Accurately predicting the geomechanical properties change in methane hydrate-bearing sediments (MHBS) during hydrate dissociation is the key to ensuring high efficient and safe exploitation of gas hydrate resources. During gas production, phase transformation of hydrates into water and gas induces damage in MHBS, thereby significantly altering reservoir mechanical properties. This study develops a bounding surface model for MHBS incorporating hydrate dissociation based on the thermodynamics principle. Hydrate dissociation within sediments is conceptualized as a damage process governing the cohesion degradation in MHBS. By linking damage evolution to hydrate saturation, dissociation process and stress state, a damage cohesion formulation related to hydrate saturation change is obtained with the aid of Weibull distribution, and a bounding surface model for MHBS incorporating dissociation is subsequently established. Compared with triaxial test results for MHBS before and after hydrate dissociation, it is confirmed that the model can predict the strain hardening/softening behavior and the dilative/contractive volumetric response of MHBS arising from dissociation-induced changes in hydrate saturation. The results indicate that hydrate dissociation degrades hydrate-mediated cementation, resulting in irreversible reductions in peak strength, residual strength, and elastic modulus. The extent of degradation accelerates as dissociation proceeds, leading to a pronounced deterioration of the macroscopic mechanical properties of MHBS. The proposed model provides an effective constitutive tool for predicting reservoir deformation and stability under coupled shear-dissociation processes during gas hydrate production, thereby supporting geomechanical assessment for the safe and efficient exploitation of gas hydrate resources.

Natural gas hydrate is an important emerging strategic resource, but low permeability makes the horizontal well length a key factor limiting productivity. A prediction model for friction torque of deepwater riserless drilling strings was established, and the segmented friction coefficient of hydrate horizontal wells was inverted and applied to the Shenhu hydrate reservoir. The results show that the main limiting factor for the extreme extension length of natural gas hydrate horizontal wells is the mechanical extreme extension length. The main affecting factor of the mechanical extreme extension length is the running limit of the screen pipe. The friction coefficient is the most significant factor affecting the mechanical extreme extension of horizontal wells, with the friction coefficient inside the casing in the high-build-rate section being the largest. The research identifies the primary factors governing the limit extension during horizontal well construction. The findings provide theoretical guidance for reservoir selection, well site determination, and wellbore configuration optimization in hydrate development. Ultimately, this contributes to maximizing single-well productivity and advancing the commercialization of hydrate resources.

Extraction and utilization of groundwater is essential for meeting the demand of drinking water, agricultural irrigation, and industrial processes. Nevertheless, groundwater extraction frequently leads to land subsidence and damage to nearby facilities. The groundwater extraction process would be more complex in coastal areas, where gassy soils rich in discrete gas bubbles are widely distributed. This paper introduces a newly developed method to examine the land subsidence process induced by groundwater extraction. Considering the impact of gas bubbles on the compressibility and deposition process on anisotropic permeability of gassy soil, a new seepage equation is developed. With the aid of the integral-transform method and extended precise integration method, a precise solution is obtained. Extensive parametric investigations were conducted to compare the deformation behavior in gassy and saturated soils and further to explore the effects of saturation degree and anisotropic permeability on land subsidence processes. This paper presents a fundamental solution for analyzing the deformation behavior of gassy soil induced by groundwater extraction, which can be further treated as a kernel function of the boundary element method or finite-element method to evaluate the service process of structures built on gassy soils. The findings shed a new light on prevention and control of extracting groundwater.

Natural gas hydrates are increasingly recognized as a promising alternative to traditional energy. The intricate thermo-hydro-chemo-mechanical (THMC) coupled process during in-situ hydrate exploitation poses substantial challenges for numerical modeling. This paper develops a THMC model to investigate the reservoir behavior during the hydrate dissociation based on the open-source FEM code OpenGeoSys, characterized by robust convergence. Locally, a nonlinear complementary problem (NCP) approach is employed to handle the strong nonlinearity associated with phase (dis)appearance, and a fourth-order Runge-Kutta (RK4) method is utilized to calculate the hydrate saturation. Globally, persistent primary variables are introduced to uniformly describe single-/two-phase flow. The THMC coupled process of the first offshore hydrate production test in the South China Sea has been investigated using this model. Results show that a local temperature rise appears in the free gas layer for heat convection from the underburden layer and the dissociation front exhibits a triangular distribution due to heat convection and liquid-gas migration. Moreover, inhomogeneous reservoir deformation and low gas production rate/gas-water ratio occur, where the gas production can be notably enhanced by decreasing permeability in the underburden layer. These findings provide valuable insights into optimizing the efficiency and safety of gas hydrate exploitation.

