Presence Of Methane Hydrate Research Articles

Isolation and Characterization of Ureolytic Calcifying Bacteria from Methane Hydrate-Bearing Marine Sediments for Bio-cementation Application

In bio-calcification, microbes precipitate calcium carbonate (CaCO3), forming versatile solid substances that promotes eco-friendly materials and reduce carbon emissions. Marine bacteria can generate bio-cements to strengthen dikes and combat coastal erosion. However, the role of marine bacteria in generating bio-cements for enhancing coastal structures and combating erosion is not fully understood. This study investigates the potential of CaCO₃ precipitating bacteria isolated from methane hydrate-bearing marine sediments. Five calcifying marine bacteria were isolated using Christensen's urea agar from marine sediments collected from Gawadar coastal, Pakistan. Bacterial strains induced CaCO3 precipitation producing urease enzymes. Strains were identified as Pseudomonas putida, Bacillus altitudinis, Vibrio sp., Bacillus sp., and Vibrio plantisponsor. Energy-dispersive X-ray spectroscopy, scanning electron microscopy, and X-ray diffraction were applied for the identification and differentiation of calcite and vaterite precipitates. The growth of isolates and precipitation potential were observed optimum at 5% NaCl and pH 9.5-11. Bacillus altitudinis (ST4SD3) and Bacillus sp. (ST4SD1) produced more soluble Ca2+ (8532.53 mg/l and 7581.98 mg/l) as compare to other isolates at higher pH 10 and pH 11, favorable for CaCO3 precipitation. It is concluded that marine ureolytic bacteria possess significant potential for bio-cementation, which can stabilize methane hydrate-bearing sediments, improve soil properties, protect coastal regions from erosion, and crucial in the methane cycle, a greenhouse gas. We recommend further exploration of such bacteria's applications in marine construction and sediment stabilization to enhance the robustness and longevity of coastal infrastructures. Furthermore, such bacteria could also be beneficial in extracting gas from unconsolidated methane hydrates containing sediments.

Temperature and Pore Pressure Dependencies of the Mechanical Behavior of Methane Hydrate–Bearing Fine-Grained Sediment

Temperature and Pore Pressure Dependencies of the Mechanical Behavior of Methane Hydrate–Bearing Fine-Grained Sediment

An Elastoplastic Constitutive Model Considering Bonding Effects for Methane Hydrate-Bearing Sand

An Elastoplastic Constitutive Model Considering Bonding Effects for Methane Hydrate-Bearing Sand

Effect of Hydrate Saturation on Triaxial Shear Mechanical Properties of Methane Hydrate-Bearing Sediment

Abstract As a clean energy source, natural gas hydrate resources are essential to the development of energy in the twenty-first century. However, the degree of consolidation of the hydrate reservoir is low, and complex accidents such as wellbore instability, reservoir collapse, and gas production leakage will be caused if improperly exploited. The laboratory employs a self-developed triaxial shear test equipment that operates under high pressure and low temperature conditions. This equipment is specifically designed to determine the mechanical parameters and strength characteristics of methane hydrate bearing sediment (MHBS) samples. It is used to create water-saturated hydrate deposits with varying levels of saturation. The synthesis of MHBS samples with varying saturations is achieved by precisely regulating the amount of water. The saturation of MHBS is then determined and confirmed by the utilization of the “water content method” and the “gas collection method.” Triaxial shear tests are conducted on MHBS to examine the impact of hydrate saturation on the mechanical parameters and strength behaviours of MHBS. The experimental findings demonstrate that the presence of hydrate crystals enhances the robustness of the depositing medium. The stress-strain curves of MHBS have a hyperbolic shape, indicating strain hardening features. The stress-strain curves may be categorized into three distinct stages: the initial elastic deformation, followed by a slow shift to the initial yield deformation, and ultimately leading to strain hardening. As axial strain rises, the behaviour of the MHBS transitions progressively from elastic deformation to plastic deformation. The shear strength and initial yield strength of MHBS enhance with higher hydrate saturation. The reason for this is that as the hydrate saturation rises, the influence of hydrate crystals on the enhancement of sediment strength becomes more pronounced. However, the initial yield strain of each MHBS sample is between 0.5% and 1.5%, and the difference is not significant. When the hydrate saturation rises gradually, the cohesion of MHBS enhances. Nevertheless, the hydrate saturation has a very minor impact on the internal friction angle.

