Reserves Estimation for a Coalbed Methane Well
Authors: Hsieh F.S. , Vega C. , Vega L. 
Published: March 2010

A linear material-balance plot constructed based on "P/Z vs. 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 methane 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 a P/Z plot using "Z*" instead of the measured Z values. The values of Z* are calculated using the Z* function. The Z* function allows for calculating Z* values that are smaller than the measured Z values except the value at the initial pressure. The 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 the material-balance method proposed by Hsieh et al and the measured Z values for a coalbed well. As presented in this paper, we found that such an evaluation method not only revealed the production characteristics of a coalbed methane well but also provided us with a method for forecasting future production rates. Subsequently, this technique can be used to estimate the gas reserves for a coalbed methane well as illustrated in the example well.

Introduction

Material-balance evaluations have been commonly applied to estimate the original-hydrocarbon-in-place for conventional reservoirs. When 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 with the intercept on the x-axis given as the OGIP or the ultimate gas reserves. The expected recovery is the value of the cumulative production, Gp, at the abandonment pressure. However, should Equation (1) be applied to a coalbed methane well, the plot bends to the right (upward), similar to that of a gas well experiencing water influx, indicative of additional pressure support.

The simplicity of the P/Z plots has led to many efforts to extend this approach to coalbed methane wells. Of these, the Z* function approach of King(1) is the most familiar one. In the Z* function approach, the measured Z values 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.