This experimental study evaluates the capability of dilute nonionic surfactants for wettability alteration and enhanced oil recovery applications in carbonates.Firstly the compatibility of surfactant was determined by performing aqueous stability tests in both low salinity and high salinity brines followed by surface tension measurements to determine critical micelle concentrations (CMC). Phase behavior, and interfacialtension measurements were conducted using 0.1 wt% surfactant and crude oil at reservoir temperature. The contact angle measurements were performed using carbonate surfaces and the nonionic surfactant at dilute concentrations to evaluate wettability alterationin carbonates at reservoir conditions. Zeta potential measurements were also carried out across calcite-brine/surfactant, and crude oil-brine/surfactant interfaces to confirm the wettability alterations induced by the surfactant. Lastly microscopic dynamicliberation experiments were conducted using dilute concentrations of surfactant and polymer chemicals in both high salinity and low salinity brines to quantify the effects of wettability alteration on 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 bevery low in the range of 0.0020 to 0.0025 wt% 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 the range of about 0.05 mN/m with 0.1wt% surfactant 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 marginalwettability 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, by using 0.1 wt% surfactant in the high salinity water. Such results confirm the effectiveness of nonionic surfactant in high salinity water to alter the wettability of carbonates at dilute concentrations. The microscopic equilibrium degree ofcrude 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. These consistent findings obtained from different experimental techniquesclearly point out that dilute nonionic surfactant combined with dilute polymer in conventional high salinity injection water can become one potential cost-effective chemical EOR solution for oil recovery in carbonate reservoirs.