Impact of Wettability Alteration on CO₂ Residual Trapping in Oil-Wet Sandstone at Reservoir Conditions Using Nuclear Magnetic Resonance

Oil reservoirs have hydrophobic surfaces, wherein surface-active chemical components from the oil phase deposit onto the surface of the rock matrix and render it hydrophobic. This pore-scale property drastically impacts the Darcy-scale (macro-scale) flow functions, including P c and K r, in porous media. However, quantitative measurement of CO 2 in oil-wet formations via capillary trapping remains a prime challenge despite extensive lab research. This is primarily due to inconsistent existing experimental data sets in the literature. Moreover, substantial insecurities yet exist in relations of foreseeing the wettability of the CO 2 -rock at reservoir conditions–which are detrimental for CO 2 trapping. Thus, we used the robust Nuclear Magnetic Resonance (NMR) T 1 - T 2 2D Map to image the fluid configuration and measured T 2 to systematically quantify Wettability Indices (WIs) using the United States Bureau of Mines (USBM) scale in an oil-wet San Saba to predict carbon capture and storage capacity (CCS) and containment integrity. The T 1 / T 2 ratio was used to measure the microscopic wettability subsequent to each flooding mechanism, and the P c - S w and K r - S w characteristic curves subsequent to drainage/imbibition cyclic water saturations were measured as well. Comparison of the results was made with analogous water-wet core results, and general consistency was found. The NMR T 2 response was also correlated to physical properties, Pore Size Distributions (PSDs), and a 12% residual CO 2 trapping was measured which is significantly lower than that in an analogous water-wet sandstone. Importantly, the NMR T 2 distribution measurements demonstrated that water was displaced from small pores by CO 2 flooding, whereas in the water-wet analogue rocks, capillary trapping occurs in the large pores. This work thus provides a comprehensive data set on the effect of pore-scale properties on Darcy flow functions which ultimately aid and advance industrial-scale implementation of CGS and EOR project schemes in oil-wet reservoirs.