(669c) Surface Tension Behavior of Aqueous Solutions of a Propoxylated Surfactant and Interfacial Tension Behavior Against an Un-Preequilibrated Crude Oil

Equilibrium interfacial tensions (EIFTs) are important in determining the oil recovery efficiency of surfactant-based enhanced oil recovery (EOR) processes. The dynamic IFTs (DIFTs), and the adsorption mechanisms are also important for the initial stages of EOR and for screening various surfactants for potential field uses. We report on the DIFT and EIFT behavior of a commercial anionic surfactant, S13D, which is a single-chain propoxylated sodium sulfate salt. The solubility in water is over 25 wt%, and the solubility in the brine used is over 20 wt%. The synthetic brine used is similar to the one present in an actual oil reservoir. It contains 0.95 wt% salts, primarily NaCl, and NaHCO₃ with smaller concentrations of CaCl₂, MgCl₂, BaCl₂, KCl, Na₂SO₄, and MnCl₂. The low-viscosity crude oil samples used were prepared from oil produced from a reservoir in the Illinois Basin.

For comparison purposes, the dynamic and equilibrium surface tensions, DSTs and ESTs, and DIFTs and EIFTS, were measured, with either the emerging bubble or drop method, and often with the spinning bubble or drop method. Only the latter method was used for the ultralow IFT measurements. DST has a simpler adsorption mechanism than DIFT. DSTs were measured in water and in brine at surfactant concentrations ranging from 0.00001 wt% to 1.0 wt% (0.1 ppm to 10,000 ppm). The DST data revealed long equilibration times, ranging from 24,000 s to 1 s. The times were shorter in brine than in water. Several surface tension relaxation tests after surface area perturbation were done, for testing the stability of the STs, and establish the validity of the ESTs. The EST data reveal a cmc of about 12 ppm in water and 1 ppm in brine. The molecular areas of the surfactant in the adsorbed monolayer were estimated as 1.2 nm² in water and 1.0 nm² in brine.

The un-pre-equilibrated DIFTs and EIFTs in water and brine against crude oil drops were also measured for the same concentration range with similar area perturbations to test for the stability and accuracy of the EIFTs. In water, the lowest observed EIFTs were 1 mN/m. In brine, the EIFTs were much lower above the cmc, reaching ultralow values, less than 0.01 mN/m. The timescales of IFT equilibration were slightly longer than that for ST equilibration. The results suggest that the low and ultralow IFTs are associated with adsorbed monolayers at the aqueous/crude oil interface.