P/Z Abnormally Pressured Gas Reservoirs

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ABSTRACT The P/Z versus cumulative gas production plot is a commonly accepted method used to obtain original gas in place in volumetric (no water drive) gas reservoirs. Many petroleum engineers apply this method to abnormally pressured reservoirs which can lead to errors of up to 100% in the original gas in place extrapolation. The problem is that the conventional P/Z plot in abnormally pressured reservoirs must be adjusted for rock and water compressibility to obtain reasonable estimates of original gas in place. A normally pressured reservoir will have an initial pressure gradient between 0.43 and 0.50 psi/ft. Reservoirs with pressure gradients greater than this are abnormally pressured. In the Gulf Coast area of Louisiana and Texas, pressure gradients can be very close to overburden pressure gradients with some of the gas reservoirs having gradients of 0.85 psi/ft. or higher. In abnormally pressured reservoirs the reservoir fluids support additional pressure as is evident by pressure gradients of 0.85 psi/ft. The gas compressibility can actually approach formation compressibility in abnormally pressured reservoirs. This makes it important to include both formation and water compressibility in the P/Z plots when trying to estimate original gas in place in abnormally pressured reservoirs. This paper presents a new technique for predicting performance of an abnormally pressured gas reservoir. This method maintains the old straight line relationship for conventional P/Z's in normally pressured reservoirs and allows direct extrapolation of original gas in place in abnormally pressured reservoirs by accounting for rock and water compressibility in the adjustment factor term. Thus, in abnormally pressured reservoirs, the early prediction of original gas in place from the P/Z curve is one of the main reasons for using this method. This method has been successfully applied to three abnormally pressured reservoirs. They are the North Ossun Field in Louisiana, the Anderson "L" gas reservoir in South Texas, and to a Louisiana offshore gas reservoir. The results of the application of the new method and a comparison of predicted performance in abnormally pressured reservoirs using this technique and the conventional P/Z method will be presented in this paper.