Effect of "Rich" Gas Composition on Multiple-Contact Miscible Displacement A Cell-to-Cell Flash Model Study
Authors: Fussell D.D. , Shelton J.L. , Griffith J.D. 
Published: December 2007

Abstract
A cell-to-cell flash model was used to simulate the transition, or mixing, zone between a reservoir oil and several "rich" gases for multiple-contact miscible displacements. The transition-zone properties that control The oil recovery efficiency properties that control The oil recovery efficiency were determined and are junctions of the solvent concentration in the rich gas. Results indicate that the optimum use of solvent corresponds to a solvent concentration near the minimum enrichment level.

Introduction
Many papers have been published on the characteristics and applications of miscible displacement of oil with hydrocarbon fluids. The distinction is clearly made between first-contact miscible displacement, where the injected hydrocarbon fluid is miscible in all proportions with the oil, and multiple-contact miscible displacement, which occurs as a result of component transfer between hydrocarbon phases during flow in the porous media. Two general types of multiple-contact porous media. Two general types of multiple-contact miscible displacements are condensing gas or "rich" gas drive, and vaporizing or high-pressure gas drive. This paper concentrates on the rich gas drive, where the main feature is the transfer of intermediate components or solvent from the injected rich gas to the oil phase. The advantage of condensing gas drive in comparison with first-contact miscible displacement is the reduced concentration of the valuable solvent component in the injected hydrocarbon fluid. The effect of solvent concentration on the displacement process is an important factor in optimizing the use of solvent.
Several studies have reported the use of compositional models to investigate the condensing gas drive process. These studies were useful in obtaining a better understanding of the component transfer between the liquid and vapor phases, and its effect on the transition zone existing between the injected hydrocarbon fluid and the reservoir oil. These studies did not define subzones within the transition zone or present methods to evaluate the properties of these subzones. A detailed numerical investigation of the effect of solvent concentration in the injected rich gas upon the behavior of the transition zone has not been reposed previously. The purpose of this work is to obtain this information, which then can be applied to the simulation of reservoir performance and the design of laboratory experiments. performance and the design of laboratory experiments. A cell-to-cell flash model described by Metcalfe et al. was used in this study.

OBJECTIVES
The principal objective of this study is to present a method that can be used to investigate the physical properties of the transition zone existing between properties of the transition zone existing between the rich gas and the reservoir oil in miscible displacements. Only the effect of rich gas composition on these physical properties will be demonstrated. However, the method allows investigation of the effect of other variables, such as relative-permeability characteristics, pressure, and the composition of the reservoir oil.
Knowledge of the physical properties of the transition zone is considered a first step toward proper design of rich gas, multiple-contact oil proper design of rich gas, multiple-contact oil recovery methods. Additional studies are necessary for the design. Though these studies are mentioned within the paper, they are beyond the scope of this work.

DESCRIPTION OF STUDY
The study involved simulating the displacement of a reservoir oil with five different rich gases. The composition and properties of the fluids are given in Table 1. The reservoir temperature was 231 degrees F and the reservoir pressure was 2,250 psia for all simulations.
The cell-to-cell flash model with the "phase mobility option" was used to simulate the oil recovery method. One hundred cells were used. The phase mobility option uses phase mobilities, (kr/) phase, to determine the relative volume of each phase flowing from a particular cell.
SPEJ
P. 310