Miscible Gas Displacement of Multicomponent Oils
- R.T. Johns (University of Texas, Austin) | F.M. Orr Jr. (Stanford University)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1996
- Document Type
- Journal Paper
- 39 - 50
- 1996. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.3.4 Scale, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 5.6.4 Drillstem/Well Testing, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.3.2 Multiphase Flow, 5.4.9 Miscible Methods, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 4.5.7 Controls and Umbilicals
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This paper extends the theory of multicontact miscible displacements in four-component systems to displacements by a single gas component of an oil containing any number of components. Multicomponent solutions are developed using the method of characteristics in the absence of dispersion and are shown to consist of a series of successive pseudoternary displacements. Example analytical solutions are constructed and compared to experimental results for a synthetic ten-component oil displaced by CO2 as given by Metcalfe and Yarborough.
The minimum pressure for miscibility (MMP) for the synthetic oil is determined to be substantially below pressures used in the experiments and is below the bubble-point pressure. We show that multi-contact miscibility (MCM) can be achieved even when an oil exists as two phases. The displacement of the synthetic oil by CO2 is demonstrated to have features of both condensing and vaporizing mechanisms (C/V). Furthermore, the key tie line that controls development of miscibility is the crossover tie line that divides the condensing and vaporizing portions of the displacement path (composition route).
Extension of the theory for displacements of real oils with an arbitrary number of components is also discussed. For displacements by CO2 of an oil that contains only CH4, C2 and heavier components, we demonstrate that the most downstream crossover tie line will control the development of miscibility. For displacements by the most volatile component (i.e. pure N2 or CH4 injection) of the same oil, however, displacements are purely vaporizing and miscibility is controlled by the tie line that extends through the oil. A comparison of the component volatilities of the oil relative to the injection gas determines which one of the tie lines will control the development of miscibility. A procedure is given that can be used to find the key tie line that controls miscibility and also to estimate the MMP from an equation-of-state (EOS).
When gas is injected at high pressure into a porous medium containing oil, components in the oil transfer to the vapor phase, and components in the injection gas dissolve in the oil phase as chemical equilibrium is established. The fraction of a component present in the liquid or vapor phase depends on its volatility, because more volatile components preferentially partition into the vapor phase. The resulting liquid and vapor phases move through the porous medium at different flow velocities that depend on the local pressure gradient, the phase viscosities, and the saturation of each phase. As a result, the vapor phase contacts oil with some composition different from the equilibrium liquid composition. Additional component transfers then take place as equilibrium is established again. The combined effects of phase equilibrium and two-phase flow, therefore, induce a series of composition changes that are equivalent to the separations that occur during chromatography, with the most volatile components being displaced faster. If the pressure is high enough, or if the injected gas composition is chosen appropriately, those chromatographic separations can lead to multicontact miscible displacement (MCM) in which a very efficient local displacement of oil by gas is achieved.
Most analyses of the mechanisms of miscible floods have been based on the idealized behavior of three-component systems. According to three-component theory, only two types of displacements are possible, vaporizing or condensing gas drives.
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