Fundamental studies of crude oil–surface water interactions and its relationship to reservoir wettability

A fundamental understanding of the wettability of rock surfaces by crude oil and brine requires a good description of the intermolecular interactions between the different phases in contact. The aim of our study was to directly measure those interactions and to establish their implications for observed wetting behavior. A Surface Forces Apparatus (SFA) was used to measure the adsorption on and the forces between two molecularly smooth mica surfaces confining thin films of crude oil, brine or both, at different conditions of pH and salinity. By modifying one or both surfaces by controlled pre-adsorption of surface-active polar species from the crude oil, the interaction between mica and oil in brine, or oil and oil in brine was also determined. From contact angle measurements a ‘wettability map’ was established for the system investigated showing the regions of different contact angles or ‘wettability’ as a function of pH and ionic strength. This wettability map was found to be correlated with the independently measured surface forces and adsorption. The relative importance of different colloidal forces (van der Waals, electrostatic, etc.) on the observed wetting behavior was established, and in some cases quantitative agreement between measured contact angles and measured adhesion forces was obtained. It is shown that in addition to van der Waals and electrostatic forces (included in the DLVO theory), polymer-like interactions of adsorbed asphaltene layers are involved in the overall process. It was further established that the concept of an intrinsically oil-wet or water-wet surface or reservoir cannot, in general, be correct: depending on the nature of the oil and brine, different species in a crude oil will adsorb on the mineral surfaces and at the oil–water interface, thereby determining the wetting behavior of a reservoir or rock.