Hydrate Reservoir Research Articles

Digital rock-based investigation of conductivity mechanism in low-resistivity gas hydrate reservoirs: Insights from the Muli area's gas hydrates

Gas hydrate reservoirs in the Muli area of China are characterized by complex lithology, high hardness, developed micro-fractures, and low porosity-permeability, in which an unusually low-resistivity phenomenon is observed. This phenomenon has led to considerable uncertainty in determining the gas hydrate layers and measuring the gas hydrate saturation. Therefore, it is necessary to understand the underlying factors appropriately to optimize exploration and production. In the present study, digital rock technology was combined with field-scale well-logging data to analyze the conductivity mechanism and identify the causes of the observed low resistivity. The obtained results and sensitivity analysis indicated that the rock porosity was the main controlling factor, and then the formation water salinity, pore structure, micro-fractures, clay components, heavy minerals, and gas hydrate pore-habits were the other significant parameters in the order of high to low significance with the corresponding significance values of 3.64, 2.51, 1.97, 1.72, 1.36, 1.04, and 0.56, respectively. Moreover, wells DK-8, DK11-14, and DK12-13 were taken as examples to analyze the formation mechanism of low-resistivity gas hydrate, and the results showed that the high reservoir porosity, developed micro-pores, high irreducible water saturation and good pore connectivity are the main reasons for the low-resistivity gas hydrate in DK-8; and micro-fractures and the banded distribution of clay are responsible for the low-resistivity gas hydrate in wells DK11-14 and DK12-13, respectively. These findings provide fundamental information for accurate interpretation of the key well-logging data in the study area as well as in similar reservoirs and help improve the exploitation of gas hydrate reservoirs both in the permafrost regions and marine environments.

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.

Heterogeneous and Coexisting Hydrate Phases─Formation under Natural and Laboratory Conditions

Natural gas hydrates are non-stoichiometric, crystalline solids composed of water and gas molecules. Dependent upon the source of the hydrate-forming gas, the structure and composition of the occurring natural gas hydrates may vary. In nature, the existence of structure I, structure II, and structure H hydrates containing predominantly methane but also other hydrocarbons, H2S, or CO2 could be verified. Interestingly, the number of reports on coexisting hydrate phases with different structures and compositions in natural gas hydrate reservoirs has increased in recent years. However, it has not yet been clearly clarified what leads to the formation of these coexisting hydrate phases. In the present study, we analyzed natural gas hydrate samples spatially resolved using Raman spectroscopy to check whether these natural samples only show heterogeneity with regard to their cage occupancy and composition or whether they already show coexistent phases. The samples available to us from the Hikurangi margin and the Cascadian margin showed strong fluctuations in cage occupancy and composition within the individual crystals but no coexisting phases. With complementary experiments, we were able to show that gas hydrates with a heterogeneous composition formed from a complex feed gas mixture. Furthermore, we were able to verify experimentally that coexisting phases may form when an initial methane hydrate phase was exposed to a complex gas mixture.

Effect of Hydrate Microscopic Distribution on Acoustic Characteristics during Hydrate Dissociation: An Insight from Combined Acoustic-CT Detection Study

Geophysical detection techniques are important methods in marine gas hydrate exploration and monitoring, because the small-scale distribution of hydrates has a large impact on the wave velocity. The acoustic response characteristics of hydrate micro-distributions have strong significance for monitoring the hydrate dissociation process. In this paper, experiments simulating the hydrate dissociation process were carried out in a self-developed experimental device combining X-ray computed tomography (X-CT) scanning and ultrasonic detection, which allowed the acoustic wave characteristics and X-CT scanning results to be simultaneously obtained during the hydrate dissociation process. This study found that the hydrate dissociation stage is divided into three stages. The hydrate begins to dissociate at spots where it comes into touch with sand particles early in the dissociation process. The main factor affecting the acoustic wave velocity of hydrates in this stage is changes in the microscopic distribution of hydrate. In the middle stage, a large amount of hydrate decomposes, and the main factor affecting the acoustic wave velocity of hydrate in this stage is the change in hydrate content. In the later stage of hydrate dissociation, the hydrate distribution pattern consists mainly of the pore-filling type, and the hydrate micro-distribution at this stage is the main factor affecting the acoustic wave velocity. This study will be of great significance for understanding the microscopic control mechanism of hydrate reservoir geophysical exploration.

