Alkaline/Surfactant Flood Potential in Western Canadian Heavy Oil Reservoirs

Abstract

Abstract For heavy oil reservoirs (with oil viscosities ranging from 1,000 to more than 10,000 mPa·s), primary production and waterflood can only recover 5 to 10% initial oil-in-place (IOIP) due to the unfavourable mobility ratio of the water phase to the oil phase. Heavy oils usually have a relatively high content of organic acids, which can be neutralized by alkalis to form in-situ surfactants. With the assistance of these in-situ surfactants, an oil-in-water emulsion with a much lower viscosity than heavy oil can be generated. In this way, the heavy oil is entrained in the water phase and produced out of the reservoir. An initial study was carried out to evaluate the feasibility of alkaline/surfactant (A/S) flooding for western Canadian heavy oil reservoirs. The integrated approach included extensive emulsification tests, oil/brine interfacial tension measurements, viscosity measurements, and sandpack flood tests. The experimental results showed that the dynamic interfacial tension of oil/water can be lowered to an ultralow level (< 0.01 dyne/cm) by an alkaline solution and a very dilute concentration of surfactant, leading to easy emulsification of heavy oil in formation brine under slight interfacial disturbance. A series of sandpack flood tests were carried out to investigate the recovery performance of A/S flooding for five western Canadian heavy oils with viscosities ranging from 650 to 18,000 mPa·s at 22°C. Tertiary oil recoveries in sandpack flood tests were between 20 to 30% IOIP. The results of these sandpack flood tests suggest that A/S flooding is a promising enhanced oil recovery process for thin heavy oil reservoirs, in which thermal processes are not suitable. Introduction Emulsification of crude oil in displacing water is one of the mechanisms of alkaline flooding for conventional oils.[1–3] Physico-chemical processes, such as chemical reaction and mass transfer, near the oil-water interface can result in low transient interfacial tension (IFT) and interfacial instability. With the help of some mechanical shear or disturbance, oil can be broken into fine droplets or emulsified into the water phase.[3[ Cash et al.[4] investigated spontaneous emulsification behavior and its potential for enhancing oil recovery. He concluded that spontaneous emulsification would occur between a hydrocarbon phase and an aqueous solution of petroleum sulfonate. The small oil droplets thereby mobilized may subsequently coalesce with other ganglia of residual oil, creating a local region of enhanced oil saturation. Once the local oil saturation is increased sufficiently, the oil will be mobilized. Further coalescence can take place and an oil bank can be formed, leading to the improvement of sweep efficiency of injected water. McAuliffe[5] studied the oil-in-water (O/W) emulsion flow in porous media. When an O/W emulsion was injected, a greater amount of emulsion first entered the more permeable zones. The flow of emulsion became restricted due to the "Jamin" effect[6] if the pore throats were smaller than the sizes of the oil droplets. Water then began to flow into less permeable zones, resulting in greater sweep efficiency. McAuliffe's experimental results showed that O/W emulsions could effectively reduce the water permeabilities of sandstone cores if the initial water permeabilities were less than 2 darcies (D). The permeability reduction caused by injecting emulsion was retained even when the emulsion was followed by many pore volumes of water injection. The oil/water interfacial tension behavior in alkaline flooding was widely studied for conventional oils. In coreflood tests for an oil with API gravity of 27, Thigpen et al.[7] added surfactant to the alkaline solution to reduce the oil/water interfacial tension. The surfactant was soluble in both the aqueous solution and the reservoir oil but more soluble in the former. The addition of surfactant made alkaline flooding more efficient in recovering the waterflood residual oil. Rudin et al.[8] investigated the effect of added surfactant on interfacial tension and spontaneous emulsification in alkali and acidic oil systems. They used two oils with viscosities of 3.75 and 52 mPa·s, respectively, and an artificial brine containing NaCl and alkalis. They found that adding surfactant reduced the equilibrium IFT to an ultralow value. The pH range for ultralow IFT and for spontaneous emulsification was also widened. The addition of surfactant also caused a higher interfacial resistance to mass transfer, which reduced the rate of acid ionization, resulting in a longer period of low dynamic interfacial tension.