Surfactant Flooding 1: The Effect of Alkaline Additives on IFT, Surfactant Adsorption, and Recovery Efficiency

Abstract

Abstract This paper examines the effects of alkaline additives on dilute surfactant systems for improved oil recovery. The study was limited to the determination of the effects of alkaline additives on interfacial tension (IFT), surfactant adsorption or retention in Berea cores, and improvement in oil recovery. The alkaline chemicals studied were sodium silicates, sodium phosphates, sodium carbonate, and sodium hydroxide. In addition, optimal salinities and surfactant average equivalent molecular weight for the recovery of two midcontinent crude oils were determined through a combination of IFT determinations and oil displacement tests. The laboratory results show that the alkaline chemicals have two major effects. First, IFT is reduced further by the high pH surfactant/alkali solution combinations, and second, certain alkaline species significantly reduce surfactant retention. This leads to recoveries of residual oil from 40 to 70% with surfactant solutions containing only 0.25 wt% surfactant. Introduction The use of dilute aqueous surfactant systems for enhanced oil recovery has been documented for several decades, but the more pertinent work has been performed in the past 15 years. Gogarty, and Tosch outlined Marathon's early studies on the Maraflood TM process, which employs what is commonly known as a micellar/polymer system for enhanced recovery. In the area of dilute surfactant systems, a considerable body of work has been published in which alkaline chemicals have been added to improve recovery efficiency. A number of processes have been patented in which excess alkaline chemicals have been injected following the initial injection of organic acids. Other systems in which an alkaline preflush preceded the injection of the surfactant slugs have been reported. Similar processes, which can be summarized in a general manner have been reported by several research teams. All these systems are individually different but employ the same general principles in their operation. These principles are as follows.Injection of a sacrificial alkaline chemical, such as sodium carbonate or sodium tripolyphosphate, in a saline solutionto reduce the hardness ion level,to reduce surfactant adsorption or retention, andto provide an optimal salinity for the surfactant slug.Injection of a dilute surfactant slug containing alkaline chemicals, such as sodium carbonate and/or sodium tripolyphosphate, with the proper concentration of sodium chloride added to provide the minimal IFT for optimal oil recovery.Injection of a drive fluid that may or may not contain a polymer additive for increased mobility control. Several field trials of low-tension waterfloods have been reported. In most cases, increased production was observed from watered-out areas. The tests were considered to be technologically successful but not necessarily economically feasible at the time of the test. Some of the disadvantages of dilute surfactant systems can be traced to complex and, in most cases, deleterious effects of hardness ions, mineral surfaces, and highly saline reservoir fluids on the effectiveness of the surfactant in mobilizing the oil phase. Some of these disadvantages can be overcome by proper selection of surfactants that are less sensitive to hardness ions and more tolerant of high salinities. However, most of these surfactant systems are much more expensive than the petroleum sulfonates and still would be subjected to considerable adsorption or retention by the reservoir substrate. Thus, the addition of alkaline chemicals to the surfactant system provides an economical solution that overcomes the adverse conditions mentioned previously. SPEJ P. 503^