Numerical Study on the Effect of Localized Fluid Pressurization on Shear and Hydraulic Behavior of a Natural Fracture in Granite

Abstract

ABSTRACT: Mechanical and hydraulic properties of fractures are strongly affected by the exploitation of enhanced geothermal system (EGS) and unconventional reservoirs. This study attempts to reveal the evolution of shearing properties and fluid pressure heterogeneity of natural fractures, possibly caused by mud loss during drilling and hydraulic stimulation. A series of triaxial shear-flow and injection-driven shear numerical simulations were carried out using a coupled hydro-mechanical pore network method (PNM) in DEM. Fracture surface profiles were generated according to the X-ray computed tomography scanning data of open and closed joints of rock cores retrieved from a 4.2-km-deep well at the Pohang EGS site. We developed an algorithm to update the orientations of joint contacts automatically and overcome the mismatch between joint roughness and contact direction especially at large shear displacement. A refreshing fluid grids approach based on the PNM is modified by Delaunay triangulation and Kriging interpolation. The results indicate that injection has caused pressure heterogeneity in the pressurized area. Localized shear rupture could occur in the area with highly concentrated pore pressure. These characteristics also emphasize the significant cause of fluid pressure heterogeneity in rupture propagation in a fracture, likely resulting in unstable slip of critically stressed preexisting faults after small fluid pressure perturbations. 1. INTRODUCTION Mechanical and hydraulic properties of fractures are strongly affected by the exploitation of enhanced geothermal system when hydraulic stimulation is utilized to enhance the permeability of hot dry rock (Rathnaweera et al., 2020). Anthropogenic fluid injections into rock crust can generate seismicity (Guglielmi et al., 2015; An et al., 2020), which brings more challenges to exploit this clean energy. For instance, an Mw 5.5 earthquake, one of the most damaging events in South Korea, was induced by hydraulic stimulation in 4.2-km-deep granodiorite. After 2 years’ hydraulic stimulation, highly localized fluid pressurization and perturbation induced by hydraulic stimulation destabilized and activated the fault where the main shock happened (Kim et al., 2018; Gorigli et al., 2018). Many injection-driven shear tests experiments and numerical computation have been carried out to investigate injection-induced fracture instability. Ji et al. (2020, 2021) carried out triaxial shear-flow experimental tests and COMSOL numerical simulations to recover fluid pressure heterogeneity on rock fractures. Fluid pressure heterogeneity on a rock fracture may induce localized shear rupture and propagates to activate the entire fracture. Analytical models that could describe the decrease of frictional strength of a rock fracture and predict potential seismic events have been proposed (Wei et al., 2021; Ji et al., 2021).