Natural gas hydrates is a promising alternative energy resource, widely buried in marine sediments. During hydrate exploitation, the exsolution of dissociated gas into bubbles creates a foam flow, which impedes fluid migration and increases the risk of geotechnical hazards. This study proposes a theoretical model for gas exsolution from hydrate dissociation and combines it with the population balance equation to characterize bubble generation, burst, and migration. The proposed theoretical model has been integrated into OGS-Hydrate, enabling it to capture bubble exsolution and the consequent increase in flow resistance during hydrate exploitation through the implementation of a foam flow model. This model is validated against Masuda's experimental data, accurately capturing the gas pressure accumulation. Building on this, the study investigates how bubble seepage and gas exsolution behavior affect mass and heat transfer, as well as hydrate dissociation. Results demonstrate that: (i) gas migration during hydrate exploitation occurs in three stages, including an initial rapid gas migration stage, a subsequent impeded flow stage dominated by foam flow characteristics, and a final secondary rapid migration stage with accumulated gas pressure overcoming seepage resistance; (ii) the dominance of foam flow seepage is governed by initial gas saturation, sediment permeability, and dissolved gas exsolution; (iii) stronger deformation occurs in the near-well region during field-scale exploitation. Specifically, this seepage mechanism is enhanced by a lower initial gas saturation, greater sediment permeability, and a higher critical gas saturation. This study highlights the crucial role of hydrate dissociation in multiphase flow evolution and offers novel insights into the gas-liquid migration mechanism during hydrate exploitation.

Natural gas hydrates are widely recognized as a promising alternative to conventional fossil fuels. During depressurization, production efficiency gradually declines due to mass and heat transfer limitations within the maximum recoverable zone. This study integrates analytical solutions with numerical simulations to quantitatively evaluate field-scale production efficiency decline and identify high- and low-efficiency dissociation zones. Several indicators are proposed to characterize the effective production duration and the high-efficiency production zone in horizontal well depressurization. A stratified weighted average method is developed to extend the analytical model, enabling consideration of geothermal and pressure gradients as well as horizontal well configurations. By analyzing the mass and heat transfer characteristics of different reservoir types and well configuration strategies, optimal well configurations are recommended. The recoverable zone with enhanced permeability and the critical well length required for commercial production for field-scale in the South China Sea hydrate reservoir are predicted. Results show that placing the horizontal well in the middle gas hydrate-bearing layer (GHBL) promotes long-term production and yields a higher gas water ratio. During hydrate exploitation through horizontal well depressurization, the recovery factor (R out) varies slightly with permeability (0.18-0.21). However, with sufficient heat supplementation, R out increases markedly and continues to rise with increasing permeability. Therefore, a combined strategy of permeability enhancement and heat injection is recommended for commercial exploitation. Energy return on investment (EROI) analysis indicates maximum economic efficiency when the GHBL permeability reaches 144 mD; however, considering fracturing costs, targeting permeability below this value is advisible. These findings provide theoretical guidance for optimizing gas hydrate production via depressurization.

