Enhanced Oil Recovery (EOR) methods include injection of different fluids into reservoirs to improve oil displacement. These processes can be divided into two main categories: thermodynamical and hydrodynamical ones. Analytical models for 1-D displacement of oil by gas have been developed during the last 15 years. It was observed from semi- analytical and numerical experiments that several thermodynamic features of the process (MMP, key tie lines, etc) are not dependent of transport properties. The model for one-dimensional displacement of oil by miscible fluids is analyzed in this paper. The main result is the splitting of thermodynamical and hydrodynamical parts in the EOR mathematical model. The introduction of a potential associated with one of the conservation laws and its use as an independent variable reduces the number of equations. The reduced auxiliary system contains just thermodynamical (equilibrium fractions of each phase, sorption isotherms) variables and the lifting equation contains just hydrodynamical (phases relative permeabilities and viscosities) parameters while the initial EOR model contains both thermodynamical and hydrodynamical functions. So, the problem of EOR displacement was divided into two independent problems: one thermodynamical and one hydrodynamical. Therefore, phase transitions occurring during displacement are determined by the auxiliary system, i.e. they are independent of hydrodynamic properties of fluids and rock. For example, the minimum miscibility pressure (MMP) is independent of relative permeabilities and phases viscosities. The splitting technique may be used for the solution of Riemann problems and for non-self-similar displacement of oil by rich gas solvent slug with lean gas drive considering ideal behavior of the fluids, i.e. constant distribution coefficients. For 3-D displacements, the splitting is valid if and only if the total mobility is constant. It allows the application of the obtained 1-D analytical solutions in streamline simulators. The technique reduces significantly the amount of calculations for sensitivity study of gasflooding processes with respect to transport properties: auxiliary thermodynamic problem may be solved once for given reservoir and injected compositions; variation of relative permeabilities and viscosities should be performed just in the solution of one transport equation.
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