Evolution of Fracture Aperture under Combined Effect of Stress and Flow

Abstract

Fracture apertures may decrease or increase by different mechanical and chemical mechanisms when the fractures are subject to stress and flow. A model is presented to describe evolution of fracture aperture mediated by dissolution and precipitation. The model accounts for the fact that dissolved minerals carried by flowing water along the fracture can not only diffuse into and out of the adjacent rock matrix but also at first diffuse into the stagnant water zone existing in part of the fracture plane and then from there into and out of the rock matrix adjacent to it. This simple model allows us to gain some insights into which processes and mechanisms have the larger impact on the fracture aperture under different circumstances. The analytical solution in Laplace domain is used to study fracture closure/opening rate in a pseudo steady state procedure. It is found that the times involved for any changes in fracture aperture are very much larger than the times needed for concentrations of dissolved minerals to reach steady state in the rock matrix, the stagnant water zone and the flow channel. Moreover, it is shown that diffusion into the rock matrix, which acts as a strong sink or source for dissolved minerals, clearly dominates the rate of concentration change and consequently the rate of evolution of the fracture aperture.