Geochemical alterations of fractures and the environmental and policy implications

Abstract

Fractures provide preferential pathways for transport of environmentally-relevant fluids and solutes in subsurface energy, waste disposal and groundwater management. Fracture geometries and hydrodynamic properties are subject to geochemical alterations. Acid-driven geochemical alterations of fractures are of special interest because when acidic fluids are introduced, their interactions with minerals, especially carbonates that are abundant in various geological environments can result in substantial geometrical and hydraulic changes in a short time frame. Deeper knowledge of geochemical alterations of fractures is critical for accurate projections of fluid migration in subsurface fractured media and the ultimate fate of substances of interests.

This dissertation is dedicated to investigating acid-driven geochemical alterations of fractures and their environmental and policy implications. The findings of the simulation and experimental studies at core scale under conditions relevant to geologic carbon sequestration demonstrated the importance of mineralogy and influent chemistry in controlling fracture alteration. In presence of mineral spatial heterogeneity, CO2-brine-rock interactions can lead to complex fracture geometry alterations. Consequently, fracture permeability is enhanced by mineral dissolution, but the extent of increase is less-than-expected because of increase in fracture surface roughness resulted from preferential dissolution of calcite. In contrast, channeling in carbonate fractures leads to rapid increase in fracture permeability despite the reduced overall reaction. This alteration is affected by influent chemistry, as higher reactivity of the fluid results in stronger channeling. Additionally, a two-dimensional reactive transport model was developed to simulate channelization, which improves our capability of predicting fracture alterations. Collectively, the findings provide important insights on upscaling of reaction and permeability evolution caused by sub-grid scale fracture alterations, which are needed for large-scale simulations.

The dissertation also includes reservoir scale analyses (Chapter 7 and 8). The results highlighted the inherent lateral and vertical variability of subsurface hydrodynamic properties, and provided statistics valuable for reservoir scale models. In addition, from an economic and policy perspective, it was demonstrated that ensuring low permeabilities of potential leakage pathway is critical for CCS deployment in our energy system.