PVT Characterization of a Wet Gas Reservoir and its Impact on Reserves Estimate

Abstract

Abstract Wet gas reservoirs with very low Condensate Gas Ratio (CGR) are very simple hydrocarbon system to deal with. However, accurate reserves calculation requires an accurate knowledge of gas compressibility factor (z) at reservoir Pressure and Temperature. In the absence of laboratory PVT data, very often correlations are used to estimate Bg (formation volume factor) for these reservoirs. Then accuracy of those estimates remains a question which may have significant impact in the Gas Initial in Place (GIIP) calculation. In this paper we review our experience in estimating and validating Bg for a wet gas reservoir system. Wellhead samples were taken from exploration, delineation and then development wells. Initial GIIP was calculated from Bg estimated using a popular Standing-Katz correlation. Then more rigorous equation of states (EOS) were used to compare the values against those derived through z-factor correlations. Peng-Robinson and Soave-Redlich-Kwong (SRK) EOS based estimates were about 8% apart. A laboratory PVT study was carried out to accurately measure formation volume factor at various pressure and temperature including the reservoir P and T. Results showed that Peng-Robinson EOS over estimates Bg by 1%, while Standing-Katz z-factor correlation and SRK EOS underestimate this by 3% and 7% respectively. The use of detailed PVT data for Reservoir Engineering calculations provides more accurate GIIP, 3% higher than the simple compressibility factor based gas equation of state in this case.