Impact of Temperature Variations on Shaft Wall Fracture and the Mechanical Properties of Hot Dry Rock

Wellbore instability remains a critical challenge in deep drilling operations, particularly in Hot Dry Rock (HDR) reservoirs characterized by high temperatures and complex stress regimes. Unlike traditional hydrocarbon reservoirs, HDR formations experience significant thermal shock when low-temperature drilling fluids circulate, exacerbating wellbore failure risks and complicating failure mode prediction. This study investigates the evolution of wellbore failure modes under transient temperature conditions. First, a modified failure pressure evaluation model is established, incorporating thermal stress into traditional failure criteria to reveal the mode transitions. Subsequently, high-temperature triaxial compression and Brazilian splitting tests were conducted on granite cores from the Gonghe Basin, Qinghai Province, to determine the evolution of the mechanical properties. Finally, the coupled effects of in situ stress, temperature, and well trajectory on failure pressure were analyzed. Experimental results demonstrate that while HDR compressive strength increases with confining pressure, it degrades significantly with rising temperature; notably, the rock exhibits a transition from brittle to ductile behavior and strain-softening characteristics. Theoretical analysis reveals that cooling induces a reduction in fracture pressure, driving a critical shift in failure mode from shear (Mode 3) to tensile (Mode 1). These findings provide a theoretical basis for optimizing drilling fluid density and trajectory design, bridging the gap between rock mechanics experiments and field stability control in HDR development.