Changes in Gas and Oil Gravity During Depletion of Oil Reservoirs and Their Effect on PVT Data and Reserve Calculations
Authors: Elsharkawy Adel M. 
Published: December 2007

Abstract
In this paper, a PVT simulator running SRK-EOS was used to study the changes in the evolved-reservoir gas gravity and produced gas gravity, during pressure depletion, for various crude oils from the Middle East. Simulation studies show that reservoir gas and average produced gas gravity change by as much as 50 %. In absence of PVT studies, initial separator gas gravity is assumed to represent reservoir gas and produced gas and used to calculate crude oil and gas properties. While this assumption results in small error in gas properties (6–8%), it greatly underestimates solution gas-oil ratio and oil formation volume factor, and overestimates crude oil viscosity. To study the effect of these changes in gas gravity on reserve calculations, a one-dimension black oil model was used to estimate the oil recovery for various PVT data sets estimated from separator gas and average produced gas. Reserve estimates show that using separator gas to represent both reservoir gas and average produced gas in calculating gas and oil PVT properties results in underestimating depletion-drive ultimate oil recovery by 40%. Laboratory measured differential PVT data were used to develop correlations for predicting reservoir gas gravity and liquid gravity. These correlations are function of easily measured field data. The gas gravity correlation has an average absolute error of 2.35% and coefficient of correlation of 95%, and the oil gravity correlation has an average absolute error of 1.0% and coefficient of correlation of 97%.
The importance of the PVT data in predicting reservoir behavior has been recognized since the early 1940's. Several PVT correlations have been published for estimating PVT properties for various crude oils from different parts in the world. A review of these correlations is presented by Elsharkawy et al. These PVT correlations are important source for PVT data when laboratory measurements are not available. All the PVT correlations use the initial separator gas or average produced gas gravity and oil API gravity to calculate the oil and gas PVT properties as a function of reservoir pressure at reservoir temperature. Quite often reservoir gas properties: formation volume factor and gas viscosity, are estimated from correlations assuming constant gas gravity and using separator gas gravity to represent reservoir gas. Such assumption results in errors in calculating gas formation volume factor and gas viscosity. Separator gas is also used to represent average produced gas gravity in calculating solution gas oil ratio, oil formation volume factor, and oil viscosity as function of pressure at reservoir temperature. Thus, the PVT correlations neglect the changes in reservoir gas gravity and produced gas gravity as a function of pressure during depletion of oil reservoirs. The use of separator gas to represent average produced gas gravity as input variable in calculating oil formation volume factor, solution gas oil ratio, and oil viscosity would result in some error in PVT data that would affect reserve calculations. Among all the input variables in any PVT correlation, gas gravity is the most questionable variable. Unfortunately, stock tank gas gravity and stock tank gas-oil ratio are rarely measured in the field. Neglecting the stock tank gas-oil ratio results in material balance calculations that are in error by as much as 20%. Correlation developed by Rollins and McCain used to estimate stock tank gas-oil ratio. However, correlation to estimate stock tank gas gravity is not available.
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