Determination of the Active Soap Number of Crude Oil and Soap Partitioning Behavior

The optimal salinity of the alkali/surfactant/crude oil system in an alkali/surfactant/polymer (ASP) flooding process was found previously to be a function of the soap/surfactant ratio. Therefore, the soap number is of great importance in formulation design and simulation of ASP flooding processes for enhanced oil recovery. However, there is as yet no established way to quantitatively determine the amount of soap in crude oil relevant to an ASP process. Soaps are the salts of fatty acids, a definition generalized here to include the salts of naphthenic acids. In this paper, we present a method to determine the amount of “active soap”, which consists only of soap that partitions into the aqueous phase at low ionic strength and transfers into the oleic phase at high ionic strength. Two fast and accurate methods, aqueous-phase potentiometric titration and two-phase colorimetric titration, were used to determine the water-soluble active soap number (WSASN), a measure of the active soap. Both methods were proven to be sufficiently precise by titrating a model oil containing known concentrations of oleic acid, both with and without isopropyl alcohol (IPA) present. The total soap number (TSN) with IPA present and water-soluble soap number (WSSN) and WSASN of a crude oil without IPA were measured in Na₂CO₃ and NaOH solutions. The partition of soap between oil and brine phases was also investigated. It was found that the partition coefficient of water-soluble active soap (WSAS) is near unity at optimal salinity as determined by IFT measurements, a result that supports the use of WSASN to represent the amount of active soap. Moreover, it was found that the logarithm of optimal salinity versus soap fraction for a soap/surfactant mixture followed the previously proposed mole fraction mixing rule more closely when WSASN was used than if total acid number (TAN) or TSN were used as in previous studies. It was also found that the values of WSSN and WSASN measured at room temperature were different from those measured at high temperature and that the soap generated by NaOH was more hydrophobic than that generated by Na₂CO₃. Results of this work are helpful for formulation design and simulation of ASP flooding processes