Co-solvent Partitioning and Retention

Abstract

Favorable microemulsion rheology is required for achieving low surfactant retention and economic viability of chemical EOR. Co-solvents play a pivotal role in obtaining favorable microemulsion rheology as well as many other aspects of chemical EOR. We measured the partitioning of co-solvents between phases to better understand their behavior and how to select the best co-solvent for chemical EOR. There is an optimal co-solvent partition coefficient for microemulsion systems. Commercial co-solvents used for chemical EOR are actually mixtures of different components. We used HPLC to measure the partitioning of the constitutive components of phenol ethoxylate co-solvents between oil and water phases and between microemulsion and excess oil and water phases. These measurements show that the components partition independently and the partitioning of individual components is often different from the average. The co-solvent partition coefficients between oil and water were systematically evaluated as functions of the number of ethylene oxide groups, number of propylene oxide groups, temperature, salinity, and the equivalent alkane carbon number (EACN) of the oil. Novel alkoxylate co-solvents were also evaluated for chemical EOR. The novel alkoxylate co-solvents can be more effectively tailored to match the characteristics of different crude oils. Coreflood experiments were conducted to investigate co-solvent transport and retention. Co-solvents were identified that showed excellent performance and low retention. et al., 1989; Strey and Jonströmer, 1992;Wennerström and Olsson, 2014). Co-solvents widen the type III window and reduce both the shear viscosity and the interfacial viscosity, which is the basis for the process called alkaline-co-solvent-polymer (ACP) flooding (Fortenberry et al., 2015). Co-solvents are one tool used to optimize formulations. Solairaj et al. (2012) performed multivariable regression analysis on a dataset of multiple corefloods with optimized formulations, good mobility control, and favorable salinity gradients and showed a weak correlation between co-solvent concentration and surfactant retention. Co-solvents partition between oil, water and the surfactant micellar interface. Studying the partitioning of co-solvents aids their selection. The best co-solvent is determined by taking into account factors such as cost, impact on IFT, microemulsion viscosity, aqueous stability, and partition coefficients and often requires numerous laboratory experiments.The oil-water co-solvent partition coefficient is defined as the ratio of the co-solvent concentration in the oil divided by that in the water. Co-solvents that partition equally between the oil and water phases have a partition coefficient of 1. Based on previous studies, the ideal value for the oil-water partition coefficient is 1 taking into account considerations such as minimizing the chromatographic separation of the co-solvent and surfactant when a chemical slug is injected into an oil reservoir (Sahni et al., 2010).In biochemistry and organi...