When water percolates through a carbonate bed containing faults or fractures, the solutional attack of dissolved CO2 starts to widen them. Dissolution of fractured carbonate rocks is often accompanied by the formation of highly localized flow paths. An example can be seen in the top panel of the figure, which shows the outlet face of a limestone fracture (marked by arrows). The two holes (about 10cm across) are dissolution channels or wormholes, which develop by steadily accumulating more of the flow and reactant; the spacing between the wormholes is much larger than their diameter. The lower left panel shows the inlet face of a fracture; here the wormhole spacing is much smaller, comparable to their diameter. Numerical simulations of fracture dissolution (lower right) show a similar weeding out of the flow paths as dissolution progresses.
Our research applies chemical engineering principles to the study of the evolution of geological formations. The aim of the present work is to study the development of competing flow paths in fractures and the coupling between fluid flow, reactant transport and chemical reactions at the surfaces.
We have simulated the dissolution of a single fracture, starting from random but spatially correlated distributions of fracture aperture. Our results show a surprising insensitivity of the evolving dissolution patterns and fluid flow rates to the amplitude and correlation length characterizing the initial aperture field. We connect the similarity in outcomes to a self-organization of the flow into a small number of channels, with the spacing between each channel determined by the length of the longest channel.
Figure 1. Outlet face from a limestone quarry in Smerdyna, Poland (top) and the inlet face of a vertical fracture from a limestone quarry in Katowice, Poland (lower left: photo courtesy of Daniel Koehn, University of Glasgow). Numerical simulations of the evolving fracture aperture (bottom right) show a reduction in the number of flow paths as dissolution proceeds (left to right). The images show results for different correlation lengths in the roughness of the initial aperture; from small (left) to large (right).