The secure storage of carbon dioxide (CO₂) in deep saline aquifers relies on efficient injection, distribution, and trapping within reservoir rocks. Residual trapping, where disconnected CO₂ clusters are immobilised by capillary forces, is considered one of the most reliable long-term storage mechanisms, but its effectiveness depends on sweep efficiency and pore accessibility. This study used Otway sandstone cores from the CRC-8 well to compare conventional supercritical CO₂ injection (CI) with microbubble (MB) injection under reservoir conditions. NMR T₁–T₂ mapping, T₂ distribution analysis, saturation profiling, and SCAL measurements were applied to assess fluid displacement, sweep efficiency, breakthrough timing, and storage capacity.

Results showed that MBs invaded both meso- and micropores more effectively than CI, displacing brine from regions unswept during conventional injection. After drainage, CO₂ saturation reached 39.4% for MBs compared to 23.1% for CI. Following imbibition, residual trapping was 22.5% for MBs versus 16% for CI. Pc–Sw curves indicated lower capillary pressures under MB injection, confirming easier CO₂ entry into smaller pores and a more uniform sweep. Moreover, MB injection delayed breakthrough and produced more uniform saturation profiles, reducing preferential flow through high-permeability zones.

Overall, MB injection provided superior pore-scale control, enabling deeper invasion, broader sweep, and enhanced capillary trapping. By increasing storage capacity and improving conformance in previously inaccessible pores, MB injection demonstrates strong potential as a safer and more reliable approach for large-scale geological CO₂ sequestration.

Keywords: CO₂ sequestration, microbubbles, breakthrough time, sweep efficiency, storage capacity, NMR.