Pore-scale visualizing and quantifying the impact of flow rate on dissolution front in microfluidic rough fractures

Abstract:

Dissolution front influenced by reactive flow in rock fractures is closely related to many complex geochemical processes, including CO₂ sequestration, stimulation of petroleum reservoirs and formation of limestone caves. The flow rate, which controls the dissolution regimes and dissolution hotpots, is a critical factor for improving the conceptualization and prediction of flow and transport in fractured media. However, the relation between flow rate and evolution of dissolution front in rough fractures is not well understood. Here, real-time imaging experiments on dissolving processes in microfluidic rough fractures etched from sodium chloride crystals have been performed. By varying volume flow rate Q systematically, we find that the dissolution regimes and evolution of channel shapes strongly rely on the Péclet number and Damköhler number, which, respectively, measures the relative magnitude of convective and diffusive transport of the solute and relates the surface reaction rate to the mean fluid velocity. We observe that reactive fluid tend to smooth the initially secondary roughness of fracture surfaces and the final patterns of flow channels vary from trapezoid to rectangular as Péclet number increases. We also analyze the dissolution rate of fracture surfaces along the flow direction to characterize the hotpots of the dissolution process.