Influence of fracture roughness on shear strength, slip stability and permeability: A mechanistic analysis by three-dimensional digital rock modeling
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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:
Asperity (geotechnical engineering)
Dilatant
Dilatancy is one of the key properties of the soil–structure interface. A number of monotonic and cyclic shear tests were analysed to derive the rules and the dilatancy mechanism of the interface between a structure and a gravelly soil; diverse influential factors were considered, such as soil type, roughness of the structure and normal stress. The volumetric change due to the dilatancy consists of a reversible and irreversible dilatancy component, each of which has different mechanisms, different rules and different control factors. The reversible dilatancy component is strongly dependent on the direction of the applied shear after an initial application of shear loading. The irreversible dilatancy component gradually increases owing to the continued application of shear and exhibits a close relationship with the evolution of the physical state. The volumetric change that is due to the dilatancy mainly depends on part of the tangential displacement which corresponds to the deformation of the soil that is constrained by the structure; such a part of the tangential displacement is significantly dependent on the state of the entire tangential displacement, on the normal stress and on the features of both the structure and the soil. The roughness of the structure, the behaviour of the soil and the normal stress have significant effects on the dilatancy behaviour of the interface.
Dilatant
Direct shear test
Soil structure
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