Pore-level investigations on the oil displacement mechanisms of a viscoelastic surfactant in porous media

A good understanding of pore-level oil displacement mechanisms is of great significance for the application of viscoelastic surfactants (VES) in enhanced oil recovery (EOR). The focus of this paper is on the pore-level events that happen when VES displaces residual oil in porous media. Attention was placed on the displacement dynamics and the relation with the bulk solution properties of VES. Hence, direct measurements of interfacial tension (IFT), contact angle and relative permeability were first performed. The results showed that the tested VES could reduce the oil-water IFT to a 10(-2) mN/m level and reverse an initially oil-wet surface to water-wet state. In the visual micromodel experiments, a series of displacement scenarios were created, in which the residual oil was respectively displaced by brine, glycerol, KPS (a common surfactant) and VES aiming to identify the differentiations. The recorded images showed that glycerol was mainly to correct the mobility of the displacing phase via increasing the viscosity. In contrast, KPS was able to strip the adsorbed oil film on pore walls in the water swept zone corresponding to the effect wettability alteration. The EOR mechanism of VES when flow through the porous media can be described as two effects: "dragging" the oil film from the pore wall surface by electrochemical action, and "scretching" the residual oil out of the pore throat by the viscoelastic effect. These observations proved microscopically that the EOR mechanisms of the VES were contributed to synergistic effect of reducing interfacial tension, wettability alteration, mobility correction and viscoelastic flow.