Computer Determination of Data-Matched Equilibrium Ratios
Authors: Jones Dan M. , Erbar John 
Published: December 2007

The automatic calculation of data-matched K-values makes the method described here readily adaptable to use in compositional material balance programs and in numerical simulations involving phase behavior. programs and in numerical simulations involving phase behavior. Introduction
Over the past several years, much has been written concerning the effects of phase behavior on recoveries from volatile-oil and gas-condensate reservoirs. Jacoby, Koeller and Berry and others developed and refined nomographs and correlations to simplify the voluminous and tedious calculations necessary for this work. More recently the requirements of higher precision for performance predictions and the increasing precision for performance predictions and the increasing discovery rate for these reservoirs near the critical point have brought about the development of point have brought about the development of compositional material balance computer programs and numerical reservoir simulators. One of the major problems in this computer effort is the general lack of problems in this computer effort is the general lack of a method to determine equilibrium ratios that yield a reasonable approximation of the behavior exhibited by conventional laboratory analysis of field samples. The effort that produced this paper was directed to the solution of this problem. Direct laboratory determination of equilibrium ratios is impractical because of the expense involved. Several correlations for the purpose of obtaining K-values exist, but these are usually based on simple systems of pure components and generally yield results that diverge from the experimental performance of real systems as conditions approach the critical values. Several computerized techniques are available to calculate K-values using equations of state and other approaches, but they, like the correlations, have the same limitations. The method described here was developed to avoid these limitations and to replace the manual adjustment previously necessary. An empirical method was chosen previously necessary. An empirical method was chosen since no rigorous theoretical approach was available. The automatic calculation of a data-matched set of K-values makes the method described here readily adaptable to use in compositional material balance programs and numerical simulators involving phase programs and numerical simulators involving phase behavior.

General Procedure
The experimental data to be matched are contained in the reports of conventional laboratory analysis of field samples, These reports yield numerous data about the volumetric behavior of a hydrocarbon system with changes in pressure. For example, if the analysis is a differential vaporization, the amount of gas evolved from a known amount of sample, and its density, composition and compressibility factor can be measured with reasonable accuracy at all pressures. Some liquid properties such as volume can be measured at all properties such as volume can be measured at all points, other properties can be measured at all points, other properties can be measured at all end points of the run. However, if the analysis is of the constant composition type, the liquid volume percent is the most convenient measurement except for percent is the most convenient measurement except for the end points of the run. The K-value calculation method can use either type of data for its matching criteria.
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