Phase Behaviour And Physical Properties Of Lindbergh Heavy Oil/CO2 Mixtures
Authors: Sayegh S.G. , Rao D.N. , Kokal S. , Najman J. 
Published: March 2010

The objective of this work is to acquire phase behaviour and physical property data of carbon dioxide/heavy oil systems pertaining ta enhanced oil recovery by CO2 huff-n-puff and steam stimulation.

Phase behaviour and physical property measurements were carried out on Lindbergh heavy oil CO2 mixtures at 21 °C and 140 °C, and at pressures ranging up to about 15 MPa. Furthermore, the oil samples were analyzed using gas chromatographic simulated distillation. The experimental measurements were also carried out on three of the oil's fractions that can serve as "pseudo components" during equation of state correlations Of the experimental data.

The solubility of CO2 was less in the heavier hydrocarbons than in the lighter fractions and the solubility also decreased with increasing temperature. The densities of the saturated mixtures did not change significantly as the saturation pressure was increased. The extent of viscosity reduction with increasing CO2 content was larger for the heavier hydrocarbon fractions than for the lighter ones. In addition, the viscosity was sometimes seen to increase with pressure in the liquid-liquid equilibrium region of the phase diagrams. Analysis of the residual oils indicated that in some cases a small amount of their light ends had been extracted by CO2 and/or some of the heavy ends had been depleted (presumably precipitated).

Introduction

Gas (more commonly CO2) injection into heavy oil reservoirs is becoming a viable enhanced oil recovery method(l-5). Mathematical studies of this process have been presented in the literature(6–10), as have been laboratory displacement data(11–13) and phase behaviour measurements(l4–19). The main mechanism in this process is the dissolution of the injected gas into the crude oil, thereby swelling it, reducing its viscosity, and making it more mobile. Usually the process is an immiscible one, in contrast to the more common miscible gas flooding of conventional oil reservoirs.

The correlation of the phase behaviour data of conventional crude oils is usually carried out with one of the Van-der-Waals type of equations of state. Examples of these equations are those by Soave-Redlich-Kwong(20) and Peng-Robinson(21). Ideally, these equations require certain parameters to characterize each component in the mixtures under consideration. The common practice has been to represent the crude oil with a limited number (four to six, typically) of pseudo components. Several Authors(22–27) have proposed various schemes for selecting these pseudo components and evaluating their properties based on those of the crude oil.

Most of the methods that have been developed for the characterization of crude oils were derived from data of light oil systems and these are usually directly applied to heavy oil data. This may not be an optimum approach since heavy oils have a much larger proportion of heavy ends, asphaltenes, and resins, and a much lower content of C5-C30 hydrocarbons than light oils. In addition, heavy oil reservoirs are usually shallower than light oil reservoirs and, consequently, have lower pressures and temperatures. Alternatively, enhanced oil recovery of heavy oil reservoirs is often carried out using thermal methods, such as steam or fire flooding, in which very high temperatures (&gt; 200 °C) are encountered.