Effect of Cyclic Thermal Shock on Granite Pore Permeability

Abstract

\n Geothermal development requires an understanding of changes in pore permeability caused by repeated thermal shock fatigue damage in hot dry rock. Research on the subject can aid the evaluation of the longevity and mining value of enhanced geothermal systems (EGS). However, few relevant studies are currently available. In this study, the change characteristics of pore permeability in granite following different heating–cooling cycle temperatures (250°C, 350°C, 450°C, 550°C, and 650°C) and numbers of cycles (1, 5, 10, 15, and 20 cycles) were analyzed. Results show that with increasing temperature, the uneven thermal expansion and thermal shock effect of minerals promote crack development, leading to increases in the porosity and permeability of granite, particularly at temperatures above 450°C. When the heat treatment temperature was below 450°C, the number of cycles only slightly affected the porosity and permeability; meanwhile, when the temperature exceeded 450°C, the porosity and permeability increased significantly with an increase in the number of cycles. Moreover, three-dimensional nonlinear fitted relationships among porosity (or permeability), cycle temperature, and number of cycles have been established for the first time with correlation coefficients (R2) above 0.9, which reveals the change rules of pore permeability after quenching in hot dry rock. The results can be used to evaluate the efficiency of geothermal reservoir energy extraction and aid in geothermal reservoir design.