A Minimum Miscibility Pressure Study Using Impure CO2 and West Texas Oil Systems: Data Base, Correlations, and Compositional Simulation
- F.S. Kovarik (ARCO Exploration and Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Production Technology Symposium, 11-12 November, Lubbock, Texas
- Publication Date
- Document Type
- Conference Paper
- 1985. Society of Petroleum Engineers
- 4.1.4 Gas Processing, 4.1.5 Processing Equipment, 4.6 Natural Gas, 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State, 5.3.2 Multiphase Flow, 5.2.1 Phase Behavior and PVT Measurements, 5.8.8 Gas-condensate reservoirs, 1.8 Formation Damage, 5.4.9 Miscible Methods, 4.3.4 Scale
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This paper presents results from the study to improve the impure CO2 minimum miscibility pressure (MMP) prediction capability. The MMP data base was obtained from laboratory slim tube runs. Rigorous criteria for MMP determinations were established. These data were correlated as a function of mole fraction of contaminants in CO2, as a function of injection gas pseudocritical properties, and using a modified Peng-Robinson equation of state, with average errors in the range of 2 to 6 percent. The CO2 impurities were N2, C1, C2, C3, C5. Concentrations of impurities varied from 0 to 30 mole percent.
Comparisons with literature data indicate that the correlations, although developed using only West Texas oils, can be generalized to include a wide spectrum of reservoir fluids. These correlations, when compared to others in the literature, are found to be superior for injection gas mixtures which contain nitrogen and methane contaminants.
An ARCO developed compositional simulator was calibrated using slim tube and PVT data from the West Texas oil-CO2 system. The model simulates recovery curves reasonably well and predicts the pure and impure CO2 minimum miscibility pressures pure and impure CO2 minimum miscibility pressures for West Texas oils to within 50-150 psi.
Miscible displacement processes can achieve near 100% oil recovery. The high pressure CO2 process is known to be multicontact miscible under process is known to be multicontact miscible under certain conditions through the buildup of a transition zone between the oil and displacing gas. The minimum miscibility pressure (MMP) is defined here as the pressure threshold where the flood changes fran immiscible (multiple phase flow) to miscible (single phase flow). It is the minimum pressure one needs to maximize recovery.
Methane and nitrogen impurities in the dense CO2 stream increase the MMP, C2-C5 contaminants decrease the MMP. Systems that have rapidly rising recovery versus pressure curves with sharp breakpoints, for example, pure CO2 and West Texas oils, clearly show the minimum miscibility pressure. However, it is frequently more difficult to interpret slowly rising recovery curves such as those produced from CO2 with methane and nitrogen impurities. In fact, it is not uncommon to get a several hundred pound MMP prediction spread given a single recover curve, depending only on the person interpreting the curve. The objective of person interpreting the curve. The objective of this study is to improve upon our MMP prediction capability.
There are several pure CO2 MMP correlations in the literature. Holm and Josendal's correlation is based upon Benham's method. Their data was best fit by the Benham enriched gas correlation for a 59% methane-41% propane mixture.
Yellig and Metcalfe used temperature alone as the independent variable and included a bubble point pressure correction, i.e., if the oil's point pressure correction, i.e., if the oil's bubble point pressure exceeds the estimated MMP, the miscibility pressure is set equal to the bubble point pressure of the oil. They concluded that the CO2 MMP increases at approximately 15 psi/degrees F. psi/degrees F. Alston, et al correlated the pure CO2 miscibility pressure with temperature, C5+ molecular weight, volatile oil fraction, and intermediate oil fraction. The Johnson and Pollin equation requires the oil gravity, molecular weight, and reservoir temperature.
The last two correlations were also used for impure CO2 gas streams by the inclusion of an injection gas critical property function. The Johnson and Pollin correlation, however, is limited to the binary mixtures of CO2 with N2 or CH4.
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