The Potential of Self-Sealing Fractures in Continental Flood Basalts

Summary The permanent disposal of CO2 in the subsurface is a viable strategy to mitigate global warming. Continental flood basalts (CFB) are considered unconventional CO2 storage reservoirs, with alternating vesicular and massive basalt layers acting as potential reservoirs and caprock, respectively. Basalts dissolve rapidly under acidic conditions, promoting CO2 mineralization. However, sub-vertical fractures in basalts could pose a risk of CO2 leakage, though this may be mitigated by secondary mineral precipitation, which could lead to the self-sealing of fractures. CO2-saturated water lowers the pH, causing minerals like plagioclase, pyroxene, and basaltic glass to dissolve and release ions such as Ca2+, Mg2+, and Fe2+, contributing to mineral precipitation. Two studies were conducted under subsurface conditions (800 m depth) to explore whether geochemical reactions could facilitate self-sealing. The first study examined mineral saturation and the nature of mineral precipitation (secondary carbonate or silicate minerals) on basalt wafers, while the second focused on fractured basalt cores, investigating mineral precipitation in the fractures and adjacent pore networks. Mineral carbonation in basalt was minimal under pressure, temperature, and low pH conditions. However, significant Si-Al minerals, like zeolites and smectites, formed, with precipitation outweighing dissolution, increasing grain volume.