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As an effective carbon utilization technology, the injection of N2/CO2 mixtures into coal seams has significant potential for improving coalbed methane recovery. Considering the technical barrier that injection of pure CO2 decreases the well injectivity index and pure N2 injection leads to the rapid methane recovery, a method of injecting N2‐enriched gas mixtures with a constant component is proposed. In this study, a thermo‐hydro‐mechanical (THM) coupling numerical model for enhanced coalbed methane (CBM) recovery by injecting N2/CO2 mixtures is established. This model includes complex interactions of coal deformation, competitive adsorption, ternary gas seepage, and heat transfer. The THM coupling model is first validated, and then applied to investigate the evolution of mixed gas concentrations, reservoir permeability, reservoir temperature, CH4 production, and N2/CO2 storage during N2/CO2 enhanced CBM recovery. The results show that the displacement radius and concentration of the mixed gas in the coal seam increased with gas injection pressure increase. The concentration of CH4 gradually decreased with time, and the early decline is faster than the later stage. The sweep of the N2 flow accelerates CH4 desorption and migration, promoting a reduction in reservoir temperature near the production well. Reservoir permeability evolution results from the combined effects of ternary gases (CH4, CO2, N2) competitive adsorption, gas pressure, and geostress on the coal seam within the THM fields. At the methane natural depletion stage (within 250 days), the permeability of coal reservoir first decreases and then increases. With the arrival of the N2/CO2 mixture, the permeability decreases dramatically. From the perspective of cumulative CH4 production, the optimal composition is dominated by the synergistic effect of maximizing breakthrough time and minimizing coal matrix swelling. For 30% CO2‐70% N2, the CH4 recovery ratio reached 71.76%, representing an increase of 16.67% compared to natural depletion. To reveal the coupling mechanism of ternary gases (CH4, CO2, N2) in the coal reservoir during CBM recovery, a thermo‐hydro‐mechanical (THM) coupling model of ternary gases was established, including the competitive adsorption of ternary gases, free gas diffusion and seepage, as well as nonisothermal heat transfer and coal deformation. Gas production and storage of different mixed gas injection cases were compared to optimize the composition of injected gas mixture.

In this study, we use the TOUGH-FLAC simulator for coupled thermo–hydro-mechanical modeling of well stimulation for an Enhanced Geothermal System (EGS) project. We analyze the potential for injection-induced fracturing and reactivation of natural fractures in a porous medium with associated permeability enhancement. Our analysis aims to understand how far the EGS reservoir may grow and how the hydroshearing process relates to system conditions. We analyze the enhanced reservoir, or hydrosheared zone, by studying the extent of the failure zone using an elasto-plastic model, and accounting for permeability changes as a function of the induced stresses. For both fully saturated and unsaturated medium cases, the results demonstrate how EGS reservoir growth depends on the initial fluid phase, and how the reservoir extent changes as a function of two critical parameters: (1) the coefficient of friction, and (2) the permeability-enhancement factor. Moreover, while well stimulation is driven by pressure exceeding the hydroshearing threshold, the modeling also demonstrates how injection-induced cooling further extends the effects of stimulation.

The constitutive thermo‐hydro‐mechanical equations of fractured media are embodied in the theory of mixtures applied to three‐phase poroelastic media. The solid skeleton contains two distinct cavities filled with the same fluid. Each of the three phases is endowed with its own temperature. The constitutive relations governing the thermomechanical behavior, generalized diffusion and transfer are structured by, and satisfy, the dissipation inequality. The cavities exchange both mass and energy. Mass exchanges are driven by the jump in scaled chemical potential, and energy exchanges by the jump in coldness. The finite element approximation uses the displacement vector, the two fluid pressures and the three temperatures as primary variables. It is used to analyze a generic hot dry rock geothermal reservoir. Three parameters of the model are calibrated from the thermal outputs of Fenton Hill and Rosemanowes HDR reservoirs. The calibrated model is next applied to simulate circulation tests at the Fenton Hill HDR reservoir. The finer thermo‐hydro‐mechanical response provided by the dual porosity model with respect to a single porosity model is highlighted in a parameter analysis. Emphasis is put on the influence of the fracture spacing, on the effective stress response and on the permeation of the fluid into the porous blocks. The dual porosity model yields a thermally induced effective stress that is less tensile compared with the single porosity response. This effect becomes significant for large fracture spacings. In agreement with field data, fluid loss is observed to be high initially and to decrease with time. Key Points Thermo‐hydro‐mechanical equations of fractured media The model is calibrated from the thermal outputs of two HDR reservoirs Simulation of circulation tests at Fenton Hill HDR reservoir

To study the dynamic response of saturated asphalt pavement under moving load and temperature load, 3-D finite element models for asphalt pavements with hydro-mechanical coupling and thermal-hydro-mechanical coupling were built based on the porous media theory and Biot theory. First, the asphalt pavement structure was considered as an ideal saturated fluid–solid biphasic porous medium. Following this, the spatial distribution and the change law of the pore-water pressure with time, the transverse stress, and the vertical displacement response of the asphalt pavement under different speeds, loading times, and temperatures were investigated. The simulation results show that both the curves of the effective stress and the pore-water pressure versus the external loads have similar patterns. The damage of the asphalt membrane is mainly caused by the cyclic effect of positive and negative pore-water pressure. Moreover, the peak value of pore-water pressure is affected by the loading rate and the loading time, and both have positive exponential effects on the pore-water pressure. In addition, the transverse stress of the upper layer pavement is deeply affected by the temperature load, which is more likely to cause as transverse crack in the pavement, resulting in the formation of temperature cracks on the road surface. The vertical stress at the middle point in the upper layer of the saturated asphalt pavement, under the action of the temperature load and the driving load, shows a single peak.

Fluid injection in deep geological formations usually induces microseismicity. In particular, industrial‐scale injection of CO2 may induce a large number of microseismic events. Since CO2 is likely to reach the storage formation at a lower temperature than that corresponding to the geothermal gradient, both overpressure and cooling decrease the effective stresses and may induce microseismicity. Here, we investigate the effect of the stress regime on the effective stress evolution and fracture stability when injecting cold CO2 through a horizontal well in a deep saline formation. Simulation results show that when only overpressure occurs, the vertical total stress remains practically constant, but the horizontal total stresses increase proportionally to overpressure. These hydro‐mechanical stress changes result in a slight improvement in fracture stability in normal faulting stress regimes because the decrease in deviatoric stress offsets the decrease in effective stresses produced by overpressure. However, fracture stability significantly decreases in reverse faulting stress regimes because the size of the Mohr circle increases in addition to being displaced towards failure conditions. Fracture stability also decreases in strike slip stress regimes because the Mohr circle maintains its size and is shifted towards the yield surface a magnitude equal to overpressure minus the increase in the horizontal total stresses. Additionally, cooling induces a thermal stress reduction in all directions, but larger in the out‐of‐plane direction. This stress anisotropy causes, apart from a displacement of the Mohr circle towards the yield surface, an increase in the size of the Mohr circle. These two effects decrease fracture stability, resulting in the strike slip being the least stable stress regime when cooling occurs, followed by the reverse faulting and the normal faulting stress regimes. Thus, characterizing the stress state is crucial to determine the maximum sustainable injection pressure and maximum temperature drop to safely inject CO2. Fracture stability represented by Mohr circles in the caprock and reservoir for normal faulting, strike slip and reverse faulting stress regime. The grey circles indicate the initial stress state, the orange circles correspond to the stress state when injecting CO2 in thermal equilibrium with the storage formation, and the blue circles show the stress state when injecting cold CO2. The red lines represent the yield surface.

Hydraulic fracturing is widely used in the exploitation of unconventional gas (such as shale gas).Thus, the study of hydraulic fracturing is of particular importance for petroleum industry. The combined finite-discrete element method (FDEM) proposed by Munjiza is an innovative numerical technique to capture progressive damage and failure processes in rock. However, it cannot model the fracturing process of rock driven by hydraulic pressure. In this study, we present a coupled hydro-mechanical model based on FDEM for the simulation of hydraulic fracturing in complex fracture geometries, where an algorithm for updating hydraulic fracture network is proposed. The algorithm can carry out connectivity searches for arbitrarily complex fracture networks. Then, we develop a new combined finite-discrete element method numerical code (Y-flow) for the simulation of hydraulic fracturing. Finally, several verification examples are given, and the simulation results agree well with the analytical or experimental results, indicating that the newly developed numerical code can capture hydraulic fracturing process correctly and effectively.

This study investigates the thermal-hydro-mechanical (THM) coupled damage behavior of deep coal rocks from the Benxi Formation in the Ordos Basin. By conceptualizing coal rock as a dual-porosity medium comprising fractures and matrix, a damage constitutive model was developed through the integration of the Lemaitre strain equivalence hypothesis, continuum damage mechanics, and thermodynamic principles. The model introduces damage variables and correction coefficients to characterize the synergistic effects of confining pressure, temperature, and drilling fluid infiltration. Experimental validation was performed using a custom-designed multi-field coupled triaxial testing system, with triaxial compression tests conducted across varying confining pressures, temperatures, and moisture content conditions. The results show that: (1)The proposed constitutive model successfully quantifies damage evolution under HTM coupling, where parameter q governs residual deformation characteristics and parameter n modulates post-peak stress degradation trends; (2)Drilling fluid immersion induces time-dependent mechanical deterioration, significantly reducing peak stress and elastic modulus, with increasing moisture content exacerbating nonlinear degradation effects; (3)Macroscopic failure modes transition from tensile-shear conjugate patterns to single shear planes as confining pressure decreases and moisture content increases; (4)Theoretical stress-strain curves demonstrate strong consistency with experimental data, validating the model’s capability to simulate deformation laws and damage accumulation processes. The research establishes a theoretical framework for analyzing wellbore instability mechanisms in deep coalbed methane reservoirs, providing critical insights for drilling fluid optimization and geomechanically risk mitigation strategies.

With the rapid development of urban underground space, the construction of shield-driven cross-river twin tunnels is increasing, and the complex hydro-mechanical coupling effects and twin-tunnel interactions bring huge construction risks to such projects, which have attracted more and more attention. This study aims to understand the excavation effects induced by shield driving of cross-river twin tunnels through numerical simulation. A refined three-dimensional numerical model based on the fully coupled hydro-mechanical theory is established. The model considers the main components of the slurry pressure balance shield (SPBS) machine, including support force, jacking thrust, grouting pressure, shield-rock interaction and lining-grouting interaction, as well as the detailed construction process. The purpose is to examine the excavation effects during construction, including rock deformation around tunnels, the change in pore pressure, and the response of the lining. The results show the influence range of twin-tunnel excavation on rock deformation and pore pressure, as well as the modes of lining response. In addition, this study also systematically investigates the effects of water level fluctuation and burial depth on twin-tunnel excavation. The results indicate that the increase of water level or burial depth will enhance the excavation effects and strengthen the twin-tunnel interactions. These results provide useful insights for estimating the construction impact range and degree of twin tunnels, and serve as basic references for the design of cross-river twin tunnels.

This paper presents the hydro-mechanical behaviors of a red sandstone from the Three Gorges reservoir area using laboratory testing and numerical modelling. A series of laboratory triaxial compression tests were performed on the red sandstone to understand the effect of confining stress and seepage pressure on the rock mechanical properties and permeability. The laboratory results show that the initial elastic modulus, Poisson’s ratio and peak strength of the red sandstone increase with confining stress, and decrease with the increase in seepage pressure. The change in the permeability of the red sandstone is slight during the initial and elastic deformation stages, but increases rapidly when approaching to the peak stress. Micromechanical insights on the sandstone’s hydro-mechanical behaviors were unveiled by implementing an improved pipe network flow model in the discrete element method. The improved pipe network model can well capture the stress–strain response and the permeability changes of the red sandstone that were consistent with the laboratory test results. The numerical results reveal that the distribution of flow rate, pore pressure, and force chains in rock is closely related to microcracking, and the permeability increases accordingly with the increase in the number of microcracks and their coalescence. The number of microcracks and the rock fracture angle with respect to the axial axis at peak stress increase with the effective confining stress. The damage process of the red sandstone during the hydro-mechanical loading is divided into three distinct stages based on the mechanical response and permeability evolution.

Tunnel construction usually suffers some water-filled karst caves, which can change the stress and flow fields and bring about unpredictable engineering disasters. A typical form of water inrush disaster caused by combination of steep karst fissures and concealed karst cave is proposed in this paper. The mechanism of this water inrush disaster is studied from aspects of geology and hydro-mechanical coupling response behaviors of surrounding rock. Geological conditions including lithology, attitude of rock formation, supply and runoff conditions, climatic characteristics, etc. of Qiyueshan (QYS) tunnel are analyzed to figure out causes of this disaster. A geological conceptual model is established to describe the causes from geological aspect. Then a numerical model is built through simplifying the geological conceptual model to simulate and study hydro-mechanical coupling response behaviors of tunnel surrounding rock on the platform of PANDAS software. In addition, effects of position and size of the karst cave, and distance between the tunnel and the karst cave on principal stress and displacement of surrounding rock are discussed, which can be used to predict position, size and distance of karst caves in actual engineering.