Methane Hydrate-bearing Sediments Research Articles

Effect of initial pore water content and salinity on the resistivity of methane hydrate-bearing fine sediments

Methane hydrates are widely distributed in the soft silty-clayey soils of continental margin sediments. The resistivity method is generally used to detect their distribution state and monitor their kinetic reaction process. However, there are some major details to be elaborated on as to the occurrence state and the evolution of key characteristic parameters. Based on the kinetic reaction experiment under different lithological conditions (initial water content and pore water salinity), this research records the changing resistance of natural sand porous medium system during the cooling formation of hydrates by in-situ measurement. The Archie formula is further used to study the influencing factors of the resistance of hydrate-bearing sediments and how the resistance changes during hydrate formation. The results demonstrate that these two lithological parameters can significantly affect the resistivity of the hydrate-bearing natural sand porous media system. It is found that within a certain range, NaCl in pore water will inhibit the formation reaction of hydrates, while an increase of initial water content will enhance the hydrate production amount in the system. With the hydrate formation in the pores, the porosity and skeleton composition of the natural sand porous medium change as well, coupled with the salt-removing effect, which further changes the system resistivity. During hydrate formation, the system resistivity changes regularly in the cooling stage, hydrate nucleation stage, and hydrate formation stage. Our study systematically considers the influence of these geological parameters on resistivity characterization at different reaction stages during the formation of natural hydrates. Thus, it can serve as a technical reference for hydrate reservoir logging and other related issues.

Characterization of methane hydrate extraction influenced by hydraulic fractures using a coupled thermo-hydro-mechanical-chemical model

The low permeability of the methane hydrate-bearing sediment limits the methane gas extraction. To enhance methane hydrate extraction, hydraulic fracturing can be a promising approach to improve the hydrate reservoir permeability by creating a fracture network in the reservoir. In this study, a coupled thermo-hydro-mechanical-chemical mathematical model and its numerical implementation based on finite element technology are introduced to analyze the methane hydrate extraction through fractured methane hydrate-bearing sediment considering methane hydrates dissociation, gas-water two-phase flow, heat transfer, dynamic changes of the sediment permeability, and deformation of both sediment matrix and fractures as well as capturing the interplay between them. The coupled thermo-hydro-mechanical-chemical numerical model is verified by reproducing a methane hydrates dissociation laboratory test. Finally, we conduct a series of simulations for the methane gas depressurization extraction through the sediments with the DFNs assigned as diverse geometrical characteristics. The influence of hydraulic fracture network geometrical and hydraulic characteristics on methane hydrate extraction are discussed. The results can offer a reference for enhancing the methane hydrate extraction efficiency.

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.

Mechanical behaviour and microstructure of methane hydrate-bearing sandy sediment observed at various spatial scales

Methane hydrates (MHs) are considered an alternative energy resource but also a potential source of geo-hazards and climate change. The physical/mechanical properties of gas hydrate-bearing sandy sediments are strongly dependent on the distribution of hydrates within the pore space. The purpose of this study is to investigate morphologies and pore-habits of MHs formed in sandy sediments by means of experiments that probe a wide range of scales, from the pore scale – using Synchrotron X-Ray Computed Tomography (SXRCT) and optical microscopy – to the core scale, through mechanical property measurements. The same synthetic sands are used, in which MHs are generated successively under excess gas and excess water conditions. At the macroscopic (core) scale, MH pore habits are inferred by comparing the measured sonic wave velocities to velocities calculated from rock physics models and further assessed via triaxial compression tests. Furthermore, Magnetic Resonance Imaging is used to investigate the kinetics of MH formation and distribution along the core height. The pore habits and MH morphologies are directly visualized at the pore (grain) scale by SXRCT and, with still better spatial and temporal resolution, by transmission optical microscopy, revealing some more complex morphologies than in the hydrate pore habits commonly admitted.

Multi-field coupling analysis of mechanical responses in methane hydrate exploitation with a practical numerical approach combining T + H with DEM

The methane hydrate (MH) is widely distributed in deep-sea sedimentary layers, considered as the most promising clean energy to replace conventional oil and gas in the 21st century. Large-scale exploitation of MH may lead to a series of geotechnical engineering problems. To numerically estimate the seabed stability during MH exploitation which should involve multi-field coupling simulation of complex mechanical behaviors of methane hydrate bearing sediment (MHBS), a new practical hybrid numerical method is proposed, which combines the TOUGH + HYDRATE (T + H) code, targeting multiphase flow analysis of hydrate-bearing geologic systems, with the improved Distinct Element Method (DEM). Porosity, temperature, pressure, salinity fields are exchanged between DEM and T + H to realize the thermal-hydro-mechanical-chemical (THMC) coupling and to reproduce the progressive discontinuous failure process. The results of verification show obvious uniformity between numerical and analytical results for one-dimensional (1D) consolidation and 1D heat conduction tests on soils. The paper tries to analyze evolutions of THMC fields, gas production, hydrate bond breakage and other mechanical behaviors of actual depressurization production by using this method, focusing on geotechnical risks of exploitation. In addition, study observes significant changes within certain range from the wellbore for gas production and mechanical responses in the early stage.

