Foams With Wettability-Altering Capabilities for Oil-Wet Carbonates: A Synergistic Approach

Abstract

Summary The goal of this work is to systematically study the effect of wettability alteration and foaming, either acting individually or synergistically, on tertiary oil recovery in oil-wet carbonate cores. Three types of anionic-surfactant formulations were used: alkyl propoxy sulfate (APS), which exhibited low interfacial tension (IFT), wettability alteration, and weak foaming; alpha-olefin sulfonate (AOS), which showed no wettability alteration but good foaming; and a blend of APS, AOS, and a zwitterionic-foam booster, which showed low IFT, wettability alteration, and good foaming. First, contact-angle experiments were conducted on oil-wet calcite plates to evaluate their wettability-altering capabilities. Second, spontaneous imbibitions in a microchannel were performed to study the role of IFT reduction and wettability alteration by these formulations. Third, static foam tests were conducted to evaluate their foaming performance in bulk. Fourth, foam-flow experiments were conducted in cores to evaluate potential synergism between the anionic-surfactant AOS and the zwitterionic surfactants in stabilizing foam in the absence of crude oil. Finally, oil-displacement experiments were performed by use of a vuggy, oil-wet, dolomite core saturated with a crude oil. After secondary waterfloods, surfactant solutions were coinjected with methane gas at a fixed foam quality (gas-volume fraction). Contact-angle and spontaneous-imbibition experiments showed that AOS can act as a wettability-altering surfactant in the presence of sodium carbonate, but not alone. No synergy was observed in foam stabilization by means of the blend of zwitterionic surfactant and AOS solution (1:1) in a water-wet carbonate core. Oil-displacement experiments in oil-wet carbonate core revealed that coinjection of wettability-altering surfactant and gas can recover a significant amount of oil [33% original oil in place (OOIP)] over waterflood. During foam flooding, with AOS as the foaming agent, only a weak foam was propagated in a carbonate core, irrespective of the core wettability. A blend of wettability-altering surfactant, AOS, and zwitterionic surfactant not only altered the wettability of carbonate core from oil-wet to water-wet, but also significantly increased the foam-pressure gradient in the presence of crude oil.