Shear-induced flow channels in a single rock fracture and their effect on solute transport

Abstract

The effect of mechanical shearing on fluid flow anisotropy and solute transport in rough rock fractures was investigated by numerical modeling. Two facing surfaces of a rock fracture of 194mm×194mm in size were laser scanned to generate their respective digital profiles. Fluid flow through the fracture was simulated using a finite element code that solves the Reynolds equation, while incremental relative movement of the upper surface was maintained numerically to simulate a shearing process without normal loading. The motion of solute particles in a rough fracture undergoing shear was studied using a particle tracking code. We found that shearing introduces anisotropy in fracture transmissivity, with a greatly increased flow rate and particle travel velocity in the direction perpendicular to the shearing direction. Shear-induced channels yield a transport behavior in which advection dominates in the direction parallel with shear and dispersion dominates in the direction perpendicular to shear. The shear-induced flow channels not only increase the flow connectivity, but also the transport connectivity in the direction perpendicular to shear. This finding has an important impact on the interpretation of the results of coupled hydromechanical and tracer transport experiments for measurements of hydraulic and transport properties of rock fractures.