A Type-Curve Matching Procedure For Material Balance Analysis Of Production Data From Geopressured Gas Reservoirs
Authors: Ambastha A.K. 
Published: March 2010

Abstract
The performance of a normally-pressured gas reservoir depends on gas compressibility effects and water influx. Generally, a straightforward material balance analysis is possible for a normally-pressured gas reservoir to estimate the initial gas-in-place. However, for an abnormally-pressured (or geo-pressured) gas reservoir, material balance analysis is not as simple.
Factors complicating the performance of a geo-pressured gas reservoir are water and rock compressibility effects, and shale water influx.. These factors often cause an overestimated value of the initial gas-in-place from a p/z-Gp graph of the early production performance. Over the years, several corrections have been proposed to accurately estimate the initial gas-in-place. These corrections are based on different forms of material balance equation.
This paper uses a material balance equation that applies to both normally-pressured and geo-pressured gas reservoirs. This material balance equation demonstrates that the non-linearity on a p/z-Gp graph for a geo-pressured reservoir is due to a term containing p2/z. A new type-curve matching procedure has been developed to analyze the production performance of a geo-pressured gas reservoir. An example application for using the new method is presented.

Introduction
A normally-pressured gas reservoir exhibits an initial pressure gradient close to the pressure gradient for water (generally about 9.7 to 11.3 kPa/m). The production performance of a normally-pressured gas reservoir depends on gas compressibility effects and water influx. The effects of water and rock compressibilities are usually neglected while analyzing the performance of a normally-pressured gas reservoir. However, an abnormally-pressured (super-pressured, over-pressured or geo-pressured) gas reservoir exhibits an initial pressure gradient far in excess of typical water column pressure gradient. Prasad and Rogers(1) report initial pressure gradient as large as 21.7 kPa/m for a geo-pressured gas reservoir. A typical geo-pressured gas reservoir may exhibit an initial pressure gradient between 14.7 to 19.3 kPa/m.
Though geo-pressured gas reservoirs may be encountered in all parts of the world, several geo-pressured reservoirs exist in the Gulf Coast area of Louisiana and Texas(2,3), Andarko basin, Delaware basin, and the Rocky Mountain area of the United States(1). For a deep, geo-pressured gas reservoir, water and rock compressibilities may be close to gas compressibility during the initial stages of production. Thus, any material balance analysis of early production data from a geo-pressured gas reservoir must include water and rock compressibility effects.
This study is limited to volumetric (no water influx or water production) normally- and/or abnormally-pressured gas reservoirs. This study uses a material balance equation that applies to both normally-pressured and geo-pressured gas reservoirs. This material balance equation forms the basis of a new type-curve matching procedure to analyze the production performance of a geo-pressured gas reservoir.

Material Balance Equation for Volumetric Gas Reservoirs
Bourgoyne et al.(4) derived a material balance equation for a geo-pressured gas reservoir as:
Equation (1) (Available In Full Paper)
Where:
Equation (2) (Available In Full Paper)
Equation (3) (Available In Full Paper)
Equation (4) (Available In Full Paper)
Equation (5) (Available In Full Paper)
Equation (1) and the resulting type-curve (to be discussed later) only account for the non-linear aspects of the te