Effect of Gas Saturation on Static Pressure Calculations from Two-Phase Pressure Build-Up Curves

Abstract

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 49–54. Abstract This paper presents results of a study to determine to what extent errors in estimated free gas saturation affect the results of static pressure calculations from build-up curves in two-phase systems. Use is made of the method of pressure build-up curve analysis developed by Miller, Dyes and Hutchinson, with modifications for two-phase flow advanced by Perrine. Combined fluid compressibility of gas-oil mixtures is included, with special attention given to the discontinuity that occurs in fluid compressibility at the saturation pressure and its effect on static pressure calculations from build-up curves. The sensitivity of static pressure calculations to errors in gas saturation is considered over wide ranges of formation and fluid properties and reservoir pressure conditions. It is concluded from this study that for analyses of two-phase pressure build-up curves (1) the use of undersaturated crude compressibility data, even when producing GOR's are at or near solution values, will result in very significant negative errors in calculated static pressure, and (2) except for large values of buildup curve slope and/or very low crude gravities, rather large errors in calculated gas saturation can be tolerated without significant effect on calculated static pressures. The conclusions are qualified as referring only to the effects of gas saturation and exclude possible errors in the numerous other factors that enter into the analysis of pressure build-up curves. Introduction As part of the analysis of basic data for several major Eocene reservoirs in the Bolivar Coastal field (Western Venezuela), it was necessary to analyze a large number of pressure build-up curves for oil permeability and static reservoir pressure. From performance data it became apparent that gravity segregation is very active in these reservoirs. As a result of this performance it was recognized that the free gas saturation within the oil leg of the reservoirs would be small but would be subject to certain errors in calculation due to the inaccuracy of relative permeability data in the low gas saturation region. Since all of the various methods of build-up curve analysis were originally derived for single-phase, constant compressibility systems, and since the presence of a small but rather indeterminate gas saturation would greatly affect the reservoir fluid compressibility, a study was undertaken to investigate the effect of gas saturation on the results of two-phase pressure build-up analyses.