Mechanical Characteristics of Underconsolidated Methane Hydrate-Bearing Clayed-Silty Sediments

Natural gas hydrate reserves in the South China Sea are huge, and their commercial prospects have been proved by repeated trial production. It is essential to systematically clarify the mechanical properties of hydrate-bearing sediments for the safe drilling of hydrate reservoirs. To date, most research studies focus on the undrained mechanical behavior of hydrate-bearing sandy or clayed sediments under normal consolidation conditions, but there are few reports on underconsolidated-undrained triaxial shear experiments. In this study, the present research conducts undrained triaxial shear tests on hydrate-bearing clayed-silty sediments. The results show that underconsolidated hydrate-bearing sediments appear to show a strain-hardening phenomenon, which are different from hydrate-bearing sands in Nankai Trough. The excess pore water pressure of hydrate-bearing sediments remains positive during shear. The increasing degree of consolidation will increase the failure strength and the Secant Young’s modulus E50. According to the Mohr–Coulomb criterion, the corresponding predicting formulas are established, and the cohesion increases with the increase in the degree of consolidation, while the internal friction angle is not sensitive to the degree of consolidation. In addition, the failure strength and the Secant Young’s modulus E50 increase with the increase in initial effective confining stress before shearing.

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.

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.

Methane migration, sea ice melting and spatial distribution of chlorophyll-a in the Barents Sea

The paper considers the theoretical and practical aspects of the relationship between the processes of sea ice melting and methane migration, and their influence on the distribution of chlorophyll-a concentration as an indicator of productivity and the level of quantitative development of the phytoplankton community at the spring stage of the succession cycle. In the spring of 2018 and 2019 the thermohaline characteristics of waters and the content of chlorophyll-a in the Barents Sea were studied. The area of work covered both areas with the most probable release of methane from near-bottom gas hydrates, as well as background areas, where the presence of methane hydrates is unlikely. In 2018, the phytoplankton community of the study area was in the spring flowering stage. In the area of the most probable influence of methane, the average concentrations of chlorophyll-a were approximately 2 times higher than in the background area. High concentrations of the pigment were associated with the thawed layer, which may indicate the release of methane from the ice. In 2019, low concentrations of chlorophyll-a in the edge zone excluded the stage of spring flowering of the phytoplankton community, which did not allow us to trace the possible effect of the release of methane hydrates on the concentration of chlorophyll-a.

Influence of Temperature and Pore Pressure on Geomechanical Behavior of Methane Hydrate-Bearing Sand

Geomechanical behavior of methane hydrate-bearing sediment (MHBS) plays a major role in evaluation of the stability of a hydrate reservoir. A series of triaxial compression tests were conducted on MHBS to study the influence of temperature and pore pressure conditions on its mechanical behaviors. The experimental results show that temperature and pore pressure have a significant effect on the stress–strain curve, stiffness, and strength of MHBS. As temperature decreases and/or pore pressure increases, the stress–strain curve manifests an enhanced strain-softening characteristic, stiffness, and strength. Furthermore, MHBS cohesion also tends to exhibit a significant increase, but its internal friction angle almost remains constant with decreasing temperature and/or increasing pore pressure. These findings imply that the change in temperature and pore pressure affects the strength of MHBS, which occurs predominately due to change in its cohesiveness. To describe these impacts, a phase state parameter is introduced to characterize the temperature and pore pressure conditions. Meanwhile, three empirical formulas for relating the secant modulus, strength and cohesiveness to phase state parameter are presented. Good agreement between simulation and measured data indicates that the phase state parameter can effectively describe temperature and pore pressure conditions. The proposed empirical formulas are able to address the influences of temperature and pore pressure conditions on geomechanical characteristics of MHBS.

