Recovery of a Saskatchewan Heavy Oil Using Alkaline Solutions

Abstract

Abstract The feasibility of employing various alkaline agents (sodium hydroxide, sodium metasilicate and sodium orthosilicate) to enhance the recovery of a specific Saskatchewan heavy oil from the Kindersley region has been investigated. The dynamic interfacial tension behaviors of solutions of these alkalis in contact with the crude oil were measured for a range of concentrations and temperatures, and the most interfacially active formulations were then tested for their oil recovery efficacies by conducting oil displacement experiments in unconsolidated linear sand packs at 25 °C and 65 °C. The most promising of the three alkalis tested was found to be sodium hydroxide, which was capable of producing very significant enhancements in oil recovery relative to that obtained with pure waterflooding. Introduction The so-called heavy oil belt of Saskatchewan covers a wide area that stretches from Lloydminster in the north to Kindersley in the south. The estimated ultimate oil in place amounts to some 4 × 109 m3. In a recent appraisal of this huge reserve, Reid(1) suggested that 20% of the oil in place could be recovered by using enhanced oil recovery techniques. To date, only thermal processes such as in-situ combustion and steamflooding/stimulation are being applied to these reservoirs. However, since the density of the crude ranges from 11 to 20 °API and since the average temperature of these reservoirs is about 20 °C, the viscosity of the crude at reservoir conditions will be of the order of 1000 mPa.s or higher. Another significant characteristic of these deposits is the occurrence of the oil in relatively thin zones. According to Selby et al.(2), the thinness of the formations coupled with the large depths and low permeabilities makes thermal processes unsuitable for about 50% of the heavy oil deposits of Canada, the U.S. and Venezuela. Amongst the various non-thermal processes, alkaline flooding appears to be the most attractive. Alkaline reagents are abundant and quite cheap compared with conventional surfactants. Their effectiveness for the recovery of acidic crudes has been known since 1927 by virtue of the work of Nutting(3) and Atkinson(4). It is now generally accepted(5–8) that alkaline reagents react with surface active materials present in the crude resulting in the in-situ formation of surfactant soap species. The adsorption of these generated surfactants at the oil/water/sand interfaces often results in a drastic reduction of the interfacial tension and/or in a change in matrix wettability. The end result is the mobilization of residual oil trapped in the fine pores of the reservoir sand. Selby et al.(2) listed 15 recent field alkaline flood tests in Canada and the U.S. However, only one of those tests was considered successful, while the majority were abandoned. One reason for this dismal performance is the inadequate understanding of the complex interactions that occur between alkaline reagents, crude oil, formation water and matrix minerals. Even basic oil-water interfacial tension (IFT) behavior becomes very complex when dealing with crude oil/caustic systems. Several researchers(9,10) have reported variations in IFT of one or more orders of magnitude when crude oil was contacted with caustic solutions for contact times ranging between 10 and 60 minutes.