Gas production from unconsolidated formations is often challenged by sand production and low reservoir permeability. All past 11 field trials of gas production from hydrate reservoirs encountered various degrees of sand production, with five of them suspended due to severe sand production. This study investigates the potential of synthesizing open-cell polyurethane (PU) as a sand control filter and hydraulic fracturing proppant in water-saturated clayey sediments. The results show that the properties of formed polyurethane are heavily affected by constituent proportions, additives (e.g., catalyst and surfactant), the mixing sequence, and the hardening time. Simultaneous injection of catalyst and a premix (of toluene diisocyanate TDI, 1,4-butanediol BDO, and surfactant) into water can form polyurethane with consistent final volumes and correspondingly stable engineering properties. The stiffness and strength of polyurethane increase rapidly during the first 48 h of hardening and then stabilize at an unconfined compressive strength of UCS = similar to 1 MPa. The morphology of formed polyurethane, which is in localized chunks around the injection port as sand filters or in planar fractures propagating over a greater distance as proppants, can be manipulated by the injection flux, the amount of catalyst, and in situ effective stresses. This technique of using in situ synthesis of open-cell polyurethane, forming fracture proppants reaching the far field, followed by chunk-like filters around the injection ports, provides a novel solution for sustainable and efficient hydrocarbon recovery from unconsolidated sediments.

Gas invasion into fluid-saturated marine sediments is relevant to various geophysical and environmental processes, such as the formation of gas hydrate reservoirs and their behavior during depressurization-based gas production. The link between pore-scale mechanisms and macroscopic invasion patterns as well as their governing parameters, especially under high stress and intense flux conditions, remains unclear. We study gas invasion into water-saturated marine sediments using a coupled framework of the discrete element method and pore network modeling. We show that two dimensionless numbers, the stress number Σ and the seepage number D, govern five representative gas invasion patterns of capillary fracturing, capillary invasion, viscous fingering, seepage fracturing, and the transitional morphologies among them. The capillary number C, the stress number Σ, and the seepage number D collectively determine the shear, tensile, or cavity dominated fracturing in marine sediments. The results provide a description of gas migration regime transitions in marine sediments, which can help establish a scientific foundation to explain the development of marine cold seeps in nature and to design gas hydrate exploitation strategies.

CH4-CO2 replacement is a novel method that simultaneously enables CH4 exploitation and CO2 sequestration. The pore structure of sediments and the microscopic occurrence characteristics of CH4 and CO2 hydrates in sediments are key factors that influence the exploitation efficiency and reservoir stability of CH4 hydrate. In this study, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was employed to conduct microscopic observations and elemental analysis of CH4 and CO2 hydrate-bearing sediments. A comparative analysis of their occurrence characteristics and pore structure was performed, and the formation mechanisms of the two hydrates in sediment pores were investigated. The results indicate significant differences in the occurrence patterns of CH4 and CO2 hydrates in sediments, which are probably related to the solubility in water of the two gases. CH4 gas with lower solubility tends to form granular, pore-filling hydrates, while CO2 gas with higher solubility tends to form flaky or massive cemented and load bearing hydrates. As hydrate saturation increases, the occurrence pattern of CH4 hydrates shifts from pore-filling to patchy. In contrast, that of CO2 hydrates transitions from cemented to load bearing and further to patchy. Hydrate saturation significantly affects the pore structure characteristics of sediments. With increasing hydrate saturation, large pores are gradually divided into medium, fine, and micropores, and the pore fractal dimension initially increases and then decreases. The evolution patterns of hydrate occurrence and sediment pore structure characteristics during CH4-CO2 replacement are examined in this study, providing valuable insights into efficient hydrate exploitation and reservoir stability analysis.

Marine hydrate-bearing sediments (HBS) in the Nankai Trough and the South China Sea, characterized by high fines content and high hydrate saturation, are typically associated with very low porosity and permeability, which greatly undermines the hydrate exploitation efficiency. Inspired by the exploitation techniques of coals and shale gases, hydraulic fracturing could potentially be an effective way to improve the overall permeability of HBS and accordingly its gas production efficiency. This paper introduces a novel experimental study on the enhancement of gas production from HBS via combined hydraulic fracturing and depressurization method. The main properties examined are the viscosity of fracturing fluid and the perforated length of production well. Substantial improvement in gas production by hydraulic fracturing was observed, in terms of both the peak and long-term production rates. The most remarkable increase in peak production rate can be up to 90.4% and only half the time was required to achieve a total gas production of 70%. The optimal fluid viscosity of 500 mPa & sdot;s was identified in the present experiments. Fracturing fluids with lower viscosities would lead to only small fractures and limited increase in the overall permeability, while that with higher viscosities somewhat inhibit gas flow along fractures, both against the achievement of high gas production efficiency. In particular, sediment subsidence and sand production would be exacerbated at the presence of hydraulic fractures. Furthermore, a greater well perforated length was conductive to fracturing fluid discharge and thus facilitating gas production efficiency, in terms of not only shortening the fluid flow path but also alleviating the sand production. This study on hydraulic fracturing for HBS offers novel insights into enhancing the gas production efficiency and revealing potential engineering risks in practical applications.

