Miscible Gas Displacement of Multicomponent Oils

Abstract

Summary 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 Yarborough1. 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 multicontact 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 methane (CH4), ethane (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).