Surfactant and Surfactant-Polymer Effects on Wettability and Crude Oil Liberation in Carbonates
Authors: Ayirala Subhash , AlSofi Abdulkareem , Li Zuoli , Xu Zhenghe 
Published: November 2020

Abstract
Surfactants and polymers are used in enhanced oil recovery (EOR) to reduce interfacial tension and increase the viscosity of displacing fluid, respectively. For oil-wet to mixed-wet systems, especially carbonates, which tend to be heavily fractured, wettability becomes a key parameter that strongly affects oil recovery. Therefore, studying the impact of surfactant and surfactant-polymer chemicals on carbonate wettability is important to understand the underlying mechanisms responsible for incremental oil recovery in surfactant-polymer flooding.
In the present study, liberation kinetics of crude oil from carbonate surfaces were investigated by using a liberation cell at both ambient and elevated temperatures (70°C). The liberation cell is equipped with an optical microscope for monitoring oil liberation. In addition, a custom-designed integrated thin film drainage apparatus (ITFDA) was used to measure adhesion forces between carbonate substrates and crude oil droplets. The chemical solutions were prepared in a representative high salinity brine. Two types of surfactants: a nonionic and an amphoteric were used. A sulfonated polyacrylamide polymer was selected as it was previously proven to be tolerant to both high salinity and high temperature conditions. The chemical solutions were prepared at dilute concentrations of 1000 mg/L and 500 mg/L for the surfactant and polymer, respectively. Besides the main experimental data, i.e., adhesion forces and liberation kinetics, interfacial tensions and zeta potentials were also measured for different solutions. In the zeta potential tests, carbonate particle suspensions in brine, surfactant, polymer and surfactant-polymer solutions were used.
Oil liberation from carbonate surface is the lowest with brine and the polymer increased the degree of oil liberation. The amphoteric surfactant showed better efficiency to liberate more crude oil from carbonate surface over the nonionic surfactant. Polymer and surfactant addition to brine resulted in an oil liberation degree that is much higher than those obtained by each of the chemicals when applied individually. For solutions containing brine, polymer, surfactant, and surfactant-polymer, oil liberation degree increased at elevated temperature. Adhesion forces were very consistent with the observed oil liberation results. Adhesion force was strongest in brine, and both the polymer and surfactants further lowered the adhesion force. Accordingly, the lower adhesion force between carbonate and crude oil in aqueous solutions containing surfactant and polymer contributed to the increased oil liberation. The higher oil liberation degree obtained with the amphoteric surfactant can be explained by its ability to lower oil/water interfacial tension by two to three orders of magnitude. In addition, the surface charge of oil droplets and carbonate particles were found to be increasingly negative in aqueous solutions containing surfactant and polymer, thereby contributing to enhanced wettability alteration in crude oil-brine-carbonate systems. These microscale results indicate that trapped-oil mobilization in carbonates is governed by both wettability and capillarity; in other words, wettability alteration as well as reduction in oil/water interfacial tension would lead to increased oil liberation.
This experimental study has characterized, for the first time, surfactant, and surfactant-polymer effects on wettability and crude oil liberation in carbonates. Such enhanced understanding obtained on the microscale interactions of surfactant, and surfactant-polymer chemicals at carbonate/brine/oil interfaces can provide some guidance on how to optimize EOR formulations for carbonate reservoirs.