Limiting pathways and breakthrough pressure for CO2 flow in mudstones

Abstract

The sealing and integrity of caprocks is a critical issue in the geological sequestration of CO2. In this work, we integrate multiscale pore-size measurements and imaging techniques, to characterize macropores, microporosity, and their spatial connectivity of intact mudstone cores from an Enping oilfield in the South China Sea. Together with the information on mineral compositions, we propose clay regions in the absence of organic matter as limiting pathways for supercritical CO2 flows in the cores, which primarily consist of nanopores ranging from tens to hundred nm in diameter. The FIB-SEM tomography is used to obtain nanoporous structures of a representative domain of those limiting pathways. We simulate the primary drainage of supercritical CO2 displacing brine in the domain by using an in-house quasi-static pore-network model. The numerical predictions of capillary breakthrough pressure match the experimental data well. Our proposed method of determining capillary breakthrough pressure can substantially complement experimental measurements. Moreover, it has the potential to quantitatively link the multiscale porous structures of mudstone to its CO2 sealing.