The rheology of crude oil and carbon dioxide mixtures

Abstract

The rheology of crude oil mixtures at equilibrium with carbon dioxide (CO2) was studied at elevated pressures and temperatures, similar to those found for oil reservoir conditions. The focus of the work presented in this thesis concerns the measurement of the rheological properties of CO2 saturated mixtures of crude oil. The rheology measurements were made using a high-pressure rheometer coupled to a fluid flow system designed and built in this project. The flow system comprised a mixing vessel and fluid flow loop that allowed the test fluid to be brought into equilibrium with CO2 by stirring and circulating through the rheometer measurement geometry under the pressure and temperature required. Measurements were made for three different fluids saturated with CO2: a light crude oil from the Gulf of Mexico (GoM), Zuata heavy crude oil, and an emulsion of Zuata crude oil with deionized water. The rheological measurements for the GoM crude oil were performed at temperatures of 23 °C and 50 °C and pressures from ambient to 220 bar. The CO2 addition did not change the Newtonian behaviour of the light crude oil, but reduced its viscosity until the phase equilibrium points of CO2. Beyond the CO2 phase equilibrium points, the CO2 mixture viscosity increased with increasing CO2 pressure, which was expected when the fluid density increased without change in composition. The experiments using the Zuata heavy crude oil and its dilutions with toluene were done at temperatures from 23 °C to 50 °C and pressures from ambient to 220 bar. The Zuata crude oil was changed from a non-Newtonian fluid to Newtonian by CO2 dissolution. All of its toluene dilutions behaved as a Newtonian fluid, as well as their CO2 saturated mixtures, except for one sample. The exception was a diluted crude oil with 30 wt% toluene, which was found to be shear-thinning when CO2 dissolved into it in a certain pressure range. It is believed that the non-Newtonian behaviour in this diluted crude oil was not caused by asphaltene precipitation but instead by the formation of asphaltene micelles or by the multiphase behaviour with liquid CO2. The viscosity of the heavy crude oil and its dilutions was exponentially reduced by CO2 addition until the CO2 phase equilibrium points, above which the viscosity was increased with CO2 pressure. Furthermore, a view cell system was built to study the phase behaviour of the CO2 saturated mixtures with the GoM crude oil, Zuata crude oil and the toluene dilutions of Zuata crude. When brought to equilibrium with CO2, it was found that the CO2 solubility and the oil rich phase volume were inversely correlated to the mixture viscosity. The Zuata crude oil emulsion was prepared by mixing 50 wt% Zuata heavy crude oil and 50 wt% deionized water using a high-shear mixer. The rheology measurement of the emulsion saturated with CO2 was made at 50 °C and pressures up 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. After the viscosity jump the shear thinning effect was still observed. The CO2 dissolution not only reduced the emulsion viscosity at low shear while preserving the shear thinning behaviour, but also increased the critical shear rate at which the viscosity jump occurred. The dissolved CO2 eliminated the shear thinning effect after the viscosity jump. The emulsion viscosity jumped to a lower level than that of the original continuous phase (oil), indicating that the viscosity jump occurred due to phase inversion. However, direct evidence of phase inversion was very difficult to provide. In addition, a new correlation to evaluate Newtonian viscosity of hydrocarbons is proposed. In this correlation the hydrocarbon viscosity can be calculated based on its density. This correlation requires less experimental data to work out the parameters compared to the methods given in the literature. The proposed correlation was tested with pure alkanes, alkane mixtures and gas-saturated hydrocarbons, and the prediction gave a reasonable accuracy.