Modeling the Mechanical Behavior of Methane Hydrate-Bearing Soil Considering the Influences of Temperature and Pore Pressure

Temperature and pore pressure have a significant impact on the mechanical behavior of methane hydrate-bearing soil (MHBS). To characterize the mechanical responses of MHBS under different temperatures and pore pressures, an elastoplastic constitutive model is developed within the framework of the critical-state theory. In the proposed model, an additional variable, called the phase state parameter, is introduced to characterize the temperature and pore pressure condition. Meanwhile, the elastic parameter, plastic modulus, and tensile bonding stress are related to the phase state parameter through several empirical formulas. As a consequence, the influences of temperature and pore pressure on the stiffness, strength, and dilatancy of MHBS are addressed by the proposed constitutive model. Through comparing the predicted results and experimental data, the proposed model is demonstrated to have the ability to capture the influences of hydrate saturation, temperature, and pore pressure on the mechanical behavior of MHBS. In addition, the proposed model is used to analyze the response of MHBS to an increase in temperature or a decrease in pore pressure within the hydrate stability boundary. The result shows that the proposed model can reasonably reflect the mechanical behavior of MHBS induced by heating or depressurization procedures within the hydrate stability boundary.

Experimental Study on the Distribution Characteristics of CO2 in Methane Hydrate-Bearing Sediment during CH4/CO2 Replacement

CH4/CO2 replacement is of great significance for the exploitation of natural gas hydrate resources and CO2 storage. The feasibility of this method relies on our understanding of the CH4/CO2 replacement efficiency and mechanism. In this study, CH4/CO2 replacement experiments were carried out to study the distribution characteristics of CH4 and CO2 in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH4/CO2 replacement process could be divided into two stages: fast reaction and slow reaction, representing CH4/CO2 replacement in the hydrate-gas interface and bidirectional CH4/CO2 diffusion caused replacement, respectively. After replacement, the CO2 content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO2 consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO2 hydrate by gaseous CO2 and water almost runs through the whole experimental process. Thus, the pore scale CH4/CO2 replacement process in sediments can be summarized in the following steps: CO2 injection, CO2 diffusing into sedimentary layer, occurrence of CH4/CO2 replacement and CO2 hydrate formation, wrapping of methane hydrate by mixed CH4-CO2 hydrate, continuous CO2 hydrate formation, and almost stagnant CH4/CO2 replacement.

Application of Methane Hydrate Critical State Soil Model on Multistage Triaxial Tests of Methane Hydrate-Bearing Sediment

Natural methane hydrate, as a potential alternative energy source to fossil energy in the 21st century, is found in abundance in deep-water sediments and permafrost regions. During the production of methane gas from these sediments, the dissociation process may induce various changes to the geotechnical properties. Thus, it is important to study the geomechanical behavior of hydrate sediments and simulate the sediment deformation patterns. In this study, the effects of different sediment types on the mechanical properties of hydrate-bearing soils were investigated using multistage triaxial tests, and the methane hydrate critical state (MHCS) soil model was calibrated using an optimization-based technique. The results revealed that: (1) the strain-hardening phenomenon in methane hydrate-bearing sediments varies with the particle sizes of the host sediments; (2) the strengths of methane hydrate-bearing sediments are higher with the host sediments being pure sand, compared with those with clay-added sediments, and the strength is inversely proportional to the clay contents in these sediments; and (3) the multistage triaxial test data were fitted with the MHCS model, which illustrates the combined effects of the abovementioned factors on the model parameters and geomechanical behavior of methane hydrate-bearing sediments.

Mechanical Behavior of Methane–Hydrate–Bearing Sand with Nonlinear Constitutive Model

Mechanical Behavior of Methane–Hydrate–Bearing Sand with Nonlinear Constitutive Model

Experimental measurement and clustered equal diameter particle model of permeability with methane hydrate in glass beads

