Relationship between molecular charge distribution and wettability reversal efficiency of cationic surfactants on calcite surfaces

Wettability reversal is a critical factor in enhanced oil recovery (EOR) from predominantly oil-wet carbonate reservoirs. Cationic surfactants are able to efficiently reverse carbonate wettability due to their electrostatic interactions with the surface adsorbed carboxylate (acidic) components of the crude oil. However, details concerning the relationship between the surfactant molecular structures and their wettability alteration efficiency are lacking. In this study, we systematically probe the effect of the molecular charge distribution of quaternary ammonium type cationic surfactants on the wettability reversal efficiency. The local charges on the hydrophilic head group of the surfactants were varied by insertion of different functional groups and the charge distribution was calculated using quantum mechanical density functional theory (DFT) method. Ten modified surfactants as well as the reference octyltrimethylammonium molecule were tested using the molecular dynamics (MD) simulations. The results imply that the charge on the hydrogen atoms in the methylene group of a quaternary ammonium moiety has the most significant influence on the wettability reversal ability of the surfactants, with the Spearman rank coefficient (rho) being as high as 0.782, while charges on the quaternary ammonium group as a whole do not pose any considerable effect, with rho being -0264. The calculations further suggest some feasible chemical modifications of the surfactants leading to the highest wettability alteration efficiency (i.e., insertion of a ketone group). Additionally, the effect of alkyl chain length was explored and an optimal length was identified that suggests the importance of establishing the structural compatibility between the surfactants and the oil phase. The correlations obtained in this study can facilitate the design of high-efficiency cationic surfactants for wettability reversal in carbonate reservoirs. (C) 2020 Elsevier B.V. All rights reserved.