that most of the leaked CO2 can be secondary sequestrated only when the hydrate saturation exceeds 0.30 and/or thickness of hydrate-bearing sediment exceeds 3.0cm. The experimental investigation herein can provide technical support for avoiding environmental disasters caused by the leakage of long-term sequestrated CO2.

Compression and Deformation Characteristics of Hydrate-Bearing Sediments under High Effective Confining Pressure

The depressurization method that is used to produce methane gas can increase the effective stress in hydrate-bearing sediments and induce deformation and settlement of reservoirs, which has adverse effects on safe and stable exploitation. A series of isotropic consolidation and depressurization dissociation tests under high effective confining pressures on methane hydrate-bearing specimens were conducted to investigate compressive characteristics and deformation behaviors. Isoconsolidation test results demonstrated that the rise in effective confining pressure-induced increasing compressibility and particle crushing of sediment, while the presence of hydrates can suppress compressibility and crushing. The compression index tends to increase with increasing effective stress but decrease with increasing hydrate saturation. Dissociation test results suggested that higher effective stress during dissociation and higher deviatoric stress led to more significant deformation and particle breakage. A specimen can exhibit serious deformation (ɛv ≈ 20%) after dissociation when the effective stress is extremely high (17 MPa), even if the stress state is in the stable zone. During water recovery after dissociation, axial and volumetric strains may further increase under the combined effect of deviatoric stress and high effective confining pressure.