Abstract

Material balance analysis is a fundamental technique for estimating gas-in-place. It can be achieved using:

  1. Static material balance, using static (shut-in) reservoir pressures, where a plot of static p/z versus cumulative gas production is created to estimate original-gas-