Unique Mixtures of Anionic/Cationic Surfactants: A New Approach To Enhance Surfactant Performance in Liquids-Rich Shale Reservoirs

Surfactants are important components in fracturing fluids for helping ensure higher productivity from unconventional reservoirs. Conventional wisdom suggests that surfactant mixtures generally exhibit better performance than individual surfactants. In addition, the synergism between surfactants increases with the degree of charge difference. Some current commercial surfactants were formulated by mixing nonionic and anionic surfactants (S-n/a), but few were formulated by mixing anionic and cationic (S-a/c) surfactants because of the risk of precipitation or formulation instability. This paper discusses binary mixtures of S-a/c surfactants prepared with different mole ratios to determine their synergisms; mixtures of nonionic/anionic (S-n/a) and nonionic/cationic (S-n/c) surfactants are also compared. Surface/interfacial properties [maximum surface excess concentration (Gamma(max)), minimum molecular area (A(min)), critical micelle concentration (CMC), and Gibbs free energy (Delta G)] and interaction parameters (beta(m) and beta(s),) in both the mixed micelle and interface were quantified to demonstrate the synergistic effect between various surfactants. In addition, the potential application of these mixtures for unconventional treatments was examined with regard to emulsion behavior and column-packed oil-recovery testing. The results for the S-a/c surfactant mixtures show that, compared with the parent species, Gamma(max) of the S-a/c system is approximately one order of magnitude higher (corresponding to one order of magnitude lower in A(min)). The resultant CMC is approximately two orders of magnitude lower than that of the parent species, and the AG of S-a/c is more negative. Notably, the interaction parameters further indicate that strong synergism exists for the S-a/c, system at various mole ratios in both mixed micelle and monolayers at the interface (with an optimized ratio at 2:3), whereas for the S-a/c system, weak synergism was identified in the mixed micelle at the mole ratio of 3:2. No synergism was observed for the S-a/c system. In addition, phase-behavior testing indicated that a weak emulsion was formed in the presence of the S-a/c by use of Eagle Ford crude oil. Column-flooding testing also revealed improved oil recovery of the S-a/c system compared with individual species. The synergistic effect between S-a/c surfactant mixtures, as well as the laboratory results of the emulsion behavior and oil recovery, suggests a new practice for applying S-a/c surfactant blends for unconventional applications.