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.