Dilute surfactants for wettability alteration and enhanced oil recovery in carbonates

Abstract

The effectiveness of dilute nonionic surfactant was evaluated in this study for wettability alteration and enhanced oil recovery (EOR) in carbonates by performing a suite of measurements such as aqueous stability, surface tension, phase behavior, interfacial tension (IFT), zeta potential, contact angle, and microscopic dynamic oil liberation. Two different brines, low salinity and high salinity, were used at 0.1 wt% surfactant concentration. These measurements were conducted at elevated temperature using the rock and crude oil samples from a carbonate reservoir. The dilute concentration of polymer has also been used together with surfactant in both high salinity and low salinity brines to quantify the effects of wettability alteration on microscopic oil liberation efficiency in carbonates.The compatibility of nonionic surfactant was demonstrated in high salinity brine at reservoir temperature. The critical micelle concentrations were found to be very low in both low salinity and high salinity brines. The phase behavior results showed the formation of middle phase microemulsion and correspondingly low interfacial tensions in high salinity brine. The contact angle data indicated the ability of nonionic surfactant to significantly alter the wettability of carbonate from oil-wet to either intermediate wet or less oil-wet in high salinity brine whereas only marginal wettability alterations from oil-wet to less oil-wet were obtained in low salinity brine. The increasing negative zeta potentials and the alteration of charge polarity from positive to negative were observed at crude oil-brine and calcite/brine interfaces, respectively due to surfactant in the high salinity brine. The microscopic equilibrium degree of crude oil liberation from carbonate surface was found to be about 20% higher with high salinity surfactant-polymer solution when compared to the low salinity surfactant-polymer solution. Such higher oil release efficiency obtained with high salinity surfactant-polymer solution can be attributed to the beneficial effects of surfactant induced wettability alteration and IFT reduction mechanisms in the high salinity brine to favorably alter both capillary and adhesion forces for efficient EOR in carbonates.