Pore structure and crack characteristics in high-temperature granite under water-cooling

Abstract

The changes of porosity and microcracks of high-temperature granite under water-cooling have a great influence on the safety and efficiency of geothermal resources exploitation. To analyze the formation conditions of the pore structure and microcracks of granite specimens at different temperatures under water-cooling, the rock porosity was measured by NMR, and the rock microstructure was observed by PM and SEM. The results show that the closure of microcracks occurs at 150 °C and the grain boundary microcracks appear at 300 °C. And the porosity and pore size begin to increase, the number of pores remains stable, and intracrystalline microcracks appear at 450 °C. When T = 600 °C, macropores and trans-granular microcracks appear inside the rock. When 600 °C < T ≤ 750 °C, the increase rate of porosity decreases, the number of macropores increases, the number of micropores decreases, and the total number of pores increases exponentially. The porosity and the number of macropores increase rapidly, and the aperture of transgranular microcracks increases after T = 750 °C. The change of granite microstructure leads to the decrease of thermal conductivity, which decreases with the increasing temperature. During geothermal exploitation, the thermal conductivity of dry hot rock can be characterized by porosity, and the relationship is K=C + D × Ex.