The Role of Brine Composition on Model Oil–Clay Mineral Interfaces Examined Through Thermal Analysis

It is believed that low salinity enhanced oil recovery techniques work by altering the wettability of pore-lining minerals in an oil reservoir toward a more water-wet state. However, the underpinning mechanisms that drive this wettability change are not fully understood. In this work, thermogravimetric analysis, coupled to mass spectroscopy, was used to investigate the adsorption and desorption of organic matter on two clay minerals with different basal crystallographic structures: kaolinite and pyrophyllite. The clays were aged with a model oil mixture (decanoic acid/dodecane) and in brines of different chemical composition (pH, salinity, cation type, Na+:Ca2+ ion ratio, ionic strength). Changing the cation type led to variations in model oil sorption, with Ca2+ inducing more oil to be sorbed on the clays than Na+. In addition, it was observed that increased Ca2+ concentrations, in comparison to Na+, led to higher oil retention. Furthermore, increased oil sorption was measured with a decrease in the Na+:Ca2+ ratio. Finally, varying the ionic strength (at fixed Na+:Ca2+) showed a partial effect on the oil sorption to kaolinite but no effect on pyrophyllite. Overall, these results indicate that the presence of Ca2+ is more critical in determining the oil adsorption than the ionic strength of the aging solution.