Numerical Study of the Effect of Thermal Stimulation on Gas Production from Marine Hydrate Reservoirs

The temperature of hydrate reservoirs will decrease in a gas production process because gas hydrate dissociation is endothermic, leading to a decrease in the gas production rate as sensible heat is exhausted. Heat supply should be an effective way to alleviate the effect of temperature decrease on the production rate, by thermal stimulation, such as hot water injection into a hydrate reservoir. However, the feasibility of thermal stimulation combined with depressurization is controversial. Hence, to investigate the effect of this combined strategy, gas production performance is studied by numerical simulation based on a 2D Cartesian hydrate reservoir model with a two parallel horizontal well configuration. One of the two horizontal wells, at the top of the hydrate-bearing layers, is for gas production, and the other at the bottom of the hydrate-bearing layers is for hot water injection. The influence of the injection rate, temperature of hot water, and permeability of hydrate-bearing layers on gas production is investigated. When only the hot water injection is applied, the released gas, which originates from the dissociation of hydrates around the injection well, tends to be trapped and forms hydrates again. When the production strategy of hot water injection combined with depressurization is engaged, the hydrate dissociation shows a depressurization-dominant feature, and the production is at a relatively high rate, but it seems the hot water injection is still a negative factor in gas production. Because the hot water injection will replenish reservoir pressure and increase the water production rate, the feasibility of depressurization will be compromised. Sensitivity analyses reveal that the increased permeability of hydrate-bearing layers can enhance gas production almost proportionally. It is also found that increasing the temperature of the injected hot water only has a very limited effect on gas production.

Kinetic study of fulvic acid on methane hydrate formation and decomposition in clay-rich silty sediments

Humic substances and salts are widely distributed in marine sediments, a phenomenon that undoubtedly increases the complexity of recovering methane from hydrate. In this work, kinetic experiments were carried out to determine the role of fulvic acid (FA) on methane hydrate formation and decomposition in marine sediments. We used the mixed clay-rich silty with pore water, containing 3.0 wt% NaCl, to simulate the sediments, with experimental pressure (12.7–14.6 MPa) and temperature (8.6–16.2 °C) conditions adopted from the hydrate reservoir of Shenhu area, South China Sea (SHCS). Results revealed that FA has favorable/unfavorable effects on methane hydrate formation in the sediments, in a concentration-dependent manner. FA solutions, at concentrations of 6.0 and 10.0 wt%, inhibited the formation of methane hydrate, because the high-water holding capacity itself could reduce water activity. Conversely, 2.0 wt% FA solution accelerated methane hydrate formation by arranging surrounding water molecules with H-bonds. Interestingly, it was found that the addition of FA could significantly weaken NaCl's inhibitory effect on methane hydrate formation, while water conversion ratios increased ∼2/3-fold relative to NaCl without FA solution containing systems. These results suggested that presence of FA in marine sediments promoted hydrate reformation during reservoir production. Besides, we found that there is a synergistic effect between FA and NaCl in the process of methane decomposition.

Optimization of the pressure drop produced during CO2 replacement in hydrate reservoirs: Balance between gas removal and preservation of structures

The replacement of methane molecules with a theoretical equal number of CO2 molecules, is one of the most promising opportunities for the exploitation of natural gas hydrate reservoirs. In this work, the effect of depressurization on CH4 hydrates was analysed in depth. Different pressure drops, from 5 to 15 bar were tested and described in terms of free gas removed (desired event) and hydrates destroyed (undesired event) before the injection of carbon dioxide. A few pronounced pressure drop might be not enough to ensure an effective replacement, while an excessive one might be not sustainable due to the high quantity of hydrates destroyed before the process. In this work, variation of hydrates and free gas was evaluated before and after depressurization; it was then compared and employed to define an optimum range of pressure drop to optimize the process. It was proved that the quantity of hydrates destroyed increases with the pressure drop imposed, while the removal of free gas shows a maximum. When depressurization was close to 10 bar, only 18.57% of hydrates dissociated, while the quantity of free gas removed was three times higher (59.27%). Finally, the CO2/CH4 replacement process was carried out with the optimal depressurization degree previously defined.

The lithological characteristics of natural gas hydrates in permafrost on the Qinghai of China

The environment is seriously threatened by the methane emitted as permafrost melts. Studying deposits of natural gas hydrates that include methane is therefore important. This study presents a novel approach based on the rock Archie formula to discover the porosity and saturation of gas hydrates. The relationship between resistivity and porosity and the porosity of hydrates was studied, and the results showed that the resistivity of hydrate reservoirs was closely related to porosity and hydrate saturation, and the polarization rate was only related to the concentration of natural gas hydrates and had nothing to do with porosity. Using the multi-channel time domain induced polarization (MTIP) method, the profile with five boreholes in the Muli area of the permafrost area of the Qinghai-Tibet Plateau was observed, and the thickness of the shallow permafrost distribution and the underground structure were inferred based on the resistivity of the MTIP data. The polarization rate and hydrate saturation of the inversion assessed the presence of hydrates in the Muli region. The results show that the MTIP method can be used to detect the thickness of permafrost distribution, determine fault boundaries, reveal the distribution of natural gas transport paths, and evaluate the presence of natural gas hydrates.

Variations in terms of CO2 capture and CH4 recovery during replacement processes in gas hydrate reservoirs, associated to the “memory effect”

The replacement of methane molecules, contained within natural hydrates reservoirs, with a theoretical equal number of carbon dioxide molecules, represents a promising opportunity for the near future. However, the maximum theoretical efficiency was proved to be approximately equal to 75% and, in the real applications, it is drastically lower. This article focused the attention on the possible reasons of such difference and found in the memory effect one of them. Firstly, the formation process of pure CH 4 and CO 2 hydrates was performed and thermodynamically characterized; then, the replacement process was realized on a system excluding memory and on a system including it. In the first situation, the theoretical efficiency was reached, while, in presence of memory effect, the results were drastically less effective, both in terms of methane recovery and in terms of carbon dioxide storage . When the system did not retain memory of previous formation processes, the quantity of CO 2 captured ranged from 73.97 to 75.05 vol% and the quantity of methane released was approximately equal to the one of CO 2 captured. Conversely, in presence of memory effect, the percentage of CO 2 captured dropped to 26 vol% and the quantity of methane release was still lower (24.97–26.03 vol%). • The influence of memory effect on the CO 2 /CH 4 replacement process was studied. • The memory effect was confirmed to be a kinetic promoter for hydrates formation. • The memory of previous CH 4 hydrates formation lowered the replacement efficiency. • Both the quantity of CH 4 released and CO 2 stored drastically decreased. • In absence of memory, the percentage of CO 2 storage reached the theoretical value.

Influence of Solid Particles on Wellbore Parameters in Solid Fluidization Production Process

Objective: To efficiently exploit the hydrate reservoir, based on the problem of a large amount of sand production in the current hydrate reservoir exploration process, combined with the solid fluidization well testing and production method. Methods: A gas-liquid-solid three-phase flow model is established and hydrate decomposition is taken into consideration. The changes in parameters of wellbore pressure, gas volume fraction, liquid volume fraction, temperature and the decomposition rates of wellbore hydrate under the conditions of different rates of sand production are analyzed. Results: The results show that with the increasing rate of sand production during the solid fluidization well testing and production, the pressure of the wellbore is lifted, gas volume fraction declines, liquid volume fraction declines, and the temperature of the wellbore does not change. Moreover, increasing the rate of sand production also leads to the delay of the position where hydrate begins to decompose. Conclusion: With the decrease in hydrate saturation, the pressure of the wellbore goes up, gas volume fraction is lifted, solid volume fraction rises, and decomposition rate of hydrate declines.

Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading

Under the action of dynamic loadings such as earthquakes and volcanic activities, the mechanical properties of gas-hydrate-bearing sediments will deteriorate, leading to a decrease in the stability of hydrate reservoirs and even inducing geological disasters such as submarine landslides. In order to study the effect of dynamic loading on the mechanical properties of hydrate sediments, triaxial compression tests of numerical specimens were carried out by using particle flow code (PFC2D), and the macro-meso mechanical behaviors of specimens were investigated. The results show that the loading frequency has a small effect on the stiffness of the hydrate sediment, while it has a large effect on the peak strength. The peak strength increases and then decreases with the increase in loading frequency. Under the same loading frequency, the peak strength of the hydrate sediment increases with the increase in loading amplitude, and the stiffness of the specimen decreases with the increase in loading amplitude. The maximum shear expansion of the specimen changes with the movement of the phase change point and the rearrangement of the particles. The maximum shear expansion of the specimen changes with the movement of the phase change point and the change of the bearing capacity of the particles after the rearrangement, and the more forward the phase change point is, the stronger the bearing capacity of the specimen in the plastic stage. The shear dilatancy angle and the shear dilatancy amount both increase linearly with the increase in loading amplitude. The influence of loading frequency and amplitude on the contact force chain, displacement, crack expansion, and the number of cementation damage inside the sediment is mainly related to the average axial stress to which the specimen is subjected, and the number of cracks and cementation damage of the sediment specimen increases with the increase in the average axial stress to which the sediment specimen is subjected. As the rate of cementation damage increases, the distribution of shear zones becomes more obvious.

Modeling the Mechanical Behavior of Methane Hydrate-Bearing Sand Using the Equivalent Granular Void Ratio

For the safe extraction of methane from hydrate reservoirs, modeling the mechanical behavior of the methane hydrate-bearing soil properly is crucial in order to enable designers to analysis hydrate-dissociation-induced geotechnical failures. Hydrate morphology is one of major factors affecting the mechanical behavior of soil containing hydrate. This paper presents a new constitutive model for methane hydrate-bearing sand (MHBS) using the equivalent granular void ratio as a state variable, which can quantify the effects of the pore-filling and load-bearing hydrate morphology under a unifying framework. The proposed model is a combination of generalized plasticity and an elastic damage model so as to take into account the observed frictional and bonding aspects of MHBS, respectively. By using the concept of state-dependent dilatancy, the equivalent granular void ratio is formulated and adopted in the generalized plasticity model. In addition, a nonlinear damage function is implemented to elucidate the degradation of hydrate bonds with respect to shearing. Compared with the basic generalized plasticity model for host sand, only three additional parameters are required to capture key mechanical behaviors of MHBS. By comparing the triaxial test results of MHBS synthesized from a range of host sands with a predicted behavior by the proposed model, it is demonstrated that the new model can satisfactorily capture the stress–strain and volumetric behavior of MHBS under different hydrate saturations, confining pressures, and void ratios.

Investigating the effective permeability evolution as a function of hydrate saturation in the hydrate-bearing sands using a kinetic-theory-based pore network model

Permeability governs the fluid flow of hydrate-bearing sediment and affects the efficiency of natural gas production from hydrate reservoirs. The permeability in hydrate-bearings sediments is estimated empirically and appears to vary widely for sediments. This study focused on the sandy hydrate-bearing sediments and intended to elucidate the evolution of effective permeability by the mean of pore network modeling. A hydrate kinetics theory-based pore network model (KT-PNM) has been developed, in which the simulation of hydrate formation in porous media is implemented by employing two sub-processes, i.e., hydrate nucleation and hydrate growth. This KT-PNM has been applied to simulate hydrate formation in different pore networks. The permeability reduction exponent (N) for sandy hydrate-bearing sediments is determined to fall in the range of 3–4 based on the simulations of seven sandy samples. An empirical equation used for predicting the effective permeability in sandy hydrate-bearing sediments has been further proposed. The developed model and simulations are hoped to provide valuable insights of pore-space effects on hydrate into permeability prediction for studying the fluid seepage in hydrate-bearing sediments and facilitate the numerical simulation of gas production from the hydrate reservoirs.

Experimental study on sand control gravel particle gradation optimization in mud hydrate reservoir

Sand production significantly influences the safety and stability of gas hydrate exploitation in reservoirs with high mud content. Gravel packing is an effective method for sand control. The median grain size ratio (D50/d50) values and fine particle content of gravel pack are important factors in sand control during natural gas hydrate exploitation. This paper focuses on the sand blocking effect and permeability characteristics of different grades of gravel. The sand retention test was carried out on four samples with different ratios. The optimal D50/d50 value and fine particle content for gas hydrate production were obtained. A laser particle size analyzer and geotechnical test found the reservoir and gravel layer pore structure change. The results show that the optimal GSR was 24.5, and the optimal fine particle content was about 38%. The permeability decline of the gravel layer occurs in the early stage of production, and the fine particles in the gravel will slow down the degree of seepage damage. For gravel with the same particle size range and different median particle sizes, it has little effect on sand control accuracy but mainly on permeability. The research in this paper can provide a reference for selecting sand control gravel for natural gas exploitation in marine areas.

Geological Reservoir Characterization of a Gas Hydrate Prospect Associated with the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

Geological reservoir characterization is essential for accurate evaluation of gas production performance from gas hydrate reservoirs. Particularly, the understanding of reservoir architecture and heterogeneity is of great importance since these are considered as major controls on fluid hydrodynamic and thermodynamic conditions. This study deals with well log and three-dimensional (3-D) vertical seismic profile (VSP) data acquired from the Hydrate-01 Stratigraphic Test Well within the 7-11-12 prospect, Prudhoe Bay Unit, Alaska North Slope and reports on the results of geological/geophysical evaluation related to the geological structure and reservoir properties of the 7-11-12 prospect. The structural trends of the target reservoirs, based on well correlations, are mostly consistent with the predrill prediction using the surface seismic data, and infer the existence of subseismic faults cutting through the Hydrate-01 well. The 3-D VSP data confirm a down-to-the-east normal fault that offsets the reservoir units across the Hydrate-01 well, which is concordant with the well identification of the same fault, and indicate a northeast-dipping relay structure associated with the overstepping normal faults. The edge enhancement attribute associated with discontinuity generated from the 3-D VSP data shows small faults/fractures, possibly as part of a complex fault network within the imaged normal fault system. These results reveal that the 3-D VSP data provide detailed structural information that is not present from the surface seismic data. The Hydrate-01 well log data confirm the occurrence of gas hydrate at high saturation in the two targeted sand units (B1 and D1 sands), and the comparison to a nearby pre-existing well (7-11-12 well) shows the same general trend in gas hydrate saturation as a map of seismic impedance generated from surface seismic data. The well log data also suggest that the base of gas hydrate occurrence in the Hydrate-01 and 7-11-12 wells is almost aligned at the same depth in both of the targeted B1 and D1 sand reservoirs. Especially for the D1 sand in the Hydrate-01 well, the resistivity logs show a sharp transition from high gas hydrate saturation to fully water-saturated within the D1 sand, suggesting a common gas hydrate/water contact. The results of this study will be used to construct the geological models needed for reservoir simulation studies and they can provide important insights into the geological factors that control the occurrence of gas hydrate on the Alaska North Slope.

Experimental Study on the Permeability Damage Mechanism of Gravel Pack Sand Control Medium in Mud Hydrate Reservoirs

Yu, L.; Han, Z.; He, J., and Li, G., 2022. Experimental study on the permeability damage mechanism of gravel pack sand control medium in mud hydrate reservoirs. Journal of Coastal Research, 38(6), 1104–1115. Coconut Creek (Florida), ISSN 0749-0208. The high argillaceous gas hydrate reservoir contains high silt and fine sand content. During the mining process, the fine silt sand will cause blockage of the gravel sand control medium, resulting in reduced medium permeability, that is, penetration damage. Penetration damage will reduce productivity and affect the long-term effectiveness of gravel. For the penetration damage problem of gravel sand control medium, the sand blocking simulation experiment of different reservoir water content, experimental pressure, and gravel specifications were carried out by self-developed sand blocking experiment system. A laser particle size analyzer and electron microscope were used to measure and observe the deposition of sand particles in the upper, middle, and lower layers of gravel layer after the sand retention experiment. The law of permeation damage and the mechanism of media permeation damage were summarized. The research results show that the blockage of sand control media can be divided into four stages: the beginning of blockage, the intensification of blockage, the dredging of blockage, and the balance of blockage. Reservoir water content, experimental pressure, and gravel size all influence permeability damage of gravel. The primary and secondary relationship of the influence of these factors on permeability damage is gravel size, experimental pressure, and reservoir water content. The pressure mainly affects the invasion depth of reservoir sand. The reservoir water content mainly affects the degree of permeability damage. The optimum particle size median ratio D50/d50 is about 25, and the medium has strong antipermeability damage ability and good sand retaining effect.

Sequential Formation of CO2 Hydrates in a Confined Environment: Description of Phase Equilibrium Boundary, Gas Consumption, Formation Rate and Memory Effect

Since 1980, one of the most promising solutions for the exploitation of natural gas hydrate reservoirs was found to be the replacement of methane with carbon dioxide in order to improve the efficiency of methane recovery and, at the same time, permanently store carbon dioxide. However, the process efficiency is still too low and far from reaching technical maturity and becoming economically competitive. In this sense, studying the intrinsic properties of CO2 hydrates formation and dissociation processes may help in better defining the reasons for this low efficiency and finding feasible solutions. This work deals with carbon dioxide hydrates formation in a natural silica-based porous medium and in fresh water. A lab-scale apparatus was used for experiments, which were carried out consecutively and with the same gas–water mixture in order to detect the possible occurrence of the “memory effect”. Six tests were carried out: the quantity of gas available for the formation of hydrates led to an initial pressure equal to 39.4 bar within the reactor (the initial pressure was 46 bar; however, the dissolution of CO2 in water during the first test caused a reduction in the quantity of gas available for the process). Each experiment started and ended at temperatures equal or higher than 20 °C. Considering the local pressures, these temperatures ensured the complete dissociation of hydrates. Besides thermodynamic parameters, the gas consumption and the rate constant were evaluated throughout the whole of the experiments. Conversely to what is asserted in the literature, the results demonstrated the weak persistence of the memory effect at a temperature slightly above 25 °C. As expected, ice formation competed with hydrates; however, during tests, it caused the partial release of carbon dioxide previously trapped into hydrates or dissolved in water. Finally, the rate constant completely agreed with the labile Cluster Theory and proved that primordial clusters and hydrate crystals formed and dissociated during the whole test. The first phenomenon was predominant during the formation phase, while the opposite occurred during the following step. The rate constant was found to be an effective parameter to quantify differences between measured and real equilibrium conditions for the system.

Sand Migration Simulation during Gas Production from Gas Hydrate Reservoir at Kuparuk 7–11–12 site in the Prodhoe Bay Unit, Alaska

Sand Migration Simulation during Gas Production from Gas Hydrate Reservoir at Kuparuk 7–11–12 site in the Prodhoe Bay Unit, Alaska

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-bearing sediment of the South China Sea: Microstructure and mechanical characteristics

The successful offshore hydrate production tests in the South China Sea proved the high productivity of clayey-silty hydrate reservoirs. However, the mechanical characteristics of these clayey-silty sediments have not been comprehensively investigated since the concerns about the productivity of such low-permeability reservoirs. In this study, based on the sediment matrix from the South China Sea, specimens were remolded with a porosity of 41% and hydrate saturation of 30% to perform a comprehensive mechanical investigation from micro to macro. Microcosmically, the hydrate in the clayey-silty sediment presents patchy distribution, being particle-displacive and particle-wrapped segregated hydrate morphology. Macroscopically, the compression and swelling indices of hydrate-bearing sediments in the South China Sea are obtained for the first time, which are larger than that of the hydrate reservoir in Krishna-Godavari Basin. The increasing effective confining pressure can slightly reduce the strain-softening and dilation characteristics of the hydrate-bearing sediments in the South China Sea. What's more, the failure strength and stiffness increase with an increasing effective confining pressure, while the failure volumetric strain firstly increases and subsequently decreases with an increasing effective confining pressure. These results are essential input parameters to establish a constitutive model and numerical simulations.