Enhancing CO2 Storage Efficiency in Depleted Hydrocarbon Reservoirs, An Approach to Optimize Reservoir and Caprock Properties

Abstract The success of CO2 geo-storage in depleted hydrocarbon reservoirs hinges on optimizing reservoir rock trapping efficiency while ensuring caprock integrity. While existing research predominantly emphasizes caprock stability, this study addresses the critical gap in reservoir-specific optimization by investigating the coactive roles of interfacial tension (IFT) reduction and wettability alteration in sandstone-anhydrite systems. Core samples from a depleted Polish hydrocarbon reservoir (sandstone reservoir and anhydrite caprock) were analyzed under reservoir conditions (45°C, 15 MPa). For the reservoir rock, sodium dodecyl benzene sulfonate (SDBS) surfactant at 0.05 wt% reduced CO2-brine IFT by 40%, as measured via the pendant drop technique. This IFT reduction promoted residual trapping by enabling the formation of smaller, stable CO2 ganglia, validated through core flooding experiments. In the anhydrite caprock, baseline capillary threshold pressure was determined by scaling mercury intrusion porosimetry data. Post-treatment with silica nanoparticles (100–1000 mg/L) and SDBS (0.05 wt%), contact angle measurements under variable pressures revealed a wettability shift from CO2-wet (>110°) to water-wet (<60°) as lower contact angles means higher capillary threshold pressure which indicates a more effective seal that is crucial for preventing the upward migration of CO2. These data were extrapolated to calculate the maximum supported CO2 column height, a key metric for storage capacity. Atomic force microscopy (AFM) highlighted surface topography modifications post-treatment. The results demonstrate that dual optimization, IFT reduction in the reservoir, and nanoparticle-mediated wettability alteration in the caprock enhance storage capacity by 25% and reduce capillary breakthrough risks by 50%. These findings advance practical strategies for large-scale CO2 storage in similar depleted reservoirs by highlighting the importance of optimizing reservoir properties, alongside rigorous caprock integrity management.