Ionic liquids for low-tension oil recovery processes: Phase behavior tests

This is the accepted manuscript of the following article: Rodriguez-Escontrela, I., Puerto, M., Miller, C., & Soto, A. (2017). Ionic liquids for low-tension oil recovery processes: Phase behavior tests. Journal Of Colloid And Interface Science, 504, 404-416. doi: 10.1016/j.jcis.2017.05.102Chemical flooding with surfactants for reducing oil-brine interfacial tensions (IFTs) to mobilize residual oil trapped by capillary forces has a great potential for Enhanced Oil Recovery (EOR). Surface-active ionic liquids (SAILs) constitute a class of surfactants that has recently been proposed for this application. For the first time, SAILs or their blends with an anionic surfactant are studied by determining equilibrium phase behavior for systems of about unit water-oil ratio at various temperatures. The test fluids were model alkane and aromatic oils, NaCl brine, and synthetic hard seawater (SW). Patterns of microemulsions observed are those of classical phase behavior (Winsor I-III-II transition) known to correlate with low IFTs. The two anionic room-temperature SAILs tested were made from common anionic surfactants by substituting imidazolium or phosphonium cations for sodium. These two anionic and two cationic SAILs were found to have little potential for EOR when tested individually. Thus, also tested were blends of an anionic internal olefin sulfonate (IOS) surfactant with one of the anionic SAILs and both cationic SAILs. Most promising for EOR was the anionic/cationic surfactant blend of IOS with [C12mim]Br in SW. A low equilibrium IFT of 2 10 3 mN/m was measured between n-octane and an aqueous solution having the optimal blend ratio for this system at 25 CA. Soto acknowledges the Ministry of Economy and Competitiveness (Spain) for financial support throughout project CTQ2015-68496-P (including European Regional Development Fund advanced funding)S

Insights on Foam Transport from a Texture-Implicit Local-Equilibrium Model with an Improved Parameter Estimation Algorithm

We present an insightful discussion on the implications of foam transport inside porous media based on an improved algorithm for the estimation of model parameters. A widely used texture-implicit local-equilibrium foam model, STARS, is used to describe the reduction of gas mobility in the state of foam with respect to free gas. Both the dry-out effect and shear-dependent rheology are considered in foam simulations. We estimate the limiting capillary pressure Pc* from fmdryvalues in the STARS model to characterize foam film stability in a dynamic flowing system. We find that Pc* is a good indicator of foam strength in porous media and varies with different gas types. We also calculate Pc* for different foaming surfactants and find that foam stability is correlated with the Gibbs surface excess concentration. We compare our improved parameter estimation algorithm with others reported in literature. The robustness of the algorithm is validated for various foam systems

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