Mechanistic Insights into CO2 Residual Trapping in Oil-wet Reservoir Rocks Using NMR Core-Flooding

Wettability-altering organic molecules occur naturally in deep saline aquifers and depleted hydrocarbon reservoirs, where they adsorb onto mineral surfaces over geological time scales. Their presence renders the rock hydrophobic (CO 2 -wet) which is detrimental for capillary trapping mechanisms and CO 2 geostorage capacity and containment security. Nevertheless, significant uncertainties still exist in predicting CO 2 rock wettability and how trace concentrations of such organic acids impact rock-CO 2 wettability and residual CO 2 trapping efficiency. We, thus, investigated the microscopic wettability of the CO 2 -brine-rock system after each sequential step and quantified CO 2 residual trapping, using a non-destructive robust NMR core-flooding on oil-wet sandstone under reservoir conditions. The NMR T 1 −T 2 2D images and T 1 /T 2 ratios used to quantify the microscopic wettability and visualize the fluid fluctuations, measured T 2 relaxation time to demonstrate the fluid displacement dynamics and provide direct insight into reservoir-scale flow functions, Pore-Size Distribution (PSD) to evaluate storage capacity and accessible pore volume, and Saturation Profile (SP) to show spatial distribution of fluids along the core and fluid front propagation. CO 2 -wet sample achieved significantly lower trapping efficiency (S CO2,r = 18%) compared to the analogous water-wet sample (S CO2,r = 31%). This decrease in trapping efficiency is attributed to CO 2 molecules occupy smaller pores, restricting snap-off and ganglia formation, and forms continual surface layers allowing CO 2 to steadily desaturate. Interestingly, trapping predominantly occurred in meso- (r < 0.1< 10 μm) and micropores (r < 0.1 μm) in the CO 2 -wet rock, whereas the vast majority of the trapped CO 2 was found in macropores (r >10 μm) in the water-wet analogue. The results present inclusive dataset on the impact of organic acids on pore-scale residual trapping, which is closely correlated to reservoir-scale trapping, and ultimately aids to advance the design and optimization of industrial-scale CGS and CO 2 -EOR project-schemes in sandstone reservoirs.