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

• Remobilization of residually trapped CO2 can occur as a result of reservoir pressure decline. • 3D X-ray microtomography was used to characterize CO2 remobilization under pressure depletion. • Remobilization was observed at a higher saturation than residual saturation. • Ostwald ripening enhanced remobilization by allowing local reconnection of trapped gas. A decrease in reservoir pressure can lead to remobilization of residually trapped CO 2. In this study, the pore-scale processes related to trapped CO 2 remobilization under pressure depletion were investigated with the use of high-resolution 3D X-ray microtomography. The distribution of CO 2 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 CO 2 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 CO 2 storage are discussed: an increase in saturation beyond the residual value is required to mobilize the gas, but Ostwald ripening can allow local reconnection of hitherto trapped gas, thus enhancing migration and may reduce the amount of CO 2 that can be capillary trapped in storage operations. [ABSTRACT FROM AUTHOR]