A pore–scale investigation of surface roughness on the evolution of natural fractures during acid dissolution using DBS method

Natural fractures are the essential pathways by which water, hydraulic fracturing fluids, oil and gas, reactants, and contaminants can travel in the porous media. To fully understand subsurface processes relevant to the fluid transport in natural fractures, the Darcy–Brinkmann–Stokes (DBS) pore–scale method was employed to investigate the effects of fracture surface roughness on the reactive transport and dissolution mechanisms of porous media with natural fractures. The advection–diffusion equation described the transport of reactants, and the incorporated mass balance equation dynamically tracked the fracture evolution during acid dissolution. The model has been developed based on the OpenFOAM platform, a CFD open–source code. The partial differential equations were discretized by the Finite Volume Method (FVM) on the structured grids. We studied the dissolution mechanisms, effective surface area, and normalized porosity–permeability relationship on different surface roughness. An Indiana limestone sample with different calcite content was investigated to elucidate the impacts of unreacted minerals on acidizing process. We categorized the solid dissolution of fractures into three dissolution patterns regarding the second Damkohler number and the Peclet number—face dissolution, front dissolution, and edge dissolution—, where the front dissolution pattern and the face dissolution pattern had the shorter time to consume up the rock in the fractured system. The quantified pore–scale parameters obtained from this study are expected to improve the accuracy of Darcy–scale models. The microscopic insights from the present study can guide effective operating parameters and enhance hydrocarbon production by optimizing the acidization process.