Full Length Article
Influence of fracture roughness on shear strength, slip stability and permeability: A mechanistic analysis by three-dimensional digital rock modeling

https://doi.org/10.1016/j.jrmge.2019.12.010Get rights and content
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Abstract

Subsurface fluid injections 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. In this study, we construct numerical models using discrete element method (DEM) 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 (three-dimensional bonded quartz particle analogs) representing a mated fracture is 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 reaching 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, possibly due to the removal of asperities and redistribution of wear products, which locally reduces porosity in the dilating fracture; and (4) Increasing shear-parallel asperity wavelength reduces magnitudes of stress drops after peak strength and enhances fracture permeability, while increasing shear-perpendicular asperity wavelength results in sequential stress drops and a delay in permeability enhancement. This study provides insights into understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.

Keywords

Fracture reactivation
Fracture permeability evolution
Fracture roughness
Roughness anisotropy
Slip stability

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Chaoyi Wang obtained his BSc degree in Civil Engineering from Tongji University, Shanghai, China, in 2012. He obtained his MSc degree in Geotechnical Engineering and his PhD in Energy and Mineral Engineering from Pennsylvania State University, USA, in 2015 and 2019, respectively. He was affiliated as postdoctoral research associate in the Department of Physics and Astronomy, Purdue University, USA, since 2019. His research interests include (1) Distributed acoustic sensing for seismic monitoring; (2) Ultrasonic imaging and interpretation in the scales ranging from microstructure to geological features; (3) High fidelity modeling of rock deformation, transport behavior, and fracture propagation; (4) Developing novel tools and techniques for laboratory research and field geoscience applications; and (5) Machine learning and artificial intelligence in geological feature recognition and earthquake prediction.

Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.