Effect of Organic Acids on CO<sub>2</sub> Trapping in Carbonate Geological Formations: Pore-Scale Observations Using NMR
Auby Baban, Mujahid Ali, Muhammad Arif, Alireza Keshavarz, Robert Amin, Stefan Iglauer
Abstract
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 (CO 2 -wet) over a geological time scale. Such hydrophobic surfaces are detrimental to residual CO 2 trapping. There is, however, a lack of comprehensive dataset about how traces of these organic molecules affect the rock′s CO 2 wettability and residual CO 2 trapping. We, thus, used in situ NMR T 1 – T 2 2D images to visualize fluid configurations in the pore network and used T 1 / T 2 ratios to assess the microscopic wettability of the rock to pore space fluids subsequent to each process step. The T 2 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 CO 2 -wet sample (14%) was significantly lower than that of the analogous water-wet sample (18%). This reduction in CO 2 trapping is due to surface macroscopic flow layers acting as conduits allowing slow desaturation of CO 2 . Importantly, in the CO 2 -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.