Mechanisms of Immiscible CO2 Injection in Heavy Oil Reservoirs, Wilmington Field, CA
Authors: Spivak Allan , Chima C.M. 
Published: December 2007

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Abstract
This paper describes the results of simulation studies made to investigate the mechanisms of immiscible CO2 injection in heavy oil reservoirs. The studies were undertaken to assist in the design and monitoring of two projects implemented in the Wilmington Field, Los Angeles County, California. An equation-of-state compositional simulator was used in these studies. The equation-of-state was adjusted to match saturation pressures and swelling data from the laboratory. Simulation runs were then made to investigate the effect of various parameters on the process. parameters on the process. The paper relates the various simulator studies to the Long Beach Oil Development Company (LBOD) project in the Tar Zone, Fault Block V and the Xtra Energy project in the Ranger Zone, Fault Block I of the Wilmington Field. Both projects use the same source of gas which is approximately 82% CO2 and 18% N2.

Introduction
References 1 and 2 discuss the geology of the Wilmington oil field. Long Beach Oil Development Company and Xtra Energy Corporation are operating immiscible gas injection projects at Wilmington and the source of gas for both projects is a Texaco refinery. The LBOD project has been in operation since March 1982 and the Xtra Energy project was initiated in August 1983. Also, project was initiated in August 1983. Also, Champlin Petroleum operates an immiscible gas injection project in the Tar Zone, Fault Block III of the Wilmington Field using pure CO2- Champlin conducted a pilot of the process commencing in March 1981 and is currently expanding the pilot area to a much larger project. Table 1 is a comparison of pertinent data for the LBOD and Xtra Energy projects. The LBOD project area has been under waterflood since 1961 and the current water cut is 94%. Therefore, the LBOD project is essentially a tertiary process. The Xtra Energy project has had only a small amount of water injection to date and therefore is a secondary process. Although the LBOD project is in the Tar Zone and the Xtra Energy project is in the Ranger Zone, the crude oil properties in the two projects are very similar. In both projects, gas will be injected alternately with water until a predetermined total amount of gas has been injected. The alternate gas and water injection will be followed by water injection only, until an economic limit is reached.

MATCHING OF LABORATORY PVT DATA
The compositional simulator uses the Peng-Robinson equation-of-state for phase equilibrium Peng-Robinson equation-of-state for phase equilibrium calculations. A six component system was considered in this work, with the components and some of their properties listed in Table 2. Laboratory Studies using crude oil samples from the Tar V were performed by Chevron Oilfield Research Company. The Peng-Robinson equation-of-state was adjusted to match saturation pressures and swelling data for mixtures of the pressures and swelling data for mixtures of the Tar V crude oil and either pure CO2 or a mixture of 82% CO2 and 18% N2 (82/18). A comparison of the laboratory determined saturation pressures and the saturation pressures obtained from the equation-of-state is shown on Figure 1. The laboratory measured and the calculated swelling test data are shown on Table 3. Excellent matches were obtained in both cases. The parameters that were modified to obtain the matches were the omega a of C25+ and omega b of N2, CO2 and C25+ in the Peng- Robinson equation-of-state. The modifications that were made were relatively minor.

OIL AND GAS VISCOSITIES
For seven mixtures of Tar V crude oil and either pure CO2 or 82/18, Chevron Oilfield Research Company measured oil viscosities at the saturation pressure and at pressures above the saturation pressure. A study of all the data showed that viscosity could be uniquely correlated with pressure and total dissolved gas.
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