Improved mathematical model for OGIP prediction

Many petroleum engineers apply the conventional formation pressure method to certain gas reservoir, especially the abnormally pressured reservoirs which can lead to errors of up to 100% in the original gas in place (OGIP) extrapolation, and there is still lack of 2-order pressure related equation to estimate the OGIP. Through introducing the transformations and Taylor expansion to the material balance equation, a new 2-order pressure related equation to calculate the OGIP is established; this new method gets advantages of conventional formation pressure method and Law of water cutting volume factor. Meanwhile, it modifies the old linear relationship for conventional P/Z’s in normally pressured reservoirs, and it allows direct extrapolation of OGIP in gas reservoirs concerning rock and water compressibility; after applying this new method to two different abnormally pressured reservoirs, comparisons of predicted results with different methods have been given, and the adaptability and affectivity of the new method have been proved.