KEYWORDSRheology, phase behavior, heavy crude oil, carbon dioxide, viscosity, non-Newtonian fluid ABSTRACTThe rheology of Zuata heavy crude oil, saturated with carbon dioxide, was studied at a temperature of 50 °C and pressures up to 220 bar. Observations of phase behavior were also reported and used to interpret the rheological data. The crude oil is very viscous and non-Newtonian at ambient pressure, but when brought into equilibrium with CO2 the non-Newtonian behavior was weakened and eventually disappeared at high CO2 pressures. When diluted with 10 wt% and 30 wt% toluene, the diluted crude oils and their mixtures with CO2 behaved as Newtonian fluids. The CO2 saturated mixture of the crude oil samples showed an exponential decrease in viscosity with increasing CO2 pressure, but an increase in viscosity at higher pressures. Observing through a view cell, the CO2 dissolution caused a swelling effect on the original crude. When saturated with CO2, the swelling effect also occurred on the 10 wt% diluted crude oil, but the volume of the oil rich phase was 2 decreased at higher pressures. However, for the 30 wt% diluted crude oil, a second liquid phase was observed on top of the oil rich phase, at pressures higher than the CO2 critical point. The mixture viscosity was inversely proportional to the CO2 solubility.

The viscosity of heavy crude oils strongly affects their producibility and recovery from oil reservoirs. Thus, the viscosity of heavy crude oils with dissolved gases at different shear rates is valuable knowledge for heavy crude oil exploitation. However, most publications only present the viscosity measurements at one specific shear rate, and only few papers in the literature have reported the rheological measurements. In this study, viscosity measurements of CO2 saturated heavy crude oils were performed as a function of shear rate, at a temperature of 25 °C and pressures up to 220 bar. The experimental apparatus is a unique circulation system which is able to measure the viscosity of CO2 saturated mixtures. A novel high pressure rheometer with a flow-through Couette geometry was developed to measure the viscosity at a range of shear rates. The 2 experimental results show that the addition of CO2 to heavy crude oil at the given temperature and pressures reduces the viscosity significantly, but the pressure dependence of the mixture viscosity changes at the phase transition of the CO2 rich phase. A modified Arrhenius equation, accounting for the effect of CO2 composition on viscosity, is applied to correlate the viscosity data.

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 rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO2, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO2 and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO2 at elevated pressures up to 220 bar and a temperature of 50 °C. The results show that CO2 addition changes the oil rheology significantly, initially reducing the viscosity as the CO2 pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO2.

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