Influence of Organic Acid Concentration on Wettability Alteration of Cap-Rock: Implications for CO2 Trapping/Storage.

Every year, millions of tons of CO2 are stored in CO2-storage formations (deep saline aquifers) containing traces of organic acids including hexanoic acid C6 (HA), lauric acid C12 (LuA), stearic acid C18 (SA), and lignoceric acid C24 (LiA). The presence of these molecules in deep saline aquifers is well documented in the literature; however, their impact on the structural trapping capacity and thus on containment security is not yet understood. In this study, we therefore investigate as to how an increase in organic acid concentration can alter mica water wettability through an extensive set of experiments. X-ray diffraction (Figure S2), field emission scanning electron microscopy, total organic carbon analysis, Fourier-transform infrared spectroscopy, atomic force microscopy, and energy-dispersive X-ray spectroscopy were utilized to perceive the variations in organic acid surface coverage with stepwise organic acid concentration increase and changes in surface roughness. Furthermore, thresholds of wettability that may indicate limits for structural trapping potential (θr < 90°) have been discussed. The experimental results show that even a minute concentration (∼10-5 mol/L for structural trapping) of lignoceric acid is enough to affect the CO2 trapping capacity at 323 K and 25 MPa. As higher concentrations exist in deep saline aquifers, it is necessary to account for these thresholds to derisk CO2-geological storage projects.