Experimental investigation of thermal cycling effect on fracture characteristics of granite in a geothermal-energy reservoir

The fracture characteristics of fine-grained granite were examined for a potential geothermal-energy reservoir. The granite was thermally cycled in a furnace between 100 degrees C and 300 degrees C and its mechanical behavior and meso-crack characteristics were analyzed. The results indicate that thermal cycling leads to decreased fracture toughness (K-eff), absorbed energy (U), longitudinal wave velocity (P-v), and increased permeability (K) in granite. These changes can be explained using the thermal fatigue accumulated damage. The ability of granite to resist fracturing is greatly reduced in the first five thermal cycles. Thermal cycling is more conducive to inducing intergranular cracks. The interconnection of intragranular and intergranular cracks causes the structure of granite to become fragmented and more likely to fail. Thus, thermal cycling deteriorates the mechanical stability of fine-grained dense granite, and allows crack networks to form more easily. The correction of crack propagation critical radius (r(c)) can improve the accuracy of the modified maximum tangential stress (MMTS) predictive fracture.