Shear strength of hydrate-bearing sediment is an essential parameter for assessing landslide potential of hydrate reservoirs under exploration conditions. However, the characteristics and simulation of this shear strength under varying dissociation conditions have not been thoroughly investigated. To this end, a series of triaxial compression tests were first carried out on sediments with varying initial hydrate saturations along dissociation pathways. Combining measured data with microscale analysis, the underlying mechanism for the evolution of shear strength in hydrate-bearing sediment was studied under varying partial dissociation pathways. Moreover, a shear strength model for hydrate-bearing sediment was proposed, taking into account the hydrate saturation and the unhydrated water content. Apart from the parameters derived from the hydrate characteristic curve, only one additional model parameter is required. The proposed model was validated using measured data on hydrate sediments. The results indicate that the proposed model can effectively capture the shear strength behavior of hydrate-bearing sediment under varying dissociation paths. Finally, a sensitivity analysis of the model parameters was conducted to characterize the proposed model. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

PurposeA fully coupled multi-physics model is established to simulate thermal, hydraulic, chemical and mechanical (THCM) behaviors of the sediments with the hydrate dissociation.Design/methodology/approachLinear hydrate-soil constitutive model; Mohr-Coulomb yield criterion; Thermal, hydraulic and chemical (THC) model.Findings(1) The linear internal friction angle model proposed in this study can more reasonably simulate the effect of hydrate saturation on the mechanical properties of sediment than the fixed internal friction angle model. (2) The established THCM model can not only accurately simulate temperature variations and gas production efficiency but also reasonably simulate effective stress reduction and volumetric contraction of sediment during the process of hydrate dissociation. (3) The dissociation of hydrate not only decreases the instability coefficient of sediment but also increases the moving distance and equivalent plastic strain of the sediment.Research limitations/implicationsThe hydrate sediment with large deformations should be considered in the future.Practical implicationsThe submarine landslide will be investigated in the future.Originality/value(1) A linear hydrate-soil constitutive model based on the Mohr-Coulomb yield criterion is proposed, which defines the internal friction angle, the elastic modulus and the cohesion as a linear function of hydrate saturation. (2) A fully coupled multi-physics THCM model is established to simulate thermal, hydraulic, chemical and mechanical behaviors of hydrate-bearing sediment.

This study explores the carbon stability in the Arctic permafrost following the sea-level transgression since the Last Glacial Maximum (LGM). The Arctic permafrost stores a significant amount of organic carbon sequestered as frozen particulate organic carbon, solid methane hydrate and free methane gas. Post-LGM sea-level transgression resulted in ocean water, which is up to 20 degrees C warmer compared to the average annual air mass, inundating, and thawing the permafrost. This study develops a one-dimensional multiphase flow, multicomponent transport numerical model and apply it to investigate the coupled thermal, hydraulic, microbial, and chemical processes occurring in the thawing subsea permafrost. Results show that microbial methane is produced and vented to the seawater immediately upon the flooding of the Arctic continental shelves. This microbial methane is generated by the biodegradation of the previously frozen organic carbon. The maximum seabed methane flux is predicted in the shallow water where the sediment has been warmed up, but the remaining amount of organic carbon is still high. It is less likely to cause seabed methane emission by methane hydrate dissociation. Such a situation only happens when there is a very shallow (similar to 200 m depth) intra-permafrost methane hydrate, the occurrence of which is limited. This study provides insights into the limits of methane release from the ongoing flooding of the Arctic permafrost, which is critical to understand the role of the Arctic permafrost in the carbon cycle, ocean chemistry and climate change. Arctic permafrost stores similar to 1,700 billion tons of organic carbon. If just a fraction of that melts, the escaping methane would become one of the world's largest sources of greenhouse gas and would severely impact the environment and the climate. Over the last similar to 18,000 years, a quarter of the stored organic carbon in the Arctic permafrost has been flooded by the rising, warm seas. This has melted the ice and degraded the permafrost. But what happens to the carbon pools? This study investigates the stability of the carbon in the Arctic permafrost following the flooding using a newly developed numerical model. Results show that microbial methane is generated and emitted to the seawater immediately following the flooding. This methane is produced by the biodegradation of the previously frozen organic carbon near the seafloor. The maximum methane emission is predicted in the shallow water near the coast where the sediment has been warmed up, but the remaining amount of organic carbon is still high. This study provides insights into the limits of methane release from the ongoing flooding of the Arctic permafrost, which is critical to understand the role of the Arctic permafrost in the carbon cycle, ocean chemistry and climate change. A numerical model is developed to simulate the coupled thermal, hydraulic, microbial and chemical processes in the thawing subsea permafrost The biodegradation of the ancient organic carbon in the thawing subsea permafrost results in seabed microbial methane emission Seabed methane emission is less likely to be caused by methane hydrate dissociation at the Arctic continental shelves

High-latitude permafrost, including hydrate-bearing frozen ground, changes its properties in response to natural climate change and to impacts from petroleum production. Of special interest is the behavior of thermal conductivity, one of the key parameters that control the thermal processes in permafrost containing gas hydrate accumulations. Thermal conductivity variations under pressure and temperature changes were studied in the laboratory through physical modeling using sand sampled from gas-bearing permafrost of the Yamal Peninsula (northern West Siberia, Russia). When gas pressure drops to below equilibrium at a constant negative temperature (about -6(degrees)C), the thermal conductivity of the samples first becomes a few percent to 10% lower as a result of cracking and then increases as pore gas hydrate dissociates and converts to water and then to ice. The range of thermal conductivity variations has several controls: pore gas pressure, hydrate saturation, rate of hydrate dissociation, and amount of additionally formed pore ice. In general, hydrate dissociation can cause up to 20% thermal conductivity decrease in frozen hydrate-bearing sand. As the samples are heated to positive temperatures, their thermal conductivity decreases by a magnitude depending on residual contents of pore gas hydrate and ice: the decrease reaches similar to 30% at 20-40% hydrate saturation. The thermal conductivity decrease in hydrate-free saline frozen sand is proportional to the salinity and can become similar to 40% lower at a salinity of 0.14%. The behavior of thermal conductivity in frozen hydrate-bearing sediments under a pressure drop below the equilibrium and a temperature increase to above 0 C-degrees is explained in a model of pore space changes based on the experimental results.