Abstract

A linear material-balance plot constructed based on "P/Z versus cumulative production" is frequently used to define the Original-Gas-In-Place (OGIP). The defined OGIP is then used to estimate the reserves. However, a linear P/Z plot can only be obtained if the well is producing free gas from a conventional gas reservoir with no water influx. For a coalbed well, the tightness and the reactive gas-rock nature deprive the gas of the needed pore spaces and the time for the material-balance to take place. Consequently, a P/Z plot constructed based on the observed apparent pressure-production-time data bends to the right, similar to a P/Z plot of a conventional gas well subject to water influx. Therefore, several authors have proposed to construct P/Z plot using "Z*" instead of the original Z values. The values of Z* are calculated using Z* function. Z* function allows for calculating Z* values that are much smaller than the original Z values. Z* function also allows for calculating Z* values that are decreasing with decreasing pressure. These characteristics allow the otherwise bending curve to converge into a straight line, with the x-axis intercept as the OGIP. This paper presents the results of conducting a material-balance evaluation using a commonly used material-balance equation and the original Z values for a coalbed well. Intuition would lead us to believe that such an evaluation would not lead to any useful conclusion. However, as presented in this paper, we found that such an evaluation not only revealed the production characteristics of a coalbed well but also provided us with a method for forecasting future production rates. Subsequently, they can be used to estimate the gas reserves.

Introduction

Material-Balance evaluations have been commonly applied to estimate the original-hydrocarbon-in-place for conventional reservoirs. Should the evaluation is conducted for a gas reservoir with no aquifer influx the material-balance equation can be simplified to the following linear equation. Equation (1) (Available in full paper)

Consequently, a common approach for determining the OGIP for a volumetric gas reservoir is to plot the historical P/Z vs. cumulative production, Gp, based on Equation (1). A linear line is expected from such a plot with the intercept along the x-axis (cumulative production) as the OGIP or the ultimate gas recovery, and the recovery as the cumulative production to any P/Z value at the abandonment pressure. However, should Equation (1) is applied to a coalbed well the plot bends to the right (upward), similar to that of a gas well experiencing influx, indicative of continue additional pressure support.

The simplicity of the P/Z plots has led to many efforts to extend this approach to coalbed wells. Of which, Z* function approach of King1 is the most familiar one. In the Z* function approach the original historical Z required in Equation (1) is replaced with Z* calculated based on the following equation. Equation (2) (Available in full paper)

King defines Ck as a function of cumulative pressure drop and other parameters.

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