Experimental study of gas recovery behaviors from methane hydrate-bearing sediments by CO2 replacement below freezing point

Natural gas hydrate (NGH) is one of the most promising clean energy sources in the future because of its huge reserves and cleanliness, and the CO2 replacement method can realize the safe extraction of NGH and the underground storage of greenhouse gases. This study investigates the influence of quartz sand size and ice content on hydrate replacement characteristics below freezing point conditions. Results indicate that, at a temperature of 271.65 K, when particle sizes range from 250 μm to 500 μm, there is an observed increase in CH4 recovery alongside a concurrent decrease in CO2 sequestration as particle size decreases. Additionally, when maintaining a fixed 500 μm particle size, the proportion of quartz sand positively correlates with CH4 recovery. Furthermore, the presence of quartz sand grains in an ice powder system can improve the replacement effect. By contrast, the presence of quartz sand significantly enhances the CO2 sequestration ratio more than the CH4 recovery ratio. In mixed ice-sand porous media, CO2 sequestration can be enhanced from 0.0496 mol to 0.1012 mol, representing a 104 % increase in CO2 sequestration molarity when compared to the pure ice conditions.

A novel semi-analytical approach for modeling multiphysics coupling throughout the entire exploitation process in methane hydrate-bearing sediments

Methane hydrates are attracting attention as a promising energy source. However, accurately simulating the multiphysics dynamics during the entire exploitation process remains a significant challenge. In this study, the complete process of drilling, casing and production was examined, and an efficient semi-analytical model was proposed to couple the seepage, temperature, hydrate dissociation, and mechanical field around the vertical production well. The model integrated permeability, heat transfer and mechanical parameters that are dependent on hydrate saturation. Additionally, factors such as gas and water production, heat absorption due to hydrate dissociation, and mechanical response of the reservoir under loading and unloading stress paths were also considered. The accuracy and practicality of the model were validated by comparing computational results for depressurization-based exploitation with those generated by the researcher's own numerical model and various numerical simulators developed by other researchers. The proposed model was also found to be computationally efficient. The model was used to simulate a range of depressurization strategies and depressurization combined heat injection schemes, and recommendations were made based on the outcomes of these simulations to enhance the safety and efficiency of hydrate exploitation.

Synergistic CH4 hydrate recovery and CO2 storage by coupling depressurization with CO2/N2 injection: A pilot-scale investigation

To ensure sustainable energy supply while mitigating climate change, combining CH4 recovery with CO2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon–neutral technology. In this study, a pilot-scale reactor (V = 22 L) was used to study the combined processes of CH4 recovery from the methane hydrate-bearing sediment by using depressurization followed by in-situ CO2/N2 injection to form mixed CH4/CO2/N2 hydrate (Mix-H) at marine NGH reservoir conditions (T = 283.8 K and P = 11.0 MPa). The bottom hole pressure (BHP = 3.2 MPa, 5.3 MPa, and 6.8 MPa) during depressurization and the setting of horizontal wellbore (HW), were systematically investigated to optimize the synergistic CH4 recovery and CO2 storage. The CH4 recovery ratio (55.4–80.6%) and the CH4 production rate (5.15–134.98 mol/d) increased as BHP decreased. After CO2/N2 injection, Mix-H formed first in the CH4-rich region in the upper section of the reactor and propagated downward to the lower. The Mix-H formation kinetics were significantly enhanced with an increase in BHP, leading to a decrease in CO2 composition in the Mix-H. The use of a horizontal wellbore (HW) facilitates a more even distribution of the CO2/N2 within sediments, resulting in a 13.5% increase in CO2 composition of Mix-H compared with no HW setting. The CO2 conversion ratio and hydrate restoration ratio reaches 52.8% and 69.4% at BHP = 5.3 MPa with HW, respectively. The technical feasibility of CO2 storage in exploited marine NGH reservoirs was first proved at pilot-scale. The results provided insights into the synergistic method of NGH exploitation and CO2 storage at the marine NGH reservoirs.

