Challenges in reactive transport modeling for prediction of geometry evolution in fractured carbonate rocks

Abstract:

Carbonate minerals are common in sedimentary rocks including in formations that serve as caprock seals. These formations are intended to stop migration of injected fluids, such as CO₂ in the context of geologic carbon sequestration, ensuring permanent isolation from the atmosphere. Fractures in caprocks may allow injected CO₂ and pressurized brine to escape. If the caprock contains substantial amounts of carbonates, flow of acidified fluids may cause substantial mineral dissolution which would increase the leakiness over time. Our research seeks to understand this process with particular attention to the evolution of fracture geometry and the implications for flow permeability. Our work combines high-pressure core flow experiments, x-ray imaging methods, reactive transport modeling, and computational fluid dynamics simulations. We have found that fracture permeability can increase substantially as a result of calcite dissolution. However, the extent of permeability increase is affected by complex alterations in fracture geometry. Newly-formed surface roughness and microporosity diminishes flow relative to what would be predicted by conventional practical models such as the local cubic law model. In contrast, channelization could lead to higher-than-expected flow rates because such fractures would stabilize open flow paths against geomechanical closure forces. Modeling these processes requires fine-scale 2D, if not 3D, reactive transport flow models that simulate not only the increase in fracture aperture but also the evolution in fracture geometry. Development of computationally-tractable reactive transport models that accurately predict reaction-induced changes in fracture permeability is an ongoing research priority in our lab.