Simulating the evolution of fracture surface alteration exposed to CO2-acidified brine

Abstract:

Understanding the flow, transport, and reaction in fractures and the evolution of fracture geometries as a result of geochemical reactions is especially relevant to geologic carbon storage. Both natural and injection-induced fractures may be abundant and thus control fluid migration in the subsurface. A second effect is that the development of low pH fluid as the CO₂ dissolves into the native brine can alter fracture geometries and thus dominant flow pathways substantially over relatively short time scales, particularly when rapidly-reacting carbonate minerals are present. Existing experimental studies performed under conditions relevant to geologic carbon storage have shown complex dissolution patterns, which depend on the flow regimes and spatial distributions of reactive minerals. One of the dissolution patterns observed is the formation of a porous altered layer in the near-fracture region that is created by preferential dissolution of a reactive phase (e.g. calcite) dispersed in the rock matrix. However, there is still a lack of predictive understanding of this phenomenon and an even more limited ability to predict how the altered layer may influence subsequent evolution of the fracture. In this study, we present a reactive transport model that captures and predicts the development of the altered layer when the fracture surfaces are exposed to CO₂-acidified brine. The model explicitly accounts for permeability heterogeneity caused by initial fracture aperture variations, and updates fracture apertures and the porosity of rock matrix in the near-fracture region based on local reactions. The simulation results lend important insights into the factors that control the evolution of the spatial distribution and thickness of the altered layer. This altered layer in turn affects flow distribution in the fracture and formation of preferential flow channels. It also has an impact on the mass transport between the fracture and the rock matrix, the accessibility of reactive minerals, and the reactions that take place subsequently. Furthermore, the model provides valuable information for future studies on how the altered layer will affect fracture geomechanical properties and the responses of the altered fractures to confining pressures that are often present in the deep subsurface.