Carbon Mineralization in Fractured Basalt

Abstract

The need to meet rising energy demands while mitigating climate change driven by associated CO2 emissions has motivated the development of geologic carbon storage systems. Until recently, most research has focused on sedimentary reservoirs that rely primarily on short-term solubility and physical trapping mechanisms, where CO2 can migrate if the structural integrity of the caprock or wellbore is compromised. This inherent leakage risk could be eliminated by leveraging the natural reactivity of basalt reservoirs, which are abundant in silicate minerals that dissolve rapidly under acidic conditions and can ultimately trap dissolved CO2 as solid carbonate minerals. Given the significant advantage of mineral trapping for long-term storage security, basalts may be the most readily deployable CO2 repositories in the near term. However, our fundamental understanding of the conditions under which this CO2 mineralization process occurs and its viability as a permanent carbon sequestration pathway remain limited. This dissertation highlights multiple series of high-pressure core flooding experiments and coupled reactive transport models designed to evaluate the effects of temperature, fluid chemistry, and flow regimes on basalt dissolution and CO2 trapping through carbonate precipitation. Results indicate that basalts can effectively mineralize CO2 at representative subsurface stress conditions, but mineralization predominantly occurred within buffered diffusion-limited zones (e.g. dead-end fractures) where reaction fronts have developed from competing geochemical gradients. Carbonate precipitation was highly localized on the reactive silicate minerals contributing key divalent cations and was significantly enhanced by elevated temperature and alkalinity. Complementary triaxial direct shear fracturing experiments with carbonate-rich shales revealed that spatial distributions of precipitates may be more significant than the total amount, as small volumes at critical fracture contact points can dramatically restrict flow. In combination, this work demonstrates how complex interactions between reservoir geochemistry and transport conditions drive the extent and location of carbon mineralization reactions in basalt fractures, which will inform selection of storage sites and injection schemes that optimize long-term CO2 trapping efficiency.