Effect of chemical corrosion on propagation of complex fracture networks under different hydraulic pressures in enhanced geothermal systems

Abstract

Chemical corrosion and hydraulic fracturing are important ways to improve the permeability of the enhanced geothermal systems. In this work, we developed a discrete element code for simulating the hydraulic fracturing of three different fracture network models after chemical corrosion. Mechanisms of the crack extension and coalescence in the field-scale models with fracture network were systematically revealed, in which the effects of corrosion time, inclination of the fracture (γ), pH value and hydraulic pressure were considered. The results show that the simulated cracking behaviors of granites agree well with those obtained through Brazilian tests and triaxial compression tests, verifying the validity of the developed discrete element code. During hydraulic fracturing, the impact of γ on the fracture network connectivity increases with the increment of hydraulic pressure (Pin). With the increase in hydraulic pressure, crack propagation accelerates whereas the connectivity of fracture network models decreases. The reasons are that the excessive hydraulic pressure causes the crack to spread in an unfavorable direction and increases the effect of γ on the connectivity of models of fracture networks. As the pH value increases from 2 to 12, the crack growth rate first decreases and subsequently raises. The numbers of cracks per time step of sparse mixed fracture networks with artificial fractures (SM), dense mixed fracture networks with artificial fractures (DM) and dense mixed fracture networks without artificial fractures (NDM) are in the ranges of 0.06601–0.1259, 0.05176–0.2072 and 0.1727–0.6846, respectively. The connectivity of SM is better than that of NDM under the same hydraulic fracturing conditions, indicating that artificial fractures can greatly improve the fracture network connectivity and the utilization efficiency of EGS.