A Semianalytical p/z Technique for the Analysis of Reservoir Performance from Abnormally Pressured Gas Reservoirs

Abstract

This paper presents the development of a new analysis tech-nique 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:ce(pi- 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 reser-voir 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. Introduction An ???abnormally pressured??? gas reservoir (sometimes called an ???overpressured??? or ???geopressured??? gas reservoir) is defined as a reservoir with pressures greater than a normal pressure gradient (i.e., >0.5 psi/ft). Bernard 2 observed that the effects of ???abnormal pressure??? behavior on reservoir engineering calculations could be ignored for cases where the pressure gradient is less than 0.65 psi/ft. A typical p/z versus Gp plot for an abnormally pressured gas reservoir will exhibit two straight lines – the first straight line corresponds to the ???apparent??? gas reservoir behavior (i.e. the abnormal pressure behavior) and the second straight line corresponds to the ???normal pressure??? behavior (Fig. 1).

A Semianalytical p/z Technique for the Analysis of Reservoir Performance from Abnormally Pressured Gas Reservoirs

“…We have developed a straightforward approach for analyzing p/z versus G p behavior for abnormally pressured reservoirs. Our approach is based on the 0.272519 0.997076 12 …”

“…Since gas compressibility is much higher at lower pressures, the effect of total water compressibility on the p/z versus G p plot is probably not significant. 12 For practical purposes, we can use a constant value of the cumulative total water compressibility in the generalized material balance equation.…”

“…4), we propose a new, semi-analytical relation to model the two straight-line p/z case. (12) Eq.12 can be used as an alternative (regression-based) approach for the analysis of production data taken from abnormally pressured gas reservoirs.…”

“…12. A summary of our analysis results is presented in Table 4 and a comparative summary of analyses (present work compared to the original analyses) is presented in Table 5.…”