Predicting dissolution patterns in variable aperture fractures: Evaluation of an enhanced depth-averaged computational model

Water-rock interactions within variable aperture fractures can lead to dissolution of fracture surfaces and local alteration of fracture apertures, potentially transforming the transport properties of fractures over time. Because fractures often provide dominant pathways for subsurface flow and transport, developing models that effectively quantify the role of dissolution on changing transport properties over a range of scales is critical to understanding potential impacts of natural and anthropogenic processes. Dissolution of fracture surfaces is controlled by surface reaction kinetics and transport of reactants and products to and from the fracture surfaces. We present a depth-averaged model of fracture flow and reactive transport that explicitly calculates local dissolution-induced alterations in fracture apertures. The model incorporates an effective mass transfer relationship that represents a smooth transition from reaction-limited dissolution to transport-limited dissolution. We evaluate the model through direct comparison to previously reported physical experiments in transparent analog fractures fabricated by mating an inert, transparent rough surface with a smooth single crystal of potassium dihydrogen phosphate (KDP). These experiments allowed direct measurement of fracture aperture during dissolution experiments using well-established light transmission techniques. Comparison of experiments and simulations at different flow rates demonstrates the relative impact of the dimensionless Peclet and Damkohler numbers on fracture dissolution and the ability of the computational model to simulate dissolution. Despite some discrepancies in the small-scale details of dissolution patterns the simulations predict the evolution of large-scale features quite well for the different experimental conditions. This suggests that the depth-averaged approach is useful for modeling fracture dissolution in the context of geological processes and applied problems such as CO2 sequestration and fracture acidization.