Geologic carbon sequestration in deep saline aquifers: Brine acidification and geochemical alterations of reactive leakage pathways

Of all geologic storage options, CO2 injection into deep saline aquifers offers the largest potential storage opportunity, as these formations are nearly ubiquitous throughout the globe and have little to no economic value. However, CO2 dissolution into formation brines will lead to acidification, which may promote acid-catalyzed mineral dissolution in both the injection formation and, more importantly, along reactive leakage pathways in overlying low permeability caprock formations. Understanding the extent of brine acidification likely to occur in these formations and its impact on caprock seal integrity is a necessary first step in evaluating leakage risk associated with geologic carbon sequestration in deep saline aquifers.;This dissertation examines several important issues related to the fate of stored CO2 for the purpose of geologic carbon sequestration and offers insight into the risks of CO2 leakage. Chapter 2 begins by addressing the degree of brine acidification that can be expected due to dissolution of CO2 and the potential co-contaminant gas, SO 2. Model results demonstrate that due to SO2 diffusion limitations within the supercritical CO2 phase, co-injection of SO2 may not lead to severe, rapid acidification as previously suggested by other researchers. This theoretical modeling study is followed by Chapters 3 and 4, which discuss the findings from a set of high-pressure experiments examining flow of CO2-acidified brine through fractured carbonate caprock samples. The findings of Chapter 3 highlight the importance of mineral spatial heterogeneity in controlling the evolution of fracture permeability during flow of acidic fluids and emphasize the vulnerability of tight carbonate formations as long-term geologic seals for injected CO2. In contrast to the findings of Chapter 3 where CO2-acidified brine caused extensive erosion along the fracture, a near-replicate experiment presented in Chapter 4 exhibited a decrease in fracture permeability during flow of CO2-acidified brine. This disparity in fracture permeability evolution between two near-replicate experiments highlights the complexities of predicting fracture permeability evolution during a CO2 leakage event. Chapter 5 extends the work presented in Chapters 3 and 4 through modeling efforts that investigate fracture permeability evolution for different mineral spatial heterogeneity scenarios.