Effect of CO2 Dissolution on the Rheology of a Heavy Oil/Water Emulsion

Abstract

During the later stages of flow from an oil well, water inevitably appears in the produced fluids. When crude oil and water are energetically mixed by constrictions in the production tubing, emulsions can form. Heavy crudes may also contain surface-active agents that can stabilize the emulsion, resulting in persistent flow problems. If carbon dioxide is injected into such a reservoir (e.g., for CO2 enhanced oil recovery), then CO2 will dissolve into both oil and water phases affecting the emulsion properties; however, this aspect has been neglected in the literature thus far. This paper presents a study of the rheology of oil/water emulsion altered by carbon dioxide. The emulsion was prepared by blending 50 wt % water and 50 wt % Zuata heavy crude oil in a high shear mixer (Silverson), resulting in a water-in-oil emulsion. The emulsion was subsequently stable at ambient conditions for several weeks without the addition of any surfactants. A high-pressure rheometer system coupled to a mixing vessel and fluid circulation loop allowed the emulsion to be brought into equilibrium with CO2, and its rheology was then measured at a temperature of 50 °C and pressures from ambient to 120 bar. The emulsion without dissolved CO2 was found to be slightly shear thinning below a critical shear rate, above which the viscosity jumped to a much lower value. The CO2 dissolution had two effects: first, it reduced the emulsion viscosity at low shear while preserving the shear thinning behavior, and second, increasing the CO2 pressure in equilibrium with the emulsion increased the critical shear rate at which the viscosity jump occurred. At shear rates above the jump, the emulsion viscosity dropped to a level lower than that of the original continuous phase (oil). It is likely that the viscosity jump occurred as a result of phase inversion; however, this was difficult to observe directly. The jump was reversed (with some hysteresis) as the shear rate was reduced again. Dissolved CO2 can influence the emulsion properties, such as phase inversion, through its action in both phases. The dissolution of CO2 in the oil phase reduced the viscosity of the oil, while dissolution into the water phase markedly changed pH and, thereby, the performance of any charged surface-active agents present in the crude oil.