Mechanisms underlying the adhesion of crude oil to mineral surfaces: Relevance of oil-brine interactions

The wettability state of mineral surface relative to water and oil is a key factor for crude oil recovery from reservoirs. It appears particularly relevant to EOR by low salinity brine, where change towards water wettability is often presented as a crucial factor. The purpose of this paper is to demonstrate the importance of the adsorption onto the mineral surface of species extracted from crude oils by the aqueous phase. The transfer of endogenous oil species during oil-water contact is evidenced by Total Organic Carbon, pH, interfacial tension and surface tension measurements for three different crude oils. The impact of the presence of these species on the wettability state of solid surfaces (silica and carbonate) is studied by using n-octane as a probe liquid. Contact angle measurements, Washburn capillary rise and flotation tests were carried out to evidence the hydrophobation of the surface. Interfacial tension is systematically taken into account to interpret contact angle and capillary rise results by calculating the adhesion tension, a quantity of thermodynamic significance. The effect of changes of water composition (salinity, pH) on oil adhesion is carefully investigated, and the efficiency of low salinity brine to decrease the adhesion of oil to the substrate, i.e. to remove the organic species adsorbed onto the surface, is evaluated. The importance of running wettability measurements with synthetic brine previously equilibrated with the crude oil is demonstrated.