Deterioration of surface roughness and hydraulic transmissivity of a rock fracture due to pressure-solution compaction

Abstract:

Results of three-dimensional hydro-mechanical-chemical (HMC) pore-scale simulations of fractures in siliciclastic rocks are presented. The coupled processes of pressure-solution at contacting asperities, diffusion of dissolved mass in the pore-fluid, and precipitation-dissolution at the free pore space, are modelled in pore and contact regions defined by self-affine fracture surfaces under confining pressure. The governing equations derive from the Boussinesq solution for elastic contact between rough surfaces and diffusion-reaction equations. No constitutive relationships are assumed relating dissolution/precipitation to changes in surface geometry and contact area; these changes arise as a direct result of the simulation. Results are compared against existing conceptual models derived from pressure solution experiments to assess the effect of scale dependency, and against theoretical models to assess the effect of assuming an idealized surface geometry. Results suggest that when using realistic stress distributions, local concentration of contact pressure leads to rapid compaction at early times, compared to the lower dissolution rates obtained by applying averaged pressures. The geomechanical model confirms that the increase in contact area reduces the energy available to deform the free fracture surface, thereby reducing elastic compaction, and leading to a non-linear relationship between dissolved asperity height, pore-volume, and transmissivity. The complex effect on static friction is discussed by quantifying two contrasting phenomena: the increase in contact area over which adhesive forces act, and the concurrent reduction in surface roughness that is responsible for interlocking between the surfaces.