Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system (EGS). This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin, Qinghai, China. The granite samples were heated to 400 degrees C and then cooled in air, water, or liquid nitrogen. The physical and mechanical properties of the thermally treated granite were evaluated, and microstructural changes were analyzed using a scanning electron microscope (SEM) and computed tomography (CT). The results indicate that cooling with water and liquid nitrogen significantly enhances permeability and brittleness while reducing P-wave velocity, strength, and Young's modulus. Specifically, liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreated samples, while reducing compressive strength by 13.6%. After thermal treatment, the failure mode of the granite shifted from axial splitting to a combination of shear and tension. Microstructural analysis revealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooled with water or air. Additionally, acoustic emission (AE) monitoring during damage evolution showed that liquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate, with numerous AE signals detected during both stable and unstable crack growth. The results suggest that liquid nitrogen induces a stronger thermal shock, leading to more significant thermal damage and promoting the development of a complex fracture network during EGS reservoir stimulation. This enhances both the heat exchange area and the permeability of the deep hot dry rock (HDR) in EGS reservoirs. The insights from this study contribute to a deeper understanding of thermal damage characteristics induced by different cooling media and provide valuable guidance for optimizing deep geothermal energy extraction. (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 license (http://creativecommons.org/licenses/by/4.0/).

Accurate prediction of hydraulic fracture propagation is vital for Enhanced Geothermal System (EGS) design. We study the first hydraulic fracturing job at the GR1 well in the Gonghe Basin using field data, where the overall direction of hydraulic fractures does not show a delineated shape parallel to the maximum principal stress orientation. A field-scale numerical model based on the distinct element method is set up to carry out a fully coupled hydromechanical simulation, with the explicit representation of natural fractures via the discrete fracture network (DFN) approach. The effects of injection parameters and in situ stress on hydraulic fracture patterns are then quantitatively assessed. The study reveals that shear-induced deformation primarily governs the fracturing morphology in the GR1 well, driven by smaller injection rates and viscosities that promote massive activation of natural fractures, ultimately dominating the direction of hydraulic fracturing. Furthermore, the increase of in situ differential stress may promote shear damage of natural fracture surfaces, with the exact influence pattern depending on the combination of specific discontinuity properties and in situ stress state. Finally, we provide recommendations for EGS fracturing based on the influence characteristics of multiple parameters. This study can serve as an effective basis and reference for the design and optimization of EGS in the Gonghe basin and other sites. (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/).

Fluid flow in fractures controls subsurface heat and mass transport, which is essential for developing enhanced geothermal systems and radioactive waste disposal. Fracture permeability is controlled by fracture microstructure (e.g. aperture, roughness, and tortuosity), but in situ values and their anisotropy have not yet been estimated. Recent advances in geophysical techniques allow the detection of changes in electrical conductivity due to changes in crustal stress and these techniques can be used to predict subsurface fluid flow. However, the paucity of data on fractured rocks hinders the quantitative interpretation of geophysical monitoring data in the field. Therefore, considering different shear displacements and chemical erosions, an investigation was conducted into the hydraulic-electric relationship as an elevated stress change in fractures. The simulation of fracture flows was achieved using the lattice Boltzmann method, while the electrical properties were calculated through the finite element method, based on synthetic faults incorporating elastic-plastic deformation. Numerical results show that the hydraulic and electrical properties depend on the rock's geometric properties (i.e. fracture length, roughness, and shear displacement). The permeability anisotropy in the direction parallel or perpendicular to the shear displacement is also notable in high stress conditions. Conversely, the permeability -conductivity (i.e., formation factor) relationship is unique under all conditions and follows a linear trend in logarithmic coordinates. However, both matrix porosity and fracture spacing alter this relationship. Both increase the slope of the linear trend, thereby changing the sensitivity of electrical observations to permeability changes. (c) 2024 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/).