Phase behavior aspects of carbon dioxide (CO2) miscible flooding in tight cores: A case study

Carbon dioxide flooding process has been a proven valuable tertiary enhanced oil recovery technique. Although the petroleum industry has been applying the technique to produce heavy oil, it can be an effective tool to yield appreciable recoveries from complicated formations to produce comparatively lighter oil. The focus of experimental studies has been on the production of heavy oil and Carbon dioxide application to a wide range of other geological conditions and for wide range of petroleum fluids has been a less traversed path. Especially the process can be applied to tight formations where normal production procedures are not economically viable. This unviability may be due to rock or rock-fluid interactive properties. A research program for evaluation of the feasibility of development of such tight formations in the Middle East has been initiated. The research work included determination of minimum miscibility pressure (MMP), CO₂ Core flooding and phase behavior investigation. Since both these processes are governed by the phase behavior; an investigation of phase behavior and PVT properties of reservoir fluids when combined with carbon dioxide are an integral part for a complete evaluation of crude oil extraction process. This paper presents the experimental and simulated phase behavior data for various mixtures of a live crude oil and carbon dioxide. The data helped in designing of the slim tube investigations and core flooding experiments. The phase behavior during a CO₂ flooding is a very complex process. Three mechanisms; oil swelling, reduction of oil viscosity, and the acidization of carbonate help obtaining better recovery in a CO₂ flooding process. First two are the mechanisms boosting the miscibility between CO₂ and reservoir oil. The other parameters which effect phase behavior during a CO₂ flooding are the temperature, pressure and rock-fluid interactive properties of the reservoir. Carbon dioxide can be first contact miscible with crude oils, but usually at very high pressure. Attaining and operating a flooding process at these pressures is not financially desirable. Multi contact miscible process is preferred as the economical process for the carbon dioxide flooding. The phase behavior of the original oil and after addition of different amounts of CO₂ was studied by performing Constant Composition Expansion (CCE) tests. The bubble point pressure determined for the original oil sample and with increasing carbon dioxide. The phase behavior properties like bubble point pressure amount of liquid for the mixture with carbon dioxide range matched well with Equation of State (EOS) simulations. A comparison of the average density of the crude oil CO₂ mixtures confirmed the swelling process. A material balance study between the injected and produced material from the core was also done for verification.