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

Abstract

KEYWORDSRheology, heavy crude oil, emulsion, carbon dioxide, viscosity, non-Newtonian fluid 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 stabilise the emulsion, resulting in persistent flow problems. If carbon dioxide is injected into such a reservoir (e.g. for CO2 EOR), then the CO2 will dissolve into both oil and water phases affecting the emulsion properties, however this aspect has been neglected in the literature so far.2 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 at 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 behaviour; 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 due to phase inversion; however, this was difficult to observe directly. The jump was reversed (with some hysteresis) as the shear rate was reduced again. The 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.

A New Diminishing Interface Method for Determining the Minimum Miscibility Pressures of Light Oil–CO2 Systems in Bulk Phase and Nanopores

“…It is found from Figure a–c that x CO 2 is quickly increased with the pressure up to a certain pressure threshold, above which x CO 2 is increased gradually and finally reaches its maximum. The gaseous CO 2 is much more easily dissolved into the oil phase than the liquid CO 2 . On the other hand, y HCs is slightly decreased at a lower pressure, which is followed by an obvious increase starting from the threshold pressure, at which a strong HCs extraction occurs because the liquid CO 2 has a stronger extraction ability than the gaseous CO 2 .…”

“…The gaseous CO 2 is much more easily dissolved into the oil phase than the liquid CO 2 . 53 On the other hand, y HCs is slightly decreased at a lower pressure, which is followed by an obvious increase starting from the threshold pressure, at which a strong HCs extraction occurs because the liquid CO 2 has a stronger extraction ability than the gaseous CO 2 . At an even higher pressure, y HCs increases much more gradually because most light to intermediate HCs in the oil phase have already been extracted.…”

Experimental Study on the Effective Viscosity of Unstable CO2 Flooding Produced Fluid with the Energy Dissipation Method

“…This is also contrary to the conclusion obtained by Johnsen and Rønningsen, which demonstrated that the phase inversion point shifts to a higher water fraction after the dissolution of light hydrocarbons, from about 80% to about 90%. As for the study of the CO 2 -dissolved emulsion, Hu et al recently designed a circulation system that could be employed to test the rheology of the CO 2 -saturated crude oil emulsion . However, the measurement process was still accomplished with a rheometer, so it is only applicable to uniform and stable emulsions, but not suitable for the unstable oil–water–gas mixed fluid.…”

“…As for the study of the CO 2 -dissolved emulsion, Hu et al recently designed a circulation system that could be employed to test the rheology of the CO 2 -saturated crude oil emulsion. 18 However, the measurement process was still accomplished with a rheometer, so it is only applicable to uniform and stable emulsions, but not suitable for the unstable oil−water−gas mixed fluid.…”

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

“…The blue line represents CO 2 flow, and the black line represents the crude oil mixtures. Reprinted with permission from Hu et al 14 . Copyright 2016 American Chemical Society.…”