Competition between cooling contraction and fluid overpressure on aperture evolution in a geothermal system

Abstract

Comprehensive understanding of aperture evolution of HDR (hot dry rock) fracture is critical for enhancing permeability in a geothermal system. Coupled HM (hydro-mechanical) and THM (thermo-hydro-mechanical) simulations are conducted to elucidate the competition between cooling contraction and fluid overpressure in fracture aperture enhancement during heat extraction. The effects of critical factors, including fracture geometry, reservoir properties, and injection settings on this competitive relationship, are systematically examined. We find that (1) fracture aperture is dominated by the competing effect of cooling contraction and fluid overpressure; the cooling-induced aperture grows as the injection time increases, declining the overpressure-induced aperture; (2) high thermal expansion coefficient and low fracture normal stiffness tremendously enlarge total and cooling-induced apertures and mildly shrink the overpressure-induced aperture; (3) decreasing fracture spacing and increasing fracture length remarkably improves the total aperture; the former increases the cooling-induced aperture while the latter enhances the overpressure-induced aperture; (4) the reduction of injection temperature increases the cooling-induced aperture and lowers the overpressure-induced aperture; the growth of flow rate expands both the cooling-induced and overpressure-induced apertures. We thus unveiled the mystery of the invisible aperture evolution during heat extraction. The findings largely benefit the optimization of mining parameters of a geothermal reservoir.