Calculation of Phase Compositions and Properties for Lean- or Enriched-Gas Drive
Authors: Dykstra Herman 
Published: December 2007

Abstract
This paper describes a method for obtaining compositions of gas and oil phases in equilibrium with each other at a given reservoir temperature and pressure as lean or enriched gas is injected into a reservoir oil not in equilibrium with the gas. The method is based on calculation of equilibrium constants, K, from the equation log Kp= A + B F, where p is pressure and F is component characterization factor. A and B are constants that vary with a composition parameter. The paper explains how A and B could be determined with a minimum amount of data. The method consists of calculating equilibrium constants as a function of liquid composition and using the K's to calculate changes in composition of injection gas as it successively contacts original reservoir oil at the gas-oil front and the change in composition of reservoir oil as it is successively contacted by the injection gas in the vicinity of an injection well. An iterative process is required. Two examples of the method are given which show how oil and gas compositions and properties are calculated with the aid of a computer.
Introduction
The calculation of recovery from lean-gas or enriched-gas injection into a saturated or under-saturated reservoir oil requires a knowledge of how phase properties change in a reservoir between injection well and producing well. To determine the phase compositions or properties experimentally would be difficult and time consuming, especially if one wanted to look at the effect of various injection gas compositions. For some fields an engineer would want to make preliminary calculations on the economics of gas injection before he makes the decision to spend a considerable sum of money for laboratory work to obtain the necessary fluid properties. A method for calculating compositions and properties based on a minimum of laboratory data is described. The minimum amount of data required is injection gas and reservoir oil composition and injection gas solubility as a function of pressure for an undersaturated oil, or the compositions of mixtures of reservoir fluid with two or more enriched gases, which mixtures have a bubble-point pressure that is the same as the expected average displacement pressure. On the other hand, if the economics appear to be attractive, more laboratory data can be obtained in the form of equilibrium constants for three or more mixtures of the reservoir fluid and injection gases. The method is based on the calculation of K's from the equation, log Kp= A + B F, where K is equilibrium constant, p is pressure and F is component characterization factor. This method of plotting K data by means of the above equation is described by Hoffman, Crump and Hocott. A and B are constants that vary with a composition parameter. It is shown how experimental data are used to determine how A and B vary with composition.
BASIS OF METHOD
EQUILIBRIUM CONSTANTS
Equilibrium constant (K) data are required for this method. The K of each component is obtained from an equation that gives the product of K and pressure as a function of component characterization factor. The characterization factor F= b(1/Tb- 1/T), where b is a constant characteristic of a given component, Tb is the boiling point in oR at atmospheric pressure of the given component, and T is the temperature of the system in oR. This method of plotting K data is described by Hoffman, Crump and Hocott. The straight-line equation of the log Kp - F plot is
log kip= log Kop + BFi
where Kop is at F= o, i designates the component, and B is the slope of the line.
SPEJ
P. 239ˆ