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Abstract

Compositional reservoir simulation is a relatively new facet of reservoir simulation. Although the mathematical equations are somewhat more complicated than for conventional reservoir simulation, the concepts can be simplified by stating that we are simultaneously modeling (1) three-phase Darcy flow, (2) the movement of each individual hydrocarbon component through the reservoir, and (3) phase equilibrium at each point in the reservoir. The phase equilibrium between gas and oil is determined by the K-values which are used for each particular temperature, pressure, and composition.

Compositional simulation is applied to a variety of reservoir fluids but the most common application is to volatile oil and gas condensate reservoirs. For these reservoirs, gas cycling, waterflooding, and primary depletion can be modeled for the purpose of making economic comparisons and making decisions.

The cost and computational speed of compositional simulation depends partly on the number of components which are being modeled. A typical computational requirement is 0.1 seconds/mesh point/time step on a CDC 6400 computer for a 12 component system.

Introduction

Fortunately, many reservoirs contain fluids which have a simple type of phase behavior and physical properties. It is sufficiently accurate for these fluids, to represent the amount of gas in solution as an empirical function of pressure only. The densities of gas and reservoir oil are related to surface densities through formation-volume-factors which are empirical functions of pressure. Viscosities are also empirical functions of pressure. pressure. Viscosities are also empirical functions of pressure. Such simple systems are sometimes called "black-oil" systems. This is the type of fluid system which has been the basis of most of the reservoir simulation technology developed in recent years.

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