A bounding surface plasticity model for methane hydrate-bearing sediments in deep seabed

This study proposes a new constitutive model to simulate the mechanical behavior of methane hydrate-bearing sediments, by modifying the SANISAND-C model originally designed for sands with low cement content. To capture the distinctive cementation effect of methane hydrate (MH), modifications are made to the state parameter and bonding degradation law of the SANISAND-C model. In addition, the initial bonding strength of MH is not only determined by MH saturation but also by a condition parameter that depends on temperature, pore pressure, and salinity. The densification effect caused by MH pore-filling and grain-coating is captured by replacing the original void ratio in the elastic and plastic moduli of the SANISAND-C model with a reduced void ratio. The model is verified through a comparison of its predictions not only with experimental data but also through discrete element method simulations that cover a wide range of loading paths including drained and undrained true triaxial tests. The model successfully captures the stiffness, strength and dilatancy enhancements induced by MH, as well as the temperature, pore pressure, and salinity-dependent behavior of sediment. Additionally, the model developed can account for different hydrate occurrence habits by adjusting a single parameter related to bonding strength.

Three-dimensional DEM investigation of mechanical behaviors of grain-cementing type methane hydrate-bearing sediment

Three-dimensional DEM investigation of mechanical behaviors of grain-cementing type methane hydrate-bearing sediment

Influence of matrix composition on the mechanical behavior of methane hydrate-bearing clayey-sandy sediments: Insights from fine and clay contents

It is extremely necessary to fully understand the mechanical properties of hydrate-bearing sediments before the commercialization of hydrate. However, few studies have explored the effect of matrix composition on mechanical behavior. In this paper, seabed clay recovered from the Shenhu Sea in the hydrate deposit area of the South China Sea and quartz sand were used as the matrix skeleton to remold hydrate-bearing clayey-sandy sediments (HBCSS). A series of triaxial tests were then carried out to investigate the effect of matrix composition (clay and fine contents) on the strength behavior of HBCSS. The results show that the fine can fill the pores and thus increase the density, which in turn enhances the strength and stiffness of HBCSS and makes the stress-strain relationship more easily softened. The lubrication of clay can significantly change the contact relationship between particles, induce strain hardening and reduce stiffness. Cohesion and internal friction angle are positively and negatively correlated with clay content, respectively. A new concept of the hardening index was proposed, which has a better quantitative description of the hardening or softening of the stress-strain relationship. The strength behavior of HBCSS is the result of the coupling of various factors.

Failure mode and the mechanism of methane hydrate-bearing clayey sand sediments under depressurization

Failure mode prediction of hydrate-bearing sediments (HBSs) is crucial for accurate evaluation of reservoir mechanical properties and safe exploitation of hydrate resources. In this study, the variation of HBS failure mode with hydrate dissociation degree (Dhd) and effective confining stress (σ3′) and its micro-mechanism were investigated. The results show that the stress-strain curve of HBS shifts from strain-softening to strain-hardening as Dhd and σ3′ increase. The failure strength and residual strength of the sample are negatively correlated with Dhd but positively correlated with σ3′. The increase of Dhd results in a reduction in the cohesion of HBS from 608.74 to 2.49 kPa and a shift in the internal friction angle from increasing to decreasing. The post-shear sample indicates a minimum porosity of 11.47% under the highest Dhd and σ3′ conditions. The increased pore volume available for particle rearrangement resulting from the increase in Dhd, and the enhanced confining effect on the sample due to the increase in σ3′, result in a volume shrinkage characterized by plastic strain-hardening failure mode. A prediction model of HBS failure mode based on the classification of strain-hardening and strain-softening degrees of stress-strain curves was developed and verified using published results.

Dual-gas co-production behavior for hydrate-bearing coarse sediment with underlying gas via depressurization under constrained conditions

Hydrate reservoirs with underlying gas show significant potential for commercial exploitation. An experimental apparatus capable of applying confining pressure and axial pressure was built to simulate the accumulation and dissociation of naturally occurring marine hydrate and a novel method for synthesizing hydrate reservoirs with underlying gas was proposed. With the formed methane hydrate-bearing coarse sediment (HBCS) with underlying gas, hydrate dissociation experiments via depressurization under constrained conditions were carried out at different HBCS proportions. The characteristics of heat transfer and dual-gas co-production during the exploitation of hydrate reservoir at various HBCS proportions were presented. Due to the influence of hydrate decomposition, the diverging temperature variations of the underlying layer and upper HBCS layer were observed at HBCS proportions of 0.21, 0.48, and 0.77. The negative effect of the outside pressure on gas production was not observed in our experimental conditions owing to the adopted coarse quartz sand deposits. The final produced gas volume increases with the HBCS proportion increasing. The produced gas percentage is negative or positive related to the HBCS proportions respectively with the extended production time. In addition, the different production behavior of the underlying gas and hydrate decomposed gas may induce the unexpected exploitation accidents such as blowouts resulting from unstable gas production rate. These findings guide the design of exploitation scheme for reservoirs with different HBCS distribution. And an exploitation method by setting the production well in the underlying gas layer/underlying sediment layer with free gas is then proposed, which can inhibit hydrate reformation and ice generation and improve the production well stability during the hydrate exploitation process.

