Thermodynamic modeling of wettability alteration induced by asphaltene - A combined theoretical and experimental study

Wettability is a paramount factor in multiphase flow through porous media influencing most processes in oil and gas recovery. It is commonly accepted that asphaltene can alter the rock wettability. However, the mechanisms via which asphaltenes affect wettability are intricate and far from being fully understood. Current asphaltene models are mainly based on 1) empirical correlations, which lack fundamental basis with limited ability to provide quantitative predictions; or 2) molecular dynamics simulations, whose practical use is hindered by high computational costs and challenges in accurately defining asphaltene molecular structures. The absence of a comprehensive model that integrates major key factors and provides both fundamental understanding and practical predictability remains a significant gap in the field. Herein, we present a thermodynamic model for the wettability alteration with asphaltenes, based on the thin film theory and the disjoining pressure principle. The model has a fundamental basis on the electrostatic, van der Waals, and structural forces at the oil/water/solid interface, while it generalizes the impact of asphaltenes into surface forces without accounting for individual molecules. To serve as inputs into the model, experiments were performed using AFM, zeta potential, etc. to characterize the fluid and mineral properties, with asphaltenes extracted from the Brutus crude. The model-predicted contact angles match well with experimental data. The impact of asphaltene on wettability was investigated under different salinities, oil types, and capillary pressures. It was found that asphaltene shifts the wettability towards more oil-wet and that the surface potential and interfacial tension are the most important factors that govern the asphaltene-induced wettability alteration.