Investigation of Shear Failure and Seepage Behavior of Fractured Hot Dry Rock Induced by Water Injection

China holds a significant portion of the world’s hot dry rock (HDR) resources, accounting for about one-sixth of the global total. Its large-scale development is crucial for achieving national carbon peaking and carbon neutrality targets. Since deep HDR typically consists of hard, dense granite, reservoir stimulation techniques like hydraulic fracturing and shearing are needed to create conductive fracture networks, enhance fluid flow, and establish an Enhanced Geothermal System (EGS). During hydraulic stimulation, water injection can induce shear slip along fractures, which increases permeability. The mechanisms behind shear dilation and seepage in high-temperature and high-pressure conditions remain a key challenge. This study focuses on granite, conducting experiments and numerical analyses on compressive, tensile, shear, and water injection-induced shear slip tests on samples that are heated to various temperatures (25°C, 200°C, 400°C, and 600°C) and then cooled to room temperature. The influence of thermal treatment on granite’s mechanical properties and permeability evolution before and after shear slip is investigated. The main research conclusions are as follows:
(1) Compressive, tensile, and shear tests were conducted on granite blocks treated at 25–600°C. Results show that high temperature significantly weakens the mechanical properties of granite, including uniaxial compressive strength, tensile strength, internal friction angle, and cohesion, with the degradation rate accelerating as temperature increases. From room temperature to 600°C, compressive and tensile strengths dropped from 150.79MPa and 5.34MPa to 60.77MPa and 1.19MPa, respectively—decreasing by 59.7% and 78%. Shear strength exhibited nonlinear changes with temperature. At 600°C, the cohesion and internal friction angle of intact granite decreased by 24.87% and 7.9°, respectively, while the cohesion of straight fractured granite dropped nearly to 0MPa.
(2) The water injection induced shear slip experiment showed that with the increase of water pressure, the shear slip process of fractured rock mass can be divided into four stages: elastic deformation stage, stable slip stage, periodic stick-slip stage and instability failure stage. In the periodic stick-slip stage, the shear stress of the fractured granite decreases instantaneously and recovers rapidly, accompanied by a significant increase in shear displacement. The shear displacement gradually increases with the increase in the number of slips, until it enters the instability failure stage, which is manifested by a sharp increase in shear displacement but the shear stress cannot be quickly recovered after it decreases. Further research found that the water pressure required for shear slip in high-temperature fractured rock mass gradually increases with the increase in treatment temperature. The water pressure at the first slip point at 25°C is 1.8MPa, while it rises to 4.5MPa at 600°C, an increase of about 150%.
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