Application and improvement of a stress partition framework-based methane hydrate-bearing sediment constitutive model for wide range confining stress

A constitutive model of methane hydrate-bearing sediment (MHBS) was recently proposed based on a stress partition framework (SPF-MHBS). To capture the mechanical behavior of MHBS under a wide range of effective confining pressures, this study introduces the equivalent granular void ratio (e*) and the particle breakage index (Br) associated with critical state variables into the original SPF-MHBS model. A comprehensive set of triaxial tests on MHBS under 0.2–20 MPa effective confining stress were simulated. The simulation results indicate that based on appropriate parameter calibration, the response of specimens under low effective confining stress can be acceptably represented using the original SPF-MHBS model. However, at high confining stress, the simulated strength of pure sand was overestimated, while that of the hydrate-bearing specimen was and underestimated. Dilatancy was also significantly overestimated at high confining stress. After introducing the e* into the original model, the performance under relatively low pressure is improved and three model parameters are reduced from the original model. After introduction of Br, the model performance is further improved at high confining stress. The improvements allow the SPF-MHBS model to provide a unified description of MHBS behavior under a wide range of confining stress.

A Fully Coupled Thermo-Hydro-Mechanical-Chemical Model for Methane Hydrate Bearing Sediments Considering the Effect of Ice

The ice generation is one of the challenges facing the methane hydrate depressurization, which, however, has not been fully addressed by existing numerical models for hydrate-bearing sediments (HBS). In this study, we develop a high-fidelity, fully coupled thermo-hydro-mechanical-chemical numerical model that incorporates the effect of ice. The model, developed using COMSOL, takes into account water–ice phase change, thermally induced cryogenic suction and constitutive relation in HBS. It is verified well against the temperature, pressure and cumulative gas production of Masuda’s experiment. The model is then employed to investigate multiphysical responses and gas/water production when ice generation is induced by setting a low outlet pressure. The results reveal that ice forms near the outlet boundary of the specimen center, leading to a reduction in intrinsic permeability and fluid velocity and an increase in the bulk modulus of ice-HBS. This enhanced bulk modulus results in higher porosity under axial load. Although the exothermic effect of ice generation promotes the hydrate dissociation, the effect on cumulative gas production is negligible after the ice melts. A negative correlation between ice saturation and water production rate is observed, indicating that a higher gas–water ratio can be achieved by adjusting the ice duration during hydrate production. The developed coupled model proves to be crucial for understanding the effect of ice on hydrate exploitation.

Integrating test device and method for creep failure and ultrasonic response of methane hydrate-bearing sediments.

Clarifying the creep behaviors of hydrate-bearing sediment (HBS) under long-term loading is crucial for evaluating reservoir stability during hydrate exploitation. Figuring out a way of characterizing deformation behaviors and their geophysical responses to HBS is the basis for modeling creep behaviors. In this study, we propose a novel device to test time-dependent deformation and the ultrasonic response of HBS under high-pressure and low-temperature. The experimental device consists of a high-pressure chamber, an axial-load control system, a confining pressure system, a pore pressure system, a back-pressure system, and a data collection system. This testing assembly allows temperature regulation and independent control of four pressures, e.g., confining pressure, pore pressure, back pressure, and axial loading. Columned artificial HBS samples, with a diameter of 39mm and a height of 120mm, can be synthesized in this device. Afterward, in situ creep experiments can be achieved by applying stable confining pressure and axial load, together with geophysical signals acquisition. During loading, the stress-strain relationships and ultrasonic data can be obtained simultaneously. Through analyzing the stress-strain relationship and ultrasonic data, the macroscopical failure and microcosmical creep deformation law of the samples can be figured out. Preliminary experiments verified the applicability of the device. The method provides some significance for field observation of reservoir failure via geophysical techniques during hydrate exploitation.

