Alteration of Calcite Wettability by Asphaltenes and Its Reversal by Surfactants

Asphaltenes are believed to be one of the main contributors to the strong oil-wet nature of carbonate mineral surfaces in contact with crude oils. While prior studies have shown that certain surfactants, particularly cationic ones, can effectively reverse the wettability of carboxylate-aged carbonates, it has remained unclear whether the same approach works when oil wetness is driven by asphaltene adsorption. To address this gap, we conducted contact angle measurements on asphaltene-aged calcite chips treated with five representative surfactants (cationic, nonionic, amphoteric, and anionic). The experiments were complemented by scanning electron microscopy (SEM) to examine the surface morphology and molecular dynamics (MD) simulations to elucidate molecular-level adsorption and desorption mechanisms. The results were directly compared to previous data obtained from identical experiments on carboxylate-aged calcite surfaces. Both experimental and computational results revealed a striking correlation between the wettability reversal of asphaltene- and carboxylate-aged calcite surfaces. Specifically, the cationic surfactant (CTAB) led to the most pronounced wettability reversal to water-wet calcite, the nonionic surfactant (DTGE) yielded a neutral-wet state, while the other types had either no effect or even strengthened the oil wetness. The simulation results revealed that asphaltenes adsorb on calcite primarily via electrostatic interactions between carboxylate groups on the asphaltenes and the positively charged mineral surfaces. This shared interaction mechanism with carboxylate-aged surfaces explains the similarity of surfactant performance. Our findings therefore demonstrate that the same types of surfactants are effective in reversing wettability regardless of whether oil wetness originates from adsorbed carboxylates or asphaltenes.