The effect of addition of ethoxylated sulfonate on salt tolerance, optimal salinity, and impedence characteristics of petroleum sulfonate solutions

Abstract

The increase in salt concentration causes very unusual changes in the optical appearance of the surfactant formulations reported in this paper. At low salt concentration, the surfactant (TRS 10-410) solutions are isotropic and clear. With an increase in salt concentration, precipitation of surfactant occurs. However, upon further increase in salt concentration, the precipitate redissolves in the solution and a stable birefringent phase forms. Upon further increase in salt concentration the birefringent phase is destroyed and phase separation occurs. The petroleum sulfonate solutions have very poor salt tolerance (about 2.5% NaCl). However, upon addition of an ethoxylated sulfonate (EOR-200) into petroleum sulfonate, the salt tolerance of the mixed surfactant formulation markedly increases. By itself, the salt tolerance for the phase separation of the petroleum sulfonate or the ethoxylated sulfonate (EOR-200) is respectively 2.5 and 18% NaCl. However, upon blending these two surfactants in a 1:4 weight ratio, the salt tolerance for phase separation is found to be 24% NaCl, suggesting a synergistic effect of blending these two sulfonates for salt tolerance. When the mixed surfactant solutions are brought in contact with an equal volume of oil, at a specific salinity a middle phase containing equal volumes of oil and brine is formed. This salinity is defined as the optimal salinity. It is observed that the optimal salinity increases with the fraction of ethoxylated sulfonate in the solution. The mixed surfactant solutions in distilled water, without added oil, were investigated using radio frequency impedence measurements. The capacitance of such solutions was interpreted in terms of surface charge density around micelles. It was shown that the micellar charge density increases with the fraction of ethoxylated sulfonate in the solution. The relevance of these findings for designing optimum surfactant formulations for oil recovery has been discussed.