Effect of Organic Acids on CO₂ Trapping in Carbonate Geological Formations: Pore-Scale Observations Using NMR

Stearic acid is an example of a carboxylic compound naturally present in geological formations (deep saline aquifers and depleted hydrocarbon reservoirs), which renders the rock hydrophobic (CO₂-wet) over a geological time scale. Such hydrophobic surfaces are detrimental to residual CO₂ trapping. There is, however, a lack of comprehensive dataset about how traces of these organic molecules affect the rock′s CO₂ wettability and residual CO₂ trapping. We, thus, used in situ NMR T₁-T₂ 2D images to visualize fluid configurations in the pore network and used T₁/T₂ ratios to assess the microscopic wettability of the rock to pore space fluids subsequent to each process step. The T₂ relaxation time was measured to demonstrate displacement processes and evaluate the trapping behavior at the pore scale, which is closely correlated to reservoir-scale flow functions. The trapping in the CO₂-wet sample (14%) was significantly lower than that of the analogous water-wet sample (18%). This reduction in CO₂ trapping is due to surface macroscopic flow layers acting as conduits allowing slow desaturation of CO₂. Importantly, in the CO₂-wet sample, trapping predominately occurred in meso- and micropores, whereas trapping in the analogous water-wet rock primarily occurred in macropores. This work thus provides a comprehensive dataset on the impact of organic acids on residual trapping at the pore scale, which ultimately helps to advance industrial-scale implementation of CGS and EOR project schemes in carbonate reservoirs.