Pore-scale characterization of residual gas remobilization in CO2 geological storage

A decrease in reservoir pressure can lead to remobilization of residually trapped CO₂. In this study, the pore-scale processes related to trapped CO₂ remobilization under pressure depletion were investigated with the use of high-resolution 3D X-ray microtomography. The distribution of CO₂ in the pore space of Bentheimer sandstone was measured after waterflooding at a fluid pressure of 10 MPa, and then at pressures of 8, 6 and 5 MPa. At each stage CO₂ was produced, implying that swelling of the gas phase and exsolution allowed the gas to reconnect and flow. After production, the gas reached a new position of equilibrium where it may be trapped again. At the end of the experiment, we imaged the sample again after 30 hours. Firstly, the results showed that an increase in saturation beyond the residual value was required to remobilize the gas, which is consistent with earlier field-scale results. Additionally, Ostwald ripening and continuing exsolution lead to a significant change in fluid saturation: transport of dissolved gas in the aqueous phase to equilibriate capillary pressure led to reconnection of the gas and its flow upwards under gravity. The implications for CO₂ storage are discussed: an increase in saturation beyond the residual value is required to mobilize the gas, but Ostwald ripening in turn can allow local reconnection of hitherto trapped gas, thus enhancing migration and may reduce the amount of CO₂ that can be capillary trapped in storage operations.