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The temperature increase induced by radioactive waste decay generates the thermal pressurisation around the excavation damage zone (EDZ), and the excess pore pressure could induce fracture re-opening and propagation. Shear strain localisation in band mode leading to the onset of micro-/macro-cracks can be always evidenced before the fracturing process from the lab experiments using advanced experimental devices. Hence, the thermal effects on the rock behaviour around the EDZ could be modelled with the consideration of development of shear bands. A coupled local 2nd gradient model with regularisation technique is implemented, considering the thermo-hydro-mechanical (THM) couplings in order to well reproduce the shear bands. Furthermore, the thermo-poro-elasticity framework is summarized to validate the implemented model. The discrepancy of thermal dilation coefficient between solid and fluid phases is proved to be the significant parameter leading to the excess pore pressure. Finally, an application of a heating test based on Eurad Hitec benchmark exercise with a drift supported by a liner is studied. The strain localisation induced by thermal effects is properly reproduced. The plasticity and shear bands evolutions are highlighted during the heating, and the shear bands are preferential to develop in the minor horizontal principal stress direction. Different shear band patterns are obtained with changing gap values between the drift wall and the liner. A smaller gap between the wall and the liner can limit the development of shear bands.HighlightsThe formulation of a coupled local 2nd gradient model considering the thermo-hydro-mechanical (THM) couplings.Validation of the model with comparison with analytical solution of thermo-elastic problem.The prediction of strain localisation pattern induced by thermal effects around a large scale drift.The analysis of the gap distance (between the drift wall and the liner) on the strain localisation process under the thermal loading.

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.

Glacial lake‐moraine dam systems are widespread in cold alpine environments such as the Qinghai‐Tibet Plateau (QTP). Without climate change, the lake‐dam system exhibits stably dynamic evolution on a hydrological annual cycle. However, climate change may drive subtle alterations in the system's evolution. We developed a fully coupled Thermal‐Hydraulic‐Mechanical simulation platform considering ice‐water phase change, showing robust performance under CMIP6‐derived boundary conditions. Using this platform, we simulated climate warming‐driven multiphysics responses and dam stability evolutions of a homogeneous, simplified conceptual model of the lake‐dam system. We identified critical temperature thresholds for permanently frozen area thawing and abrupt changes in dam stability of this lake‐dam system. Considering the current slope stability situations on the QTP, the SSP 5–8.5 climate warming scenario is conservatively anticipated to pose significant geological safety risks due to potential disaster chains from glacial lake failures. Our study provides insights into profound geological process evolutions driven by climate change. Plain Language Summary Sizable and numerous moraine‐dammed glacial lakes in cold alpine regions are increasingly threatened by climate change. This study simulated the long‐term (2020–2140) Thermal‐Hydraulic‐Mechanical coupling and stability evolution of a homogeneous conceptualized glacial lake‐moraine dam system under climate warming on the Qinghai‐Tibet Plateau (QTP). Two types of critical state‐transition points were identified in this conceptual model, marking shifts from quantitative to qualitative changes. A temperature rise threshold of 5.89°C indicates the onset of rapid shrinkage in permanently frozen area of the conceptual lake‐dam system. Another type of transition points occurs at 1.92°C and 4.48°C, corresponding to sharp year‐over‐year declines in spring and winter stability, respectively. A conservative estimation suggests that, if evaluated using stability reduction rate of the conceptualized model, moraine dams on the QTP with current stability factors below 1.19 in summer or 1.81 in winter, could fail after 120 years of intense climate warming. Considering the current stability situations on the QTP and geohazard chains resulting from glacial lake failure, uncontrolled global climate change would pose a severe threat to regional geological safety of QTP. The study has significant implications for assessing geological safety in periglacial environments and supports investigating coupling issues of climate change, geological processes, and human activities. Key Points Climate change‐driven multiphysics responses and stability evolution of a conceptual glacial lake‐moraine dam are depicted over the next 120 years Frozen area would rapidly retreat for nearly 10 years once the temperature rise crosses a certain threshold (5.89°C for the conceptual model) Spring and winter stability would rapidly deteriorate after surpassing certain temperature thresholds (1.92°C and 4.48°C, respectively, for the model)

The comprehensive exploitation of geothermal fields has an impact on the productivity of the reservoirs. To realize sustainable steam production, changes in the rock temperature need to be predicted and controlled. A coupled thermo-hydro-mechanical (THM) model employing COMSOL Multiphysics was proposed to study the characteristics of heat transfer, fluid flow, and solid deformation at the Lahendong geothermal field, in North Sulawesi, Indonesia. The numerical results were compared with analytical and measured data in order to validate the numerical simulation. Based on the results, the predicted temperatures of the production wells showed significant decrease with the production time. In addition, a reduction in the reservoir temperature leads to lower specific gross electrical power within the production well, which should significantly reduce the sustainability of the power plant.

Coupled thermal, hydraulic, and mechanical processes in porous materials play important roles in several energy and environmental technologies. The Darcy‐Brinkman‐Biot (DBB) framework has proven effective in modeling multiphase fluid flow in deformable porous solids across both pore and Darcy scales, including in systems where fractures coexist with a porous matrix. In this study, we extend the DBB framework, originally designed for isothermal conditions, to address non‐isothermal problems by incorporating an energy conservation equation. The resulting solver, hybridBiotThermalInterFoam, enables simulations of coupled multiphase fluid flow, heat transfer, and solid deformation in hybrid‐scale systems containing both solid‐free regions and ductile porous domains. The new solver is validated through comparisons with analytical solutions and, also, against established heat transfer solvers chtMultiRegionFoam and compressibleInterFoam. Further, a series of 2D and 3D case studies, including two‐phase heat transfer in solid‐free, static, or deformable porous media, highlights the solver's capacity to simulate complex flow dynamics and heat transport in systems involving high mobility ratios, viscous fingering, and fracture propagation. Our results establish the feasibility of incorporating thermal effects in simulations of a wide variety of energy geotechnics and environmental applications, including enhanced hydrocarbon recovery, soil remediation, and enhanced geothermal energy systems.

The thermal-hydro-mechanical (THM) coupling and a strong mining disturbance environment all have a significant impact on deep ground engineering excavations, which increases the risk of a rock burst disaster. The independently developed THM multi-physics coupling apparatus enables replication of intricate geological conditions in engineered rock formations, with high-strain-rate compression experiments on deep-buried rock specimens being implemented through a split Hopkinson pressure bar testing platform. The fracture surface microstructure and the pore structure of the rock after impact are characterized by SEM and NMR. Results show that the dynamic stress–strain curve exhibits nonlinear behavior, accompanied by a significant 'plastic platform area.' Under the same temperature and pre-static stress, the energy time-history evolution distribution of rock in the process of dynamic compression has the characteristics of synchronous and different amplitudes. The energy reflection coefficient and energy dissipation coefficient obey a good exponential function and a Gaussian function relationship with water pressure, respectively. 'Compression-shear crushing failure → inclined shear boundary failure → fracture failure’ is the development trend of the overall failure mode of rock when temperature and water pressure increase. The dynamic damage threshold is between 0.35 and 0.36. Microscopically, it shows a transformation trend of intergranular fracture, complex fracture, and transgranular fracture. NMR analysis reveals that elevated water pressure enhances the structural integrity of rock pores, accompanied by a reduction in pore dimensions and a progressive decline in overall porosity. A mechanical model of sliding microcrack propagation under THM coupling and impact load is constructed. By analyzing the variation patterns of crack initiation points under different operating conditions, the accuracy and adaptability of this model were validated.

In the present study, a physical model test was used to investigate the cracking characteristics of the surrounding rocks of hydraulic tunnels under high geothermal conditions. Based on similarity theory, a similar material was developed to simulate the intact and hard rocks in thermo-hydro-mechanical (THM) coupling fields; this material was used to cast a large-scale model tunnel. A new loading system, including a temperature loading system and a water pressure loading system, was designed to improve upon conventional laboratory hydraulic tunnel testing and provide a means of better understanding the fracture behavior of solid media. An acoustic emission (AE) monitoring system, thermocouples and osmometers were used to reveal the real-time evolution of the temperature field, seepage pressure field and crack propagation. The test results showed that compared to a hydraulic tunnel without high geotemperature, the critical internal water pressure of the hydraulic tunnel under high geothermal conditions decreased significantly. After the initiation of main cracks was induced by hydraulic fracturing, due to THM coupling, a certain number of secondary cracks initiated and developed between the main cracks. In this process, the trend of the secondary crack tip deviated from the radial direction, resulting in a more developed and complicated evolution of cracks (including main cracks and secondary cracks). This was different from the cracking behavior of surrounding rocks without a high geothermal gradient, in which only a few main cracks due to hydraulic fracturing were observed. An obvious multifield THM coupling effect was observed under high-geotemperature conditions, and cracks propagated and damage increased in the surrounding rocks in a discontinuous and step-like manner. The number and source locations of the cracks recorded by the AE monitoring system were in good agreement with the evolution of the temperature field and seepage pressure field. In addition, FLAC3D numerical simulations based on the thermo-hydro-mechanics and damage (THMD) model revealed that high geotemperatures and in situ stresses significantly influenced on the cracking mode of the surrounding rocks of hydraulic tunnels under high geotemperatures. The results obtained in this study provide a better understanding of the cracking characteristics of the surrounding rocks of hydraulic tunnels under high geothermal conditions and are useful for the design of hydraulic tunnels.

The Enhanced Geothermal System (EGS) constructs an artificial thermal reservoir by hydraulic fracturing to extract heat economically from hot dry rock. As the core element of the EGS heat recovery process, mass and heat transfer of working fluid mainly occurs in fractures. Since the direction of the natural and induced fractures are generally perpendicular to the minimum principal stress in the formation, as an effective stimulation approach, horizontal well production could increase the contact area with the thermal reservoir significantly. In this paper, the thermal reservoir is developed by a dual horizontal well system and treated as a fractured porous medium composed of matrix rock and discrete fracture network. Using the local thermal non-equilibrium theory, a coupled THM mathematical model and an ideal 3D numerical model are established for the EGS heat extraction process. EGS heat extraction capacity is evaluated in the light of thermal recovery lifespan, average outlet temperature, heat production, electricity generation, energy efficiency and thermal recovery rate. The results show that with certain reservoir and production parameters, the heat production, electricity generation and thermal recovery lifespan can achieve the commercial goal of the dual horizontal well system, but the energy efficiency and overall thermal recovery rate are still at low levels. At last, this paper puts forward a series of optimizations to improve the heat extraction capacity, including production conditions and thermal reservoir construction design.

The permeability contrast between the Hot Dry Rock (HDR) reservoir and the surrounding formations is a key factor governing fluid loss in Enhanced Geothermal Systems (EGS). This study thus aims to investigate its impact on system performance under varying operating conditions, and a three-dimensional thermo–hydro–mechanical (THM) coupled EGS model is developed based on the geological parameters of the GR1 well in the Qiabuqia region. The coupled processes of fluid flow, heat transfer, and geomechanics within the reservoir under varying reservoir–surrounding rock permeability contrasts, as well as the flow and heat exchange along the wellbores from the reservoir to the surface are simulated. Then, the influence of permeability contrast, production pressure, injection rate, and injection temperature on fluid loss and heat extraction performance over a 35-year operation period is quantitatively assessed. The results show that increasing the permeability contrast effectively suppresses fluid loss and enhances early-stage heat production, but also accelerates thermal breakthrough and shortens the stable operation period. When the contrast rises from 1 × 10³ to 1 × 105, the cumulative fluid loss rate drops from 54.34% to 0.23%, and the total heat production increases by 132%, although the breakthrough occurs 5 years earlier. Meanwhile, higher production pressure delays thermal breakthrough and slows transient temperature decline, but exacerbates fluid loss and reduces heat production power. For instance, raising the pressure from 17 to 21 MPa increases the fluid loss rate from 33.17% to 54.34% and reduces average annual heat production power from 25.43 to 14.59 MWth. In addition, increasing the injection rate (46 to 66 kg/s) lowers fluid loss rate but brings forward thermal breakthrough by 9 years and causes a 41 K temperature drop at the end of operation. Notably, under high fluid loss, the dynamic response pattern of heat production power shifts from a temperature-dominated “stable–breakthrough–decline” mode to a novel “rising–breakthrough–decline” mode jointly governed by both production temperature and flow rates. These findings provide theoretical support and engineering guidance for improving EGS performance.

Extraction of high-temperature geothermal resources from reservoirs is a challenge due to the complex interactions between temperature, permeability, and stress fields. Variations in physical fields are critical to thermal reservoir engineering. In this study, the coupling mechanism between temperature, permeability, and stress fields is systematically explored using theoretical analyses and numerical simulations, by using the three-field coupling model. This study models the changes in porosity and permeability evolution during geothermal extraction, emphasizing the importance of the coupling term. The effects of pressure differences between injection and production wells, reservoir temperature variations, and fluid property changes on the temperature distribution and output thermal power within the geothermal reservoir were modeled and analyzed. The results reveal the significant effect of pressure differences between wells, and show that the geothermal extraction capacity of supercritical carbon dioxide (SC-CO2) is 1.83 times higher than that of water. Based on the statistical characteristics of the distribution pattern of natural fractures within the geothermal reservoir, the study simulated the distribution of natural fractures and developed a coupled thermo-hydro-mechanical (THM) model containing natural fractures. The results show that increasing the porosity of hydraulic and natural fractures can effectively increase the geothermal production capacity, especially the increase in the porosity of natural fractures is significant. These findings provide a key theoretical basis for understanding the THM coupling mechanism during geothermal extraction, and provide substantial support for optimizing the development and use of geothermal resources.