Experimental investigation of granite properties under different temperatures and pressures and numerical analysis of damage effect in enhanced geothermal system

In this work, the variation in granite mechanical property with temperature (30 °C-150 °C) under different confining stresses (0.1-60 MPa) is experimentally investigated specifically for reservoir secondary damage of hot dry rock through a series of triaxial tests. On the basis of the test results, the damage equations of elastic modulus and Poisson's ratio of granite are obtained in accordance with damage theory. These equations are programmed into the TOUGHREACT-FLAC3D software. A field scale simulation is conducted to analyze the effect of secondary damage on the final generated electricity. The results indicated that as temperature and confining pressure gradually increase, the expansion direction of fracture turns from vertical to oblique and decreases to a single fracture plane. The mechanical properties of rock are continuously weakened with the increase in temperature. The confining pressure exerts a positive influence on elastic modulus and Poisson's ratio of granite. The rock property around the discharge section of injection well is damaged significantly and is fractured because of the high temperature difference. The reservoir secondary damage decreases the reservoir temperature gradually, thereby reducing the generated power.