The role of interfacial rheology in reservoir mixed wettability

Abstract

Since the early 1950s, industrial researchers have recognized that asphaltenic crude oil/water interfaces form so-called “rigid skins”. This work emphasizes the role that such oil/water interfacial microstructures play in establishing the mixed-wet state of reservoirs. We utilize a new oscillating-drop dynamic tensiometer that sinusoidally and infinitesimally expands and contracts a crude-oil droplet immersed in brine at a fixed frequency and measures the resulting dynamic interfacial stress from image analysis and axisymmetric drop-shape analysis. Linear viscoelastic theory permits evaluation of the dilatational interfacial elastic storage and viscous loss moduli. We find that for two crude oils, designated as Crude AS and Crude AH, immersed in synthetic sea water, the interface behaves primarily elastically and that the more asphaltenic the oil the stronger is the interfacial elasticity. Moreover, interfacial elasticity grows slowly in time over days and is clearly manifest even when “rigid skins” are not visible to the eye. Apparently, macroscopic, networked asphaltenic structures slowly evolve in time at the interface. Advancing and receding contact angles are also measured on smooth mica surfaces for the same crude oil/brine systems. We find that water advancing and receding contact angles when measured within hours are about equal (i.e., there is little hysteresis). However, aging of the drop over days dramatically alters the subsequent advancing and receding contact angles. Water receding angles grow somewhat in time, but the corresponding advancing angles increase over days towards 180° or towards complete pinning. Interestingly, the advancing contact angles for both crude oils do not depend on whether the drop is aged in the brine or in contact with the mica surface. Also, the measured, receding contact angles for both crude oils are much higher than those commonly assumed in the literature. Fascinatingly, aging kinetics of the contact angles correlates directly with the aging of interfacial elasticities and interfacial tensions. Based on in situ AFM studies of the asphaltene-coated mica surfaces, we explain why this happens. Upon rupture of the protective water film and adhesion of the oil droplet to the mica substrate, the surface underneath the oil droplet is pockmarked with water-wet patches in a Dalmatian microwetting pattern. To our knowledge the crucial role of oil/water interface aging in controlling wettability changes has not previously been recognized. Finally, by sketching various primary drainage and imbibition pore-level events, we emphasize the importance of the observed changes in contact angles towards the evolution of mixed-wet oil reservoirs.