The Effect of Pressure Dissolution and Precipitation on Fracture Permeability and Normal Stiffness

Abstract:

Mechanically-chemically mediated processes may significantly alter the morphology of rock fracture surfaces. These processes may occur either as diagenetic mechanisms over geologic timescales under in situ-conditions, or during man-made engineering processes, during which injected fluids and/or induced temperature changes can significantly accelerate these processes. Numerical simulations at the grain scale have been conducted to predict the changes in normal stiffness and transmissivity of fractures under the combined processes of pressure dissolution and free-face precipitation. The ensuing compaction mechanism is characterized by dissolution of asperities under contact, and subsequent re-precipitation of the dissolved mass over the adjacent free surfaces. The normal stiffness of the fracture increases over time, due the increase in total contact area, and an increase in the number of regions in contact. The resultant stiffness curves reflect two regimes. At low loads, contact occurs primarily over the dissolved and precipitated, smoothened surface contact region, leading to rapid, exponential-like stiffening. At high loads, previously free-surface regions are brought into contact, and their unaltered rough nature results in the traditional, linear stiffening with increasing compression. The transition between the two contact regimes is approximately given by the confining pressure acting during the compaction process. During the compaction process, a steady decline of the hydraulic transmissivity is observed, due to both the decrease in mean aperture, and the increased tortuosity caused by the additional contact regions, up to the point at which the contact zones percolate and effectively seal the fracture hydraulically. The remaining fracture porosity is hydraulically ineffective, but may be as high as a third of the initial value. It follows that both the magnitude and nature of the predicted stiffness curves differ fundamentally from those observed for freshly induced fractures used in laboratory experiments.