Degradation mechanism of crystalline rock mechanical properties under thermal-hydraulic-mechanical-chemical multifield coupling

Crystalline rock formations are commonly encountered in the deep stages of ultra-deep geothermal drilling. The degradation of their mechanical properties under the coupled effects of thermal, hydraulic, chemical, and stress fields exerts a significant impact on wellbore stability, operational safety, and efficiency. This paper elaborates on the current status of ultra-deep drilling and its downhole environmental conditions, while analyzing the mechanical characteristics of crystalline rocks. On this basis, the paper further clarifies the action mechanisms of single-field factors (including thermal field, seepage field, chemical field and stress field) during ultra-deep drilling, as well as the influence pathways of more complex multi-field coupling effects on the mechanical properties of crystalline rocks. Focusing on typical crystalline rocks (granite and gneiss), this study summarizes the microstructural evolution laws and macroscopic mechanical response behaviors under thermal shock, seepage intrusion, chemical corrosion, and stress disturbance. It also generalizes the degradation characteristics of key mechanical parameters such as strength, fracture toughness, and elastic modulus, and compares the degradation mechanisms between granite and gneiss. It is explicitly clarified that in ultra-deep drilling, high temperatures can induce the initiation of microcracks inside crystalline rocks and further promote their propagation; fluid intrusion not only exacerbates the dissolution of primary minerals but also facilitates the precipitation of secondary minerals; stress redistribution resulting from drilling-induced perturbations further promotes fracture connectivity and the formation of macroscopic failure surfaces; and multi-field synergy drives the transition of rocks from a brittle to a ductile deformation mode. Existing theories have limitations in terms of multi-field coupling mechanisms, coverage of crystalline rock types, and the long-term effects of actual drilling fluids. Future research urgently needs to strengthen fully coupled thermal-hydraulic-mechanical-chemical experiments and simulations, develop a “fully coupled thermal-hydraulic-mechanical-chemical numerical model” capable of accurately describing cross-scale damage processes and a “digital twin wellbore model” integrated with real-time drilling data, and propose an “active wellbore stability control strategy” focusing on multi-field coupling mechanisms. Ultimately, within a multi-scale and multi-field coupling framework, this study aims to establish a crystalline rock wellbore stability evaluation and degradation control method suitable for deep underground environments, providing solid theoretical support for in-depth understanding of the mechanical behaviors of crystalline rocks under multi-field coupling conditions and the safe and efficient implementation of ultra-deep drilling.