Natural gas hydrate has vast reserves worldwide and is widely regarded as an efficient alternative energy resource. Gas seepage and the evolution of seepage channels during gas production from hydrate-bearing sediments are essential for the safe and economical exploitation of natural hydrate reservoirs. This study aims to investigate the formation mechanism of gas-driven fractures in hydrate-bearing sediments during hydrate dissociation and, for the first time, their resulting influences on gas production behavior and reservoir deformation. Samples with various porosities were depressurized at different rates by using a customized chamber equipped with multiple pressure and temperature sensors and see-through windows. The results show that gas produced from hydrate dissociation can lead to cavity formation and gas-driven fractures, which facilitate rapid gas migration and production. The gas production rate increased exponentially with sediment porosity and, likewise, the postproduction subsidence. In addition, an optimal porosity with improved gas production efficiency and mild reservoir subsidence was identified. These findings provide a theoretical basis for the deployment of production wells, gas production evaluation, and assurance of reservoir safety during the commercial exploitation of marine hydrate deposits.

ABSTRACT Natural gas hydrates are the preferred alternative to traditional fossil fuels, estimated to store twice as much carbon. The gas hydrate‐bearing sediment (GHBS) is a representative degradable soil. During gas hydrate production, solid mass loss and pore liquid/gas generation occur, including both decomposition and consolidation processes of GHBS. These potentially trigger reservoir collapse, severely affecting production safety. This study develops a decomposition–consolidation model for GHBS, quantifying the solid hydrate loss by the kinetic decomposition equation and describing skeleton deformation via both modified elasticity and volumetric strain relationships. By linking hydrate saturation with the representative parameters of hydrate decomposition, mass migration, heat transfer, and skeleton deformation, decomposition degree and consolidation degree are respectively introduced to assess these processes. The decomposing and mechanical parameters are calibrated through triaxial/modeling tests. Results show that consolidation degree and decomposition degree are not synchronized in sandy hydrate‐bearing sediments, where depressurization‐induced variation of consolidation degree predominates in the early stage, while the evolution of consolidation degree lags behind decomposition degree for temperature recovery after complete hydrate decomposition; hydrate decomposition‐induced skeleton deformation, significant compared to pore pressure dissipation, remains crucial long after complete depressurization. These findings provide insights into optimizing safety of long‐term gas hydrate production.

Heated pipeline transportation is one of the most economical and effective methods for long-distance transportation of oil and gas resources.Among them,the interaction mechanism between high-temperature pipelines and soft soil foundations is the key to analyzing their service stability.During the long-term service of the heated pipeline,heat is transferred to the surrounding soft soil,triggering complex hydro-thermal-mechanical coupling behavior.This,in turn,alters the bearing capacity of the foundation,significantly affecting the stability of the pipeline in service.A model test system has been developed to simulate the interaction between high-temperature pipelines and soft soil foundations,capable of replicating temperature variations in the pipeline.Focusing on the cyclic temperature loading experienced during the start-up,shutdown,and operation of high-temperature pipelines,model tests were conducted under various conditions,including different pre-compression stresses in the foundation,pipeline heating powers,and heating methods.These tests aimed to explore the evolution of temperature and pore pressure in the soil surrounding the pipeline during its service life under cyclic temperatures.Furthermore,the study investigated the development of restraint forces in the surrounding soil and the changing bearing characteristics of the foundation.The test results indicate that during continuous heating,the pore pressure around the pipeline increases,peaking before significantly decreasing,and may even reach negative values.This phase is beneficial for pipeline stability.When subjected to cyclic temperature loading,fluctuations in temperature induce corresponding variations in the pore pressure of the foundation.It's worth noting that the amplitude of pore pressure fluctuations is smaller at the sides of the pipeline compared to the top and bottom.In tests simulating pipeline uplift,the foundation reaches its ultimate bearing capacity when the displacement reaches approximately 1.2 times the diameter of the pipe.Additionally,a higher pre-consolidation pressure offers greater bearing capacity,although there is little difference in uplift capacity between super-consolidated and normally consolidated foundation soils.T-bar tests conducted on the foundation revealed a significant enhancement in foundation strength during the high-temperature phase maintained by a single heating cycle.Moreover,an increase in foundation strength was observed even after the heating was stopped.

