A coupled model for reactive flow through deformable fractures in Enhanced Geothermal Systems

Abstract

Reactive flow through fractures results in dissolution/precipitation of minerals and thus alteration in fracture apertures/opening, affecting the flow paths in the fracture. Backed by laboratory experiments, the openings in fracture due to dissolution are most likely not stable under confining stresses, resulting in closure of fractures. In this research, a novel method to couple Thermal-Hydraulic-Mechanical (THM) with Chemical (C) processes is presented, capable of capturing the aperture closure under in situ stresses during heat withdrawal from an Enhanced Geothermal System (EGS). The dissolution process of silica is considered, resulting in a relatively uniform aperture increase in the fracture prior to applying the in situ stresses. Then, the mechanical equilibrium is solved and the final apertures are computed from the updated contact stresses on fracture surfaces. Due to the matrix compliance, in most cases the closure of the apertures induced by the uniform dissolution of silica has been observed. The results are compared against a case where the mechanical equilibrium after the dissolution process is not considered (i.e. one-way coupling of THM and C). Without mechanical feedback on the dissolution apertures, the flow in the fracture is dominated by dissolution apertures, also affecting the heat production from EGS. However, after applying in situ stresses, the effect of dissolution apertures on the heat production is diminished. Depending on the compliance of the matrix, the size of the fracture and the size of dissolution opening, the stresses are redistributed to satisfy the mechanical equilibrium, affecting the aperture distribution over the fracture.