Study of the Effect of Injection Water Quality on the Interfacial Tension of ASP/Crude Oil

Abstract

Abstract ASP (Alkali/Surfactant/Polymer) flooding has been successfully applied in several field pilots in the Daqing oil field in China. Oil recovery has improved by about 20% OOIP over recovery by waterflooding. The interfacial tension (IFT) of ASP/crude oil is one of the most important factors that control the effectiveness of ASP flooding. The effects of alkali, surfactant, polymer and oil properties on IFT were previously studied. This paper addresses the effect of injection water quality on the IFT of ASP/crude oil. This paper analyzes the inorganic ion compositions and organic compounds of injected water in different pilots in Daqing. The effect of these compositions and the suspended particles in injection water on the IFT of ASP/oil were studied. A carboxylate surfactant was used in the study. All experiments were carried out at 45 °C. Results showed that the suspended particles had little influence on IFT, and that inorganic ion compositions had some influence to a certain extent but were not the main factor affecting IFT for the low salinity injection water used in ASP flooding in the Daqing oil field. The main factor increasing the IFT is certain organic compounds in the injection water. The paper points out the main types of organic compounds influencing IFT. Some methods to lower IFT by controlling water quality are proposed. Introduction ASP (Alkali/Surfactant/Polymer) flooding technology was discovered in the early 1980's(1–3). Based on the cooperative effect of the three chemicals, ASP synergizes the three chemicals' functions and improves their efficiencies much more than singly. At the sametime, it greatly reduces the amount of the chemicals needed; especially the expensive surfactant. With crude oil having a high acid number, ASP flood field tests obtained reasonably high recovery(4,5). The crude oil in the Daqing oil field is paraffin based with a low acid number; < 0.1 mg KOH/g. In the laboratory, the Daqing oil field obtained rather high recovery for a crude oil with such a low acid number(6). Five ASP pilot tests were successfully performed in different well patterns, well spacings and reservoir conditions in the field. These pilot tests show that ASP flooding unequivocally enlarges the sweep volume, improves the displacement efficiency and produces incremental recovery of more than 20% OOIP than does waterflooding(7–9). There are many complex factors that can affect the efficiency of ASP flooding, such as the interfacial tension (IFT) of ASP/crude oil, adsorption loss of the ASP solution in the formation, the viscosity and emulsification property of the ASP solution and the compatibility of the ASP solution with the reservoir. IFT is one of the most important factors for determining the appropriate ASP injection solution. Measurement of IFT is the first step to determining the prescription for applying the ASP solution. According to capillary theory, under the conditions of the pressure gradient that can be reached through present technology, in order to activate the residual oil left by waterflooding, IFT of ASP/crude oil should be decreased to a 10−3 mN/m level.