Influence of Chitosan Salt on Capillary Pressure and Interfacial Tensions of CO₂/Brine and H₂/Brine Systems

There is increasing interest in attainment of a CO2-free global economy and net zero carbon emissions by 2050 to mitigate the negative impact of global warming and unfavorable climate change. However, the success of large-scale underground H2 and CO2 storage depends on the rock wetting behavior and dynamics of gas/brine interfacial tension (IFT), which significantly influences capillary pressure. Previous studies have demonstrated that rock wettability can be altered into a hydrophilic state using surface-active chemicals such as surfactants, nanoparticles, methyl orange, and methyl blue. However, these chemicals also showed higher propensity to reduce the gas/brine IFT, which is unfavorable for the residual and structural trapping potential of the host rock. Herein, the limestone wettability modification capacity of a polymeric surfactant (chitosan salt) and its impacts on CO2/brine and H2/brine IFT were evaluated using the pendant drop technique and by capillary pressure measurement. Results showed that the capillary pressure shifted to the right in the presence of chitosan salt solutions, indicating a reduction in the pressure needed to push water into the pore spaces of the rock. This effect increased with increasing concentrations of chitosan salt solution from 100 to 1000 ppm. Specifically, at 200 psi, the residual water saturation of seawater-saturated limestone cores increased from about 50 to 70% whereas that of deionized water-saturated limestone increased from 25 to about 40% in the presence of 1000 ppm chitosan salt concentration. The presence of chitosan salt at the CO2/water interface and H2/water interface showed no significant effects on interfacial tension. Moreover, the adsorption of DI water and seawater molecules on limestone rock was higher in the presence of chitosan salt, suggesting that the salt promotes adhesion of H2O molecules but discourages the adsorption of H2 and CO2 molecules on limestone rock. Our results generally demonstrated that chitosan salt solution can modify the wettability of hydrophobic limestone rocks, turning them into water wet while mitigating the reduction in IFT, which could increase the pressure needed to push water into the pore spaces of the host rock. Hence, saturation of geo-storage of rocks with chitosan salt solution is a promising strategy for derisking underground H2 and CO2 storage and optimizing the residual and structural trapping potential of geo-storage formations.