This paper presents the development of a new analysis technique that can be used to determine the gas-in-place for abnormally pressured gas reservoirs. This new approach requires only production data (p and Gp) and does not require a prior knowledge of formation and fluid compressibility data. Using a pressure-dependent compressibility function based on the generalized gas material balance equation, this approach can be used to model both the rock collapse and shale water influx theories presented in the literature.
In this work we introduce two new plotting functions:
c e ( p i p ) versus (p/z)/(pi/zi) and (p/z)/(pi/zi) versus Gp/G
where these plotting functions are developed using the generalized gas material balance equation for a gas reservoir under the influence of a pressure-dependent formation compressibility function (Fetkovich, et al.1) in conjunction with the two straight-line trends observed on the p/z versus Gp plot for abnormally pressured gas reservoirs. Using these new plotting functions we develop a dynamic type curve matching technique that simultaneously determines the gas-in-place (G). In addition to gas-in-place, this new technique can be used to calculate pore volume compressibility as a function of reservoir pressure.
We provide the verification of our new analysis technique using the results of numerical simulations and we demonstrate the application of this methodology using several field examples.