The Effect of Oil/Brine Ratio on Surfactant Adsorption From Microemulsion

Static adsorption measurements of petroleum sulfonates on crushed Bell Creek and Berea cores were made using fluids with the same active surfactant concentration but varying brine/oil mass ratios. The salinity of the brine was chosen such that a significant three-phase region existed in the oil/brine/surfactant/alcohol system. The surfactant adsorption was found to be independent of the structural and compositions differences among the fluids. A series of oil recovery tests in which middle-phase microemulsions were injected into waterflooded cores also were performed. The cores used in these tests had been treated to remove divalent ions accessible to fluid flow. Microemulsion slugs (1.75 to 146% PV) of equal active surfactant concentration but differing brine/oil mass ratios were injected. The total surfactant retention for this system was also found to be independent of the brine/oil mass ratio. Introduction Control of sulfonate loss is one of the single most important factors in determining the success or failure of a surfactant flooding process. In a typical surfactant flood, sulfonate costs are frequently half or more of the total project cost. As a result, this area has been studied frequently. Many authors have studied detailed adsorption mechanisms - mostly from aqueous solutions and at relatively low concentrations. A recent thesis from the U. of Texas1 and its references provide an excellent description of this type of work. The petroleum industry literature deals more with evaluating the controlling factors in actual flood implementation. Surfactant loss has been broken down into adsorption, precipitation, and phase trapping. The effects of sacrificial agents, sulfonate fractionation, divalent ions, and salinity gradients all have been investigated. Since it is not the purpose of this paper to provide a review of the literature, we have attempted only to summarize some of this literature in the form of a table (see Table 1). Surfactant retention data have been presented in terms of a wide variety of units. These fall into two basic categories: molecules per unit area, which is indicative of surface coverage, and mass per unit of pore volume, which is indicative of surfactant consumption by the reservoir. We have chosen to express both our data and all of the data in Table 1 as microequivalents per square meter (µeq/m2) and milligrams active surfactant per milliliter of pore volume (mg AS/mL PV). In many cases this involved making assumptions as to rock density, porosity, surfactant equivalent weight, etc.; however, it has the advantage of allowing direct comparison of results.