Experimental investigation on the evolution of physical and mechanical properties of hot dry rocks under elevated temperature and acidification conditions

Characterizing the evolution of the physical and mechanical properties of hot dry rock (HDR) under conditions of elevated temperature and acidification is essential for assessing the heat extraction rate and enhancing the stability of HDR reservoir in the context of enhanced geothermal systems. To investigate these phenomena, some experiments were conducted to assess changes in pore permeability and thermal conductivity following heat treatment at various temperatures, as well as triaxial loading tests after exposure to high temperature and acidification. The study separately examined the evolutions of pore permeability, thermal conductivity, deformation, damage, and compressive strength of HDR under these conditions. The results indicate that as heat treatment temperature increases, pre-existing defects in granite samples expand and new defects form, resulting in progressive increases in porosity and permeability. When temperatures exceed 300 °C, thermal conductivity decreases significantly, revealing a critical temperature range where pronounced alterations in thermal properties occur. Between 350 °C and 450 °C, the elastic modulus and compressive strength undergo substantial deterioration, indicating a threshold beyond which structural integrity is severely compromised. Acid corrosion primarily affects granite through selective dissolution and degradation of surface layers. With prolonged exposure, both elastic modulus and compressive strength decrease. However, due to granite’s inherently low porosity and permeability, acid penetration remains superficial. Consequently, bulk strength reduction following acid treatment is limited. Progressive microstructural damage occurs on dry-heated granite surfaces with rising temperatures. Thermal stress propagates both natural and secondary cracks, with microstructural deterioration becoming particularly evident at 450 °C. This explains the significant changes observed in pore permeability, thermal conductivity, and mechanical properties within this critical thermal threshold.