Catanionic Surfactants for Improving Oil Production in Carbonate Reservoirs

This paper presents the development of catanionic surfactants composed of cationic and anionic surfactants to make them high-performance products for chemical flooding in high-temperature and high-salinity carbonate reservoirs. The objective of this study is to optimize the surfactant chemistry by mixing oppositely charged anionic surfactants and cationic surfactants (CASs), which results in sig-nificant synergistic effects in interfacial properties due to electrostatic attraction to improve oil production at the given harsh conditions. The optimal mixing surfactant ratios were determined according to the brine-surfactant compatibility, microemulsion phase behavior, and the interfacial tension (IFT) between oil and surfactant solutions in high-salinity brine at 90 degrees C. Comprehensive performance of the catanionic surfactants was evaluated, including adsorption of the surfactants onto the carbonate rocks and the long -term stability at 95 degrees C. The coreflooding displacement experiments were performed using carbonate core plugs at 95 degrees C to evaluate the potential of the optimal catanionic surfactant in improving oil recovery. Three catanionic surfactants with good compatibility were developed in this study. It appeared that the synergistic effect between the mixing surfactants was enhanced with increasing temperature. Although the IFT of the individual surfactants with crude oil was between 10(-1) and 100 mN/m, a significant IFT reduction in the magnitude of 10(-2) to 10(-3) mN/m was observed by mixing the selected anionic surfactants and CASs. A salinity scan showed that the IFT values maintained a value of 10(-2) mN/m in a wide salinity range, which demonstrated the effectiveness of the catanionic surfactant. In microemulsion phase behavior studies, the developed catanionic surfactant solution in the presence of crude oil exhibited Winsor Type III emulsions. The static adsorp-tion quantities of the catanionic surfactants were lower than the values of the individual surfactants. All these indicated the feasibility of catanionic surfactants for their applications in the harsh reservoir conditions. The results of coreflooding displacement tests demonstrated significant oil recovery improvement beyond waterflooding. This work provides an efficient way to get surfactant formulations by mixing oppositely charged surfactants to obtain high performance in improving oil production under harsh conditions.