Pore Structure and Damage Evaluation of Hot Dry Rocks in Enhanced Geothermal System by Combining Electrical Resistivity, Ultrasonic Waves and Nuclear Magnetic Resonance

A liquid nitrogen cold-shock fracturing method is proposed to resolve the problem of single heat-exchange cracks generated by hydraulic fracturing in an enhanced geothermal system. A complex crack network can be constructed from the great thermal difference between cryogenic fluids and hot rocks. In this work, damage characterization that combined resistivity, ultrasonic wave, and nuclear magnetic resonance (NMR) was undertaken. The matrix continuity was analyzed ultrasonically, the fracture channel was analyzed by resistivity, and the pore structure was analyzed by NMR. The high-temperature granite damage after liquid nitrogen cold-shocking was evaluated quantitatively. The ultrasonic velocities of the granite cores decreased with high temperature, especially over 400 °C. The resistivity decreased and the largest drop was 81.86% for granite C at 600 °C. According to the NMR T 2 spectra, liquid nitrogen cold-shocking expanded the pore volume and increased the pore number. The porosity after cold-shocking increased exponentially with heating temperature, where the porosities of granites A, B, and C reached 7.21%, 4.29%, and 3.10%, respectively. The damage variables calculated by wave velocity and resistivity increased with heating temperatures. The correlation of porosity with resistivity and wave velocity was established by multiple linear regression. Liquid nitrogen cold-shocking induced a developed pore-fracture network in hot dry rocks.