Effects of Hydroxyl and Carboxyl Functional Groups on Calcite Surface Wettability Using Atomic Force Microscopy and Density Functional Theory

Abstract

Surface-active compounds, primarily in asphaltene fractions of crude oil, are responsible for binding the nonpolar oil components to mineral surfaces and, therefore, control wettability changes on reservoir rock/mineral surfaces. Surface wettability changes occur mainly through polar functional groups in these compounds, such as hydroxyl, carboxyl, or carbonyl. By using crude oil with its asphaltene fraction removed, so-called maltenes, we investigate the effect of hydroxyl and carboxyl functional groups on wettability changes of calcite surfaces. Atomic force microscopy (AFM) images show significantly increased adsorption of maltenes on calcite samples treated with two asphaltene surrogates (phenol with a hydroxyl group and benzoic acid with a carboxyl one) than that on water-treated samples. However, the adsorbate patterns are different between those two asphaltene surrogates, suggesting different aggregation mechanisms. In addition, we observed the formation of larger surface-adsorbed droplets on the phenol or benzoic acid-treated calcite samples even for relatively short exposure times (<30 min) to maltenes. Quantum-mechanical calculations show more favorable adsorption for benzoic acid onto the calcite surface both on terraces and step edges. However, when a model oil molecule adsorbs onto those two preadsorbed asphaltene surrogates, nonpolar oil molecules preferentially adsorb onto phenol on terrace sites and benzoic acid on step edges. Overall, benzoic acid changes the calcite surface wettability more significantly than phenol.