Evaluation of an Amphoteric Surfactant for CO2 Foam Applications: A Comparative Study

Abstract

Abstract In a surfactant-alternating-gas (SAG) injection, stable foams form viscous barriers and divert fluids, thereby providing conformance for enhanced oil recovery (EOR). Once foam decays, injected gas resumes preferential flow through thief zones, demonstrating the need for higher foam stability. Thus, longer foam half-lives or stability is one of the key factors determining the success of any foam-field application. The ability of surfactants to stabilize foam depends on the gas type. Many surfactants that form stable foam with nitrogen (N2) and hydrocarbon gas are not able to form a stable foam with carbon dioxide (CO2), which could be due to the presence of low pH environment in CO2 floods, relatively high solubility of CO2 in water, and CO2 permeability through liquid films. To improve the performance of CO2 floods, it is imperative to identify surfactants that can enhance the stability of CO2-foam. This work investigates an amphoteric surfactant, which is commercially available and priced similarly to other commonly used EOR foamers, for its ability to stabilize CO2-foam. Static stability and dynamic coreflood tests were conducted at high pressure and high temperature conditions, where CO2 remained in the supercritical state. The performance of the amphoteric surfactant was compared with another good foamer on the basis of foam stability and strength, both in bulk and in porous media. Dynamic adsorption tests were conducted to compare the adsorption of amphoteric and anionic surfactants on both sandstone and carbonate rock surfaces. Ways to mitigate surfactant adsorption on rock surfaces were studied. In terms of CO2-foam stability, the amphoteric surfactant performed much better than the anionic and nonionic surfactants evaluated in this study. In the presence of oil, foam stabilized by the amphoteric surfactant exhibited the longest half-life in static tests. However, the amphoteric surfactant performed similarly to other surfactants with nitrogen or hydrocarbon gas. Compared to other surfactants, foam stabilized by the amphoteric surfactant remained stable and exhibited higher apparent viscosity at high foam qualities. Foam stability at higher qualities improves the performance of SAG process as it can lengthen the gas cycle and reduce the amount of surfactant needed, a beneficial outcome when water supply is limited. We found the adsorption of amphoteric on carbonate rock to be much lower than on sandstone rock. Compared to ionic and nonionic surfactants, amphoteric surfactants are usually avoided for oilfield applications due to potential for high retention. Based on systematic evaluation, our work demonstrates the unique ability of amphoteric surfactants to enhance the stability of CO2-foams at reservoir conditions.