The Role of Alkaline Chemicals in the Recovery of Low-Gravity Crude Oils

Abstract

Summary This paper discusses the relative utility of various alkaline inorganic chemicals proposed for waterflooding recovery processes. The chemical properties of these materials are compared, with emphasis on sodium orthosilicate and sodium hydroxide. In addition, laboratory data are presented on interfacial tension (IFT) measurements and relative oil-displacement efficiency of various concentrations of sodium orthosilicate and sodium hydroxide for a low-gravity crude oil. These data were obtained to provide supplementary information for a proposed alkaline waterflooding field trial in the Wilmington field, CA. The laboratory results indicate that significant residual oil recovery from watered-out sandpacks can be obtained by using sodium orthosilicate of a concentration range from 0.2 to 0.6%. Very low recovery yields were obtained with sodium hydroxide at 0.2%, so no further tests were made at higher concentrations because of limitations on the amount of extracted core material available. Introduction Chemicals that yield highly alkaline solutions when added to injection fluids can be used in EOR systems to affect various rock and fluid parameters, such as IFT, viscosity, emulsion stability, rock wettability, fluid mobility, hardness-ion reduction, ion-exchange capacity, surfactant adsorption, phase equilibria, etc., to improve recovery efficiency. The typical chemicals used or proposed for use include the alkaline inorganic compounds such as sodium silicates, sodium hydroxide, sodium carbonate, and sodium phosphates. In alkaline waterflooding of appropriate low-gravity crude oils, oil displacement presumably is initiated by the reaction of alkaline chemicals with acidic substances in the crude oil to form a complex surfactant mixture at the oil/brine interface. To attain low IFT levels, alkaline pH values of at least 11.0 are necessary. The alkaline chemicals that produce the required high pH values at reasonable concentrations are the alkaline sodium silicates, such as sodium orthosilicate, and sodium hydroxide. These two alkaline chemicals exhibit similar interfacial behavior in soft saline solutions; however, hardness ions in the brine can produce a significant difference in their effects on IFT, emulsion stability, rock wettability, and ultimate recovery efficiency. Two alkaline waterflooding pilot field trials are in progress for which dilute saline solutions of sodium orthosilicate are being used as the injection fluid following a soft saline preflush. The first of these trials is being conducted by Aminoil U.S.A. Inc. in the Lower Main zone of the Huntington Beach (CA) field. Laboratory studies comparing the relative recovery efficiencies of sodium orthosilicate and sodium hydroxide for crude oil from this field were reported recently. The second pilot trial is being conducted by the THUMS Long Beach Co. Unit in the Wilmington field. This paper presents laboratory results on IFT measurements and oil-displacement studies performed to provide supplementary information for this field trial. The volumes of preflush, alkaline slug, and postflush used are equivalent to those being used in the field trial. Solution Properties of Alkaline Chemicals Used in Oil-Recovery Systems Alkaline chemicals used in various aspects of EOR include the sodium silicates, sodium hydroxide, sodium carbonate, and sodium phosphates. In solution, these chemicals exhibit significantly different physicochemical behavior toward hardness ions present in reservoir brines, toward crude oils, and toward reservoir rock surfaces. JPT P. 2510^