Microscale Investigation of the Impact of Surfactant Structure on the Residual Trapping in Natural Porous Media

Abstract

Residual trapping of a nonwetting phase in a porous medium is directly impacted by the relative magnitude of capillary, viscous, and gravity forces. Ratios of these forces control the sequence by which different pore-scale displacement mechanisms take place, which in turn govern the pore fluid occupancy and the residual nonwetting phase saturation. Parameters such as pore geometry, rock surface wettability, fluid–fluid interfacial tension, and fluids’ viscosities, densities, and flow rates determine the magnitude of the above-mentioned forces. In this study, we show that there is an additional set of factors that directly contributes to the distribution of fluids at the pore scale. We demonstrate that, under similar rock and fluid properties, interfacial repulsive and attractive interactions, caused by the adsorption of surface-active chemicals on fluid–fluid interfaces, can significantly alter pore-scale fluid occupancies. Using the microcomputed tomography (micro-CT) imaging technique integrated with a miniature core-flooding apparatus, we investigate the impact of surfactant structure on pore-scale fluid distributions in a limestone core sample subjected to injection of different wetting fluid solutions. Poly(ethylene oxide) (PEO), a nonionic surfactant, and ammonium alkyl ether sulfate, an anionic surfactant, are utilized as surface-active agents providing similar oil–brine interfacial tension (IFT) and contact angle (CA) values. Oil cluster analyses along with three-dimensional (3D) visualization of fluid distributions indicate that using the nonionic surfactant instead of the anionic surfactant results in the breaking up of the large and medium oil clusters into smaller and scattered ones. We propose a mechanism relating the stability of oil–brine interface to surfactant structure that is responsible for the breakup and/or coalescence of oil clusters inside the pore space. The suggested mechanism is confirmed by the micro-CT images and associated oil cluster analyses. This phenomenon affects the competition between the frequency of displacement mechanisms causing variations in remaining oil saturations.