Permeability is a key parameter for gas recovery from marine hydrate reservoirs. In this study, two kinds of BZ02 and BZ04 glass beads provided skeleton structure for methane hydrate synthesis in pressure vessel. Based on the observation of Scanning Electron Microscopy (SEM), the glass beads in the pressure vessel were considered to be equal diameter spheres and stacked in the form of orthorhombic. The water effective permeability KW of glass beads with different hydrate saturations (SH) were carried out by steady-state water injection method. Experimental results indicated that the presence of methane hydrate could cause significant decrease of KW, and the gas produced by hydrate dissociation would reduce the resistance of fluid flow in pores. The permeability ratio in the presence and absence of hydrate KrW was exponentially decreasing with the SH. A clustered equal diameter particle (CEDP) model was proposed that methane hydrate was clustered as equal diameter spheres and occupied in the center of pore space. The internal surface area of pore space with and without hydrate was simplified and the KrW-SH relationship was deduced in detail. When the saturation exponent n of BZ02 and BZ04 were recommended to be 10 and 18, respectively, the KrW in the CEDP model was fitted well with the experimental results. This model also revealed that the radius of glass beads and hydrate particles had a stronger effect on the KrW.

Experimental Investigation of the Mechanical Properties of Methane Hydrate–Bearing Sediments under High Effective Confining Pressure

A significant increase in effective stress can be induced in hydrate-bearing reservoirs when the depressurization method is applied. A series of drained triaxial shear tests were performed on hydrate-bearing sediments with various hydrate saturations to investigate their mechanical characteristics under effective confining pressures of up to 20 MPa. The results show that significant particle crushing of the host sand occurs during shearing under high pressures, and there is no remarkable effect of hydrate saturation on the degree of particle breakage. As the effective confining pressure increases, the stress–strain curves of the hydrate-bearing specimen transformed from strain-softening to strain-hardening. The peak stress ratio and internal friction angle of the sediments gradually decrease and tend to be constant with the increased confining stress, whereas the cohesion in hydrate-bearing sediments exhibits an increasing tendency. The critical state line (CSL) of hydrate-bearing sediments in the e-lnp′ space under low-to-high effective confining pressures intersects with the normal consolidation line under the same hydrate saturation. Furthermore, the CSL moves upward and rotates clockwise as the hydrate saturation increases.

Experimental Study on the Difference of Fluid Flow between Methane Hydrate-Bearing Sand and Clay Sediments

Experimental Study on the Difference of Fluid Flow between Methane Hydrate-Bearing Sand and Clay Sediments

Stress dependence of the gas permeability of montmorillonite sediments in the presence of methane hydrate

The evolution of porosity and permeability of silty clay sediments in the South China Sea has significant effect on gas hydrate production due to the stress effect of submarine overburden rocks. The gas permeability of silty clay specimens under different effective stresses was measured in a cylindrical stainless steel reaction vessel. The experimental results in this paper show that application of effective stress destroys gas permeability of methane hydrate specimens. The power function relation between permeability and effective stress and exponential function relation between porosity and effective stress are given. The experimental data show that the formation of methane hydrate improves the compressible channel of montmorillonite deposits. The permeability damage rate increases with increase of effective stress, and stress sensitivity coefficient increases first and then decreases with increase of effective stress. Finally, an effective permeability equation of sediment containing hydrate saturation and effective stress parameters is established. The results are of guiding significance for the study of the stress dependence of porosity and permeability of gas hydrate silty clay reservoirs in the South China Sea. • Power law is superior to exponential law for showing permeability stress dependency. • Hydrate formation improves the compressible space of montmorillonite sediments. • Permeability damage rate increase with increasing effective stress in sediments. • Stress sensitivity coefficient show a trend of decreasing first and then increasing. • Relational expression between effective stress and permeability is proposed.

Visualization of materials below shallow subseafloor sediment via Acoustic Mapping, an Example from Shallow Methane Hydrate-bearing Areas along the eastern margin of the Sea of Japan

Visualization of materials below shallow subseafloor sediment via Acoustic Mapping, an Example from Shallow Methane Hydrate-bearing Areas along the eastern margin of the Sea of Japan

Numerical Investigation of the Gas Production Efficiency and Induced Geomechanical Responses in Marine Methane Hydrate-Bearing Sediments Exploited by Depressurization through Hydraulic Fractures

Numerical Investigation of the Gas Production Efficiency and Induced Geomechanical Responses in Marine Methane Hydrate-Bearing Sediments Exploited by Depressurization through Hydraulic Fractures

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization