A thermo-hydro-mechanical model to evaluate the seismic properties of geothermal reservoirs

Abstract

Fractured-vuggy thermal reservoirs with complex pore spaces (stiff pores, cracks, and fractures) are typical geothermal resources for development and utilization in China. The cyclic recovery of such thermal reservoirs involves a complex thermo-hydro-mechanical (THM) coupling process. Insights into the thermoelastic effects of heating-cooling cycles on the seismic response have great potential for seismic monitoring in the cyclic recovery, which remains largely unaddressed in the literature. We intend to fill this gap by applying the double-porosity thermoelasticity theory to interpret ultrasonic measurements on granite under water-cooling conditions. We consider an isotropic porous host embedded with fractures. A plane-wave analysis yields the classical P and S waves and three slow P waves, namely the slow (Biot) P1, the slow (Biot) P2, and a thermal P. We investigate the combined effect of temperature, porous structure, and pore fluid on the thermoelastic properties of the THM process for typical granite reservoirs that experience a cold-shock process. Fractures provide the main channels for heat exchange and fluid flow. Our THM thermoelastic model describes the reservoir properties as a function of temperature associated with thermal-induced cracking, where fracture porosity is more important than the stiff (host) pore to describe the reservoir quality. We find that the thermal conductivity and specific heat have negligible effects in the seismic frequency band for the temperature range of less than 400°C, whereas the crack density significantly affects the seismic response in the heating-cooling cycles because of the additional contribution of thermal- and cold-shock-induced cracks. We further determine that the P-wave velocity and attenuation due to thermal effects under water cooling offer an important index to monitor the thermal-induced cracking and operation efficiency of the enhanced geothermal system. The THM thermoelastic model lays the foundation for active (or passive) seismic monitoring of the cyclic recovery of thermal reservoirs.