Geomechanical and heat transfer characteristics of gas hydrate-bearing sediment (GHBS) are significantly affected by hydrate dissociation during gas production from reservoirs, which is typically tens of meters in thickness. This paper presents the development of an innovative in-flight apparatus that is capable of modeling hydrate dissociation in GHBS on a geotechnical centrifuge, by which a series of model tests are conducted under normal gravity (1g) and hypergravity (100g and 80g). The effects of the hypergravity field on the development of pore pressure, soil deformation, as well as particle migration and gas production during hydrate dissociation are explored. Results show that gas released from hydrate dissociation increases excess pore pressure and changes the soil pore structure. During hydrate dissociation, the accumulation of excess pore pressure leads to the development of gas-driven fractures and the subsequent formation of a dominant seepage channel. The critical excess pore pressure of fracture formation in the 100g test is higher than that in the 1g test. The dominant seepage channel promoting fluid seepage and fine particle migration is more likely to be formed into obvious fracture structures under 100g compared with slender pipe structures under 1g. Two peaks are witnessed in gas production in the 100g test, corresponding to the stage at maximum pressure difference and at the complete formation of dominant seepage channels, which is consistent with that in the field trails. These results indicate that the formation of fracture during hydrate dissociation is beneficial to efficient gas production, while the problem of particle migration should be carefully paid attention to.

The permeability of gas hydrate-bearing sediments(GHBS)is an important factor affecting the gas-liquid transport characteristics during the processes of hydrate formation and decomposition,and is often selected as an indicator for evaluating the extraction capacity of natural gas hydrates.The changes in hydrate occurring habits and hydrate saturation in GHBS pores significantly influence GHBS permeability.Existing permeability models are mostly based on a single hydrate occurring habit,making it difficult to consider the impact of changes in hydrate occurring habits on GHBS permeability.Based on the parallel capillary tube models,considering the influence of the varying of hydrate occurring habits on pore structure of GHBS,a mixed occurring habit with grain-coating and pore-filling coexisting is proposed.A logical function with two parameters is proposed to describe how hydrate occurring habits change with saturation,and a permeability model of GHBS considering the variation of hydrate occurring habits is established.The correctness of the model is verified by comparing it with measurement data obtained from laboratory and in-situ permeability tests,and the effectiveness of the model is analyzed by comparing it with existing mathematical models.The results indicate that changes in hydrate saturation during hydrate formation usually lead to changes in hydrate occurring habits,affecting the trend of the GHBS permeability with hydrate saturation.The changes in hydrate occurring habits vary under different formation conditions,and the main characteristics of the changing process are reflected in the critical hydrate saturation corresponding to the transformation of the main occurring habit,as well as the direction and trend of changes.Compared to the existing models,this model can capture the changing characteristics of permeability when the hydrate occurring habit changes,and can better predict measurement data from both laboratory and in-situ permeability tests of GHBS.

