Study on Mechanical Properties and Synergistic Evolution Mechanism of Damage‑Permeability for Hot Dry Rock Under Thermal Shock Effect

ABSTRACT Thermal shock serves as a potential stimulation method for hot dry rock (HDR); however, the mechanisms through which it induces reservoir damage and improves permeability remain to be thoroughly investigated. To address this issue, laboratory mechanical tests under gradient thermal shock temperatures ranging from 300°C to 700°C are carried out; a thermo‐hydro‐mechanical‐damage (THMD) coupled numerical model is established; the synergistic evolution mechanisms of fracture propagation and permeability in HDR under thermal shock are analyzed; and relevant strategies and directions for optimizing HDR reservoir stimulation technologies are further discussed. Results show that the peak strength and elastic modulus of the sample continuously decline as the thermal shock temperature increases, following a three‐stage evolution pattern: slow decline, rapid decline, and slow decline. The temperature range of 400–600°C is the threshold interval with the most severe deterioration in mechanical parameters, where peak strength decreases by 36.22% and elastic modulus drops by 47.82%. The dominant driving factor is volume expansion induced by the α‐to‐β quartz phase transition, which accelerates the initiation and coalescence of internal fractures. Reservoir temperature, reservoir peak strength, and injection pressure show an exponential correlation with permeability, whereas injection temperature shows a linear correlation. The significance of each factor influencing permeability ranks in descending order: injection pressure, peak strength, reservoir temperature, and injection temperature. In practical engineering applications, the composite stimulation scheme that combines high‐pressure hydraulic fracturing to form primary fractures with low‐temperature fluid thermal shock to generate microcracks can provide theoretical support for the construction of HDR‐enhanced geothermal systems.