Channeling: A New Pore-Scale Dissolution Regime for Complex Porous Media

Abstract:

The current conceptual model of mineral dissolution in porous media is comprised of five dissolution regimes - or patterns - that develop depending on the relative dominance of flow, molecular diffusion and reaction rate during dissolution. These regimes are compact dissolution, conical wormholes, dominant wormholes, ramified wormholes and uniform dissolution. Here, we use numerical simulations on a range of stochastically generated 2D models of pore structures to show the existence of a sixth dissolution regime, called channeling, where already existing fast flow pathways are preferentially widened by dissolution. Channeling occurs only in the structurally complex cases, where the wide variance in pore size results in orders of magnitude differences in flow rate for different flow pathways. In these cases, the structures do not dissolve uniformly or develop into ramified wormholes, as is predicted by the current conceptual model, but instead dissolve only the structure lining the dominant flow pathways, leading to the formation of large channels. This focusing of dissolution and flow along only dominant pathways induces an immediate order of magnitude change in permeability with a comparatively small change in porosity. This work demonstrates that our current conceptual model of dissolution regimes is incomplete and must be modified to include channeling for accurate prediction of dissolution in applications such as geologic carbon storage and geothermal energy production.