To safely and effectively explore the natural methane hydrate, it is crucial to examine the mechanical behavior of methane hydrate-bearing sediments (MHBSs). Natural methane hydrate unevenly distributes in pores or bonds with soil particles in MHBS, changing the mechanical behavior of MHBS including stiffness, shear strength, and dilatancy. This paper presents an anisotropic critical state model for MHBS considering hydrate pore-filling and cementing effects. Based on the unified critical state model for both clay and sand, an equivalent hydrate ratio is defined to address pore-filling effect. Cohesive strength and its hardening law are introduced to characterize hydrate cementation. To describe the anisotropic behavior, the inherent anisotropy of soil particles and hydrates are modeled separately, and rotation hardening is introduced to describe the stress-induced anisotropy. Comparisons with existing triaxial tests of both synthetic and natural MHBS demonstrate that the proposed model comprehensively describes the mechanical behavior of MHBS. Detailed predictions indicate that hydrate pore-filling affects the hydrate-dependent stiffness and dilatancy of MHBS, which become more pronounced with increasing hydrate saturation. Cementing effect increases the initial stiffness and peak strength of MHBS. The pronounced influence of inherent anisotropic parameters on pre-peak stress-strain relation of MHBS is noted, and increasing hydrate saturation enhances the effect of hydrate anisotropy. These predictions contribute to a better understanding of the relation between hydrate morphologies and MHBS mechanical properties.

Gas diffusion is an important process in thermo-hydro-mechanical coupled problems of unsaturated soils, e.g., nuclear waste repositories or heated pipeline engineering, which usually strongly affects process of liquid water evaporation and water vapor condensation, migration of air and water, heat transfer, and skeleton deformation. This paper develops a simple model to assess the effects of gas diffusion on multifield coupled process of unsaturated soils, providing predicting guidance and uniform judgment rather than establishing a numerical coupled model by the traditional methods. A dimensionless characteristic number is first proposed and expressed by sediment properties and environmental conditions. Rationality of this characteristic number is then assessed and validated by numerical results, and studies indicated that the gas diffusion is crucial when the characteristic number exceeds a critical one, which is reflected in early valleys of gas/liquid pressure and effective stresses/strains induced by vapor condensation and the final maximum values. It can be found from the expression that this characteristic number is highly relevant to diffusion coefficient, permeability, temperature, initial saturation, and gas pressure. Parametric studies indicate that gas diffusion appears to be critical for unsaturated clayey soils, provided that the diffusion coefficient is larger than 10–7 m2/s and permeability smaller than 10–14 m2. The effect can be neglected for soils with diffusion coefficient lower than 10–9 m2/s or permeability greater than 10–14 m2.

Gas-bearing sediments are widely distributed in marine and lacustrine areas. The engineering properties of gas bearing sediments, containing enclosed bubbles, are significantly different with that of saturated and unsaturated sediments. This paper investigates the consolidation behaviors of gas-bearing sediments with modulus varying along depth subjected to horizontal load using an extended precise integration method (XPIM). The compressibility of enclosed bubble is introduced to a new derived seepage equation and a varying modulus along depth is considered in governing equations. With the aid of integral transformation and Taylor expansion, such problems are solved by XPIM, which is proved to be markedly efficient and precise for boundary value problems of porous media than traditional numerical methods. Detailed comparisons against analytical solutions are performed to confirm the accurateness of XPIM. Extensive parametric investigations are conducted to examine the influence of saturation degree, varying modulus along depth and type of external load on the consolidation behaviors of gas bearing sediments. The present work is conveniently utilized to evaluate the behavior of gas-bearing sediments, and subsequently the response of foundations built in gas-bearing soil areas.

Unsaturated soils with a high degree of saturation (HDS) are commonly encountered in marine and lacustrine sediments. In these soils, the gas phase generally exists in the state of discrete bubbles, which is sensitive to stress and temperature changes and can dramatically change the soil's engineering properties. This paper explores the thermal-induced behaviour of HDS soils using an efficient extended precise integration method (XPIM). Biot poroelasticity theory, extended to include thermal effects and compressibility of gas–water mixture, is employed to analyze the soil behaviour under non-isothermal conditions. Based on the Laplace–Fourier transform and Taylor series expansion, such problems can be solved by XPIM. The robustness of XPIM was confirmed by comparing the present results with analytical solutions and test data. Extensive parametric studies are undertaken to examine both the effects of soil grain thermal expansion and anisotropic permeability on soil behaviour and temperature effects on the degree of saturation ( Sr). The thermal-induced variations in Sr are more pronounced with lower initial values (e.g., 90% compared to 99%). These quantitative results demonstrate the benefits of the proposed method, which proves to be extremely efficient and several orders more precise than conventional numerical approaches, with its precision limited only by the computational effort used.