Experimental investigation on the permeability characteristics of methane hydrate-bearing clayey-silty sediments considering various factors

Permeability is a key physical parameter of hydrate-bearing sediments (HBS), which is an important basis for evaluating reservoirs and developing exploitation plans. However, few permeability studies of HBS conducted so far have involved effective stress and seepage pressure difference, which is inconsistent with the stress-seepage coupling process in actual hydrate exploitation. In this study, marine clay taken from the South China Sea and quartz sand were used to synthesize hydrate-bearing clayey-silty sediments (HBCSS). A series of water permeability tests were conducted to investigate the effects of effective stress, hydrate saturation and seepage pressure difference on the permeability of HBCSS. The results show that an increase in effective stress can cause a significant decrease in permeability, and this effect is particularly pronounced at the beginning of stress increase. The decreasing effective stress can only cause a slight rebound in permeability, as only part of the elastic strain can be recovered. The negative exponential relationship between hydrate saturation and permeability is controlled by the blockage mechanism of hydrate particles. The change in effective stress is the essence of the effect of seepage pressure difference on permeability, and this effect is attenuated with increasing hydrate cementation. Based on the experimental results, permeability models considering the coupling effect of effective stress and hydrate saturation are proposed.

Influencing Factors of Hydration Number of Synthesized Methane Hydrates in Sediment

The low-field NMR method is introduced to explore the effect of various experimental conditions on hydration number during hydrate dissociation in methane hydrate-bearing sediments. It is shown that the NMR signal peak value of water is independent of the dry density of sediment, gas pressure, and salt concentration and significantly influenced by temperature. A relationship among NMR signal peak value, substance content, pressure, and temperature is developed to estimate the hydration number of pore hydrates during hydrate dissociation. It is revealed that the hydration number (Nh) of synthesized methane hydrates remains practically unchanged during hydrate dissociation in fine-grained sediments and is trivially influenced by experimental conditions, including initial pressure, density, initial water saturation, and salt content. Our experimental results indicated that the hydration number of methane hydrates is about 6.5, which is slightly higher than that of natural hydrates.

The Failure Mechanism of Methane Hydrate-Bearing Specimen Based on Energy Analysis Using Discrete Element Method

Studying the failure mechanism of methane hydrate specimens (MHSs) is of great significance to the exploitation of methane hydrate. Most previous studies have focused on the macro or micromechanical response of MHS under different conditions. However, there are a few studies that have investigated the mechanical response mechanism of MHS based on energy evolution. Therefore, in this study, a numerical model of the methane hydrate-bearing sediments was constructed in the particle flow code (PFC) environment. Then, the numerical model was validated using the conducted laboratory tests; and a series of numerical tests were conducted under different methane hydrate saturation conditions, and the obtained results were analyzed. These results qualitatively describe the main mechanical properties of the methane hydrate-bearing sediments from the viewpoint of energy evolution. The simulation results indicated that during the shear test, the bond breaks at first. Then, the soil particles (sediments) start to roll and rarely slid before shear strength arrives at the highest value. Around the highest shear strength value, more soil particles begin to roll until they occlude with each other. Strain softening is induced by the combined action of the breakage of the hydrate bond and the slipping of soil particles. The higher the hydrate saturation is, the more obvious the strain softening is. Considering that a good agreement was observed between the numerical simulation results and the laboratory test results, it can be concluded that the numerical simulation approach can complement the existing experimental techniques, and also can further clarify the deformation and failure mechanism of various methane hydrate-bearing sediments. The results obtained from the present study will contribute to a better understanding of the mechanical behavior of the gas hydrate-bearing sediments during hydrate dissociation and gas exploitation.

Strength Behaviors and Constitutive Model of Gas-Saturated Methane Hydrate-Bearing Sediment in Gas-Rich Phase Environment

Natural gas hydrates occupy an important position in the development of clean energy around the world in the 21st century. It is of great significance to research the mechanical properties of methane hydrate-bearing sediment (MHBS). In this paper, gas-saturated MHBS were synthesized based on the self-developed triaxial compressor apparatus. The triaxial shear tests were performed at temperatures of 2 °C, 3 °C, and 5 °C and confining pressures of 7.5 MPa, 10 MPa, and 15 MPa. Results indicate that the axial strain process can be divided into three stages: initial elastic deformation, initial yield deformation, and strain softening. When confining pressure is increased, the shear strength of MHBS increases at a lower confining pressure. In contrast, shear strength appears to decrease with increasing confining pressure at a higher confining pressure. There is a negative correlation between temperature and shear strength of MHBS. The initial yield strain of MHBS increases in condition due to the increase in confining pressure and the decrease in temperature. The change in strength degradation is kept within 2 MPa. Using test data, the Duncan-Chang model was modified to describe the strength behaviors of gas-saturated MHBS. The accuracy of the model was verified by comparing calculated values with test data.