Coupling stress and reactive transport in fractures: Effects on contacting asperities, permeability and stiffness

Abstract:

The permeability and geomechanical stability of fractured rock can be altered by reactive flow that induces mineral dissolution and/or precipitation. Understanding the coupling of geochemical and geomechanical processes is critical for predicting and identifying leakage pathways for environmentally-relevant fluids in the subsurface. This study couples a two-dimensional reactive transport model with a mechanical deformation model to simulate reaction, flow and deformation in a fractured carbonate rock under subsurface confining pressures. The fracture is represented as a homogenous calcite material subjected to high-pressure reactive CO₂-acidified brine, and the dissolution reaction is modeled to be kinetically-limited by carbonic acid. Initial conditions for the simulations were based on fractured Indiana Limestone geometries obtained from xCT data. Simulation of reactive flow results in the enlargement of apertures and reduction in contact area along preferential flow paths, while apertures outside these channelized flow paths remain relatively unchanged. At high confining pressures, contact area occurred mainly in regions where channelization did not occur, resulting in a two- to three-fold reduction in the fracture specific stiffness. Moreover, at high confining stresses, channelized regions were preserved, enabling permeability to remain relatively unchanged compared to non-channelized regions, which in contrast showed an order of magnitude decrease in permeability when stressed. These simulations suggest that differences in dissolution patterns can lead to significant variations in fracture permeability and stiffness when subject to subsurface confining stresses. This work has important applications for geologic carbon sequestration, natural gas storage, hydraulic fracturing, geothermal energy and deep well injection of hazardous waste.