Wang, Chaoyi (The Pennsylvania State University / Purdue University) | Elsworth, Derek (The Pennsylvania State University) | Fang, Yi (Institute for Geophysics / Jackson School of Geosciences / The University of Texas at Austin) | Zhang, Fengshou (Underground Engineering of Ministry of Education / Tongji University )
ABSTRACT: Subsurface fluid injection can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures. Laboratory studies indicate that fracture rheology and permeability in such reactivation events are linked to the roughness of the fracture surfaces. We construct discrete element method (DEM) models to explore the influence of fracture surface roughness on the shear strength, slip stability, and permeability evolution during such slip events. For each simulation, a pair of analog rock coupons (3D bonded quartz-particle analogs) representing a mated fracture are sheared under a velocity-stepping scheme. The roughness of the fracture is defined in terms of asperity height and asperity wavelength. Results show that (1) samples with larger asperity heights (rougher), when sheared, exhibit a higher peak strength which quickly devolves to a residual strength after a threshold shear displacement; (2) these rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products; (3) long-term suppression of permeability is observed with rougher fractures, which is plausibly due to the removal of asperities and redistribution of wear products, which locally reduces porosity in the dilating fracture. This study provides insights into the understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.
Subsurface fluid injections such as hydraulic fracturing, carbon sequestration and geothermal stimulation involve injecting large volumes of fluid at high overpressures, therefore disturbs the stress field by elevating pore pressure and altering far-field stress (Elsworth et al., 2016), potentially resulting in the reactivation and potential seismic rupture of pre-existing faults and fractures. These hydraulic fractures, while creating possibility to extract hydrocarbon resources from tight shale, can be extremely vulnerable to seismic failure upon stress perturbation (Ellsworth, 2013; Walsh & Zoback, 2015; Zoback & Gorelick, 2012), causing hazardous consequences. One key question in understanding the seismic cycle is in unraveling the evolution of shear strength, stability, and permeability of fault and fractures that may contribute to dynamic slip events.