A mechanical-hydraulic-solute transport model for rough-walled rock fractures subjected to shear under constant normal stiffness conditions

Abstract

The present study developed a two dimensional mechanical-hydraulic-solute transport model for rough-walled rock fractures under constant normal stiffness boundary conditions. First, the mechanical responses of a fracture during shear including the shear stress, the normal stress, the dilation and the sheared-off area of asperities at each shear step were calculated in a mechanical module. The surface roughness was characterized by two-order asperities, i.e., waviness and unevenness, and the degradation of asperities was estimated based on the principle of wear. The mechanical module was validated by comparison with laboratory experiments. The surface geometry and the dilation behavior were subsequently incorporated into a hydraulic module to estimate the hydraulic aperture and flow rate based on the cubic law. Finally, these data were input into a solute transport module to investigate the influences of the mechanical boundary conditions on the solute transport in the fracture and the matrix at different shear displacements and time. The model linked the complex mechanisms involved in the mechanical, hydraulic and solute transport processes for fractures subjected to shear, and revealed a controlling effect of normal stiffness on the transport behavior. The results show that the normal displacement, normal stress, cumulative sheared-off area and porosity of matrix change quickly in the initial stage of shear, and gradually reach some constant values when the fracture surface is sufficiently smoothed in the residual stage. In the entire shear process, the major asperities are substantially damaged, producing abundant gouge particles that contribute to the retardation of solute transport. The shear-induced dilation and the retardation induced by gouge particles from damaged asperities played competitive roles in solute transport that resulted in the nonlinear variations in the coefficient of retardation. The model established a solid platform for estimating the mechanical-hydraulic-solute transport processes in fractures subjected to shear, upon which more sophisticated modules such as pressure solution and clogging by particles may be developed to improve the understanding on the outstanding issues involved in the coupled processes.