Effect of Sequential Heating-Cooling Cycles on the Thermomechanical Behavior of Utah FORGE and Sierra White Granites

ABSTRACT: The Core samples extracted from reservoirs are often used to determine thermo-poroelastic properties as well as to estimate the in-situ stress. The rock samples used experience significant cooling and so the impact of thermally induced mechanical changes need to be considered in these measurements. Thermal-induced microcracking of rock significantly affects geothermal reservoir rocks. Also, thermal microcracking could improve geothermal energy production. It may increase reservoir-well permeability by inducing microcracking in the rock around the injection well due to thermal expansion and contraction of preexisting natural fractures. Therefore, comprehending the behavior of thermal microcracks during heating and cooling cycles and their impact on rock properties is essential. Most laboratory studies that have considered cooling induced microcracks have not distinguished it from the effect of the heating phase. We conducted an analysis of the differences in the thermal and mechanical properties of Utah FORGE granite (quartz poor) and Sierra White granite (quartz rich) during five heating-cooling cycles: 20-60-20°C, 20-90-20°C, 20-120-20°C, 20-150-20°C, and 20-180-20°C. The triaxial stress set was 4000 psi for confining pressure and 2000 psi for axial loading. Results showed that Utah FORGE granite (UFG) experienced a significant reduction in its elastic modulus, decreasing by 12.53% at 180°C. Conversely, Sierra White granite (SWG) demonstrated a comparatively lower decrease of 4.73% in elastic modulus at the same temperature. Also, the behaviors of compressional wave velocity (Vp), shear wave velocity (Vs), and dynamic Young's modulus (Ed) were different: Utah FORGE granite had Vp, Vs, and Ed drop by 13.79%, 16.22%, and 28.96%, respectively; Sierra White granite had Vp rise by 2.11%, and Vs and Ed fall by 3.87% and 4.47%, respectively. In this paper, we examine these issues and related phenomena with reference to the specific rock textures.