Caving-enhanced geothermal system overwhelms fracturing- and pipe-enhanced geothermal system in heat extraction

Abstract

Fracturing a geothermal reservoir of hot dry rock (HDR) by the engineered stimulation is a prerequisite to efficient heat extraction of an enhanced geothermal system (EGS). However, the heat extraction performances of different EGSs employing distinct stimulation strategies remain unclear, resulting in the conundrum that which stimulation mode is optimal for heat extraction. Here, we numerically simulated the reservoir response to different stimulation modes through varying fractures/pipes assemblages, and examined the hydraulic and thermal processes and associated heat extraction performances. We found that the caving-EGS (C-EGS) with an adequately fractured reservoir yields the best heat extraction performance. Fractures/pipes in geothermal reservoirs of the fracturing-EGS and pipe-EGS inevitably produce preferential flow paths, which accelerate thermal drawdown and shorten the operation lifespan. Increasing connected/unconnected fractures can weaken the effect of preferential flow paths, postpone thermal drawdown, and therefore lead to a more remarkable heat extraction rate. Caving the geothermal reservoir into suitable-sized blocks is the most effective stimulation in maximizing heat extraction efficiency, and thus, the C-EGS owns great potential in geothermal energy commercialization. Our key findings facilitate the optimization of geothermal reservoir stimulation for large-scale HDR exploitation.