A New Compressibility Correlation for Natural Gases and Its Application to Estimates of Gas-In-Place

Abstract

Abstract This paper presents an evaluation of compressibility factor data and adiscussion of their application to the estimation of gas reserves. A correlation is presented which provides compressibility factors for use inboth two-phase and single-phase hydrocarbon systems. Accuracies comparable tothose obtained previously for single-phase systems only can be expected. Asimple means of predicting the presence or absence of a liquid phase in acondensate system of known composition is illustrated. The correlation is based on 1,030 compressibility determinations from 21hydrocarbon samples taken from eight oil fields. Of the data used, 75 per centwere from California, 15 per cent were from the Mid-Continent area, and 10 percent were from South America. The average numerical deviation of theexperimental data from this compressibility chart is 1.22 per cent. Charts and tables are included and discussed which illustrate the errorsinvolved through the misuse or nonuse of compressibility factors in estimatesof gas-in-place. Introduction The compressibility factor is a coefficient which expresses the deviation ofa gas of given composition from the Perfect Gas Laws. A factor such as this isnecessary in any calculation involving volumes of gaseous mixtures, and findsextensive application in the estimation of natural gas and condensate reserves.It is used in the decline curve, the volumetric, and the material balancemethods of gas reserves estimation. The behavior of gases at high pressures has been investigated extensively inrecent years and considerable data have been assembled on the compressibilitycharacteristics of a number of gases. Notable among the investigators of highpressure gas relationships are Kvalnes and Gaddy, Kay, Sage and Lacey, andBrown, Standing, and Katz. Other contributors are Smith and Watson, Roland andKaveler, and Stevens and Vance. The data on compressibility factors assembledby these investigators have been presented and correlated in numerous ways;particularly noteworthy is the method first presented by Kay which relatescompressibility factor ?Z? to pseudo-reduced temperature and pseudo-reducedpressure. Other investigators have used this empirical relationshipsuccessfully and numerous charts covering a large number of gases have beenconstructed on this basis. It is the purpose of this paper to evaluate compressibility factor data and toillustrate the importance of their proper use in the estimation of gas-in-placefor both gas and condensate reservoirs. T.P. 2642