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 Pc and Kr, in porous media. However, quantitative measurement of CO2 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 CO2-rock at reservoir conditions–which are detrimental for CO2 trapping. Thus, we used the robust Nuclear Magnetic Resonance (NMR) T1-T2 2D Map to image the fluid configuration and measured T2 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 T1/T2 ratio was used to measure the microscopic wettability subsequent to each flooding mechanism, and the Pc-Sw and Kr-Sw 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 T2 response was also correlated to physical properties, Pore Size Distributions (PSDs), and a 12% residual CO2 trapping was measured which is significantly lower than that in an analogous water-wet sandstone. Importantly, the NMR T2 distribution measurements demonstrated that water was displaced from small pores by CO2 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.