Evaluation of Material Balance Analysis Methods For Volumetric, Abnormally-pressured Gas Reservoirs
Authors: Ambastha Anil K. 
Published: March 2010

Abstract
Material balance analysis of production performance for an abnormally-pressured gas reservoir is complicated because of water and rock compressibility effects in addition to gas compressibility effects. An extrapolation based on an early straight line on a p/z vs. Op graph for an abnormally-pressured gas reservoir yields a significantly overestimated value of the initial gas-in-place. Several researchers have presented material balance analysis methods to obtain reasonably accurate estimates of the initial gas-in-place for abnormally-pressured gas reservoirs. These methods can be broadly grouped into two sets. The first set of methods requires a knowledge of compressibilities and analyzes production performance data to estimate the initial gas-in- place. The second set of methods attempts to obtain both an effective system compressibility and the initial gas-in-place by analyzing production performance data.
This study presents an evaluation of several material balance analysis methods assuming constant effective system compressibility for volumetric, abnormally-pressured gas reservoirs. The interrelationship between the initial gas-in-place estimates from the two methods by Hammerlindl(1) has been derived theoretically. The problem of non-uniqueness in analyzing production performance data of abnormally-pressured gas reservoirs is emphasized.
An example problem demonstrates that a small error in the initial reservoir pressure can account for a typical flat portion corresponding to the early production data on the Roach(2) plot.
The preceding observation suggests that a hypothesis of changing formation compressibility with pressure as advanced by Poston and Chen(3) is not necessary to explain the shape of the Roach(2) plot.

Introduction
Material balance analysis of production performance data for an abnormally-pressured gas reservoir should theoretically include water, rock, and gas compressibility effects. For a volumetric, abnormally-pressured gas reservoir, a p/z vs. Gp graph shows two straight lines, the first line corresponding to the early-time data and the second line corresponding to the late-time data, of distinctly different slopes. An extrapolation based on an early-time straight line on a p/z vs. Gp graph for an abnormally-pressured gas reservoir yields a significantly overestimated value of the initial gas-in-place. This paper discusses several material balance analysis methods proposed for volumetric, abnormally-pressured gas reservoirs to obtain reasonably accurate estimates of the initial gas-in-place using the early-time production-pressure data.

Currently Used Material Balance Analysis Methods for Abnormally-pressured Gas Reservoirs
All material balance analysis methods proposed for volumetric abnormaly-pressured gas reservoirs are based on a material balance equation of the following general form:
Equation (1) Available In Full Paper.
Different authors have used different expressions for effective system compressibility (ce) depending on the drive mechanisms incorporated in their analyses. The following discusses the two sets of methods proposed for analyzing production performance data for abnormally-pressured gas reservoirs.
Methods Based on a Knowledge of System Compressibility
Hammerlindl(1) presented two methods to correct apparent gas-in-place (Gipp) obtained from an extrapolation of early straight line on a p/z vs. Gp graph for an abnormally-pressured gas reservoir. Using two pressures. P1 and P2 actual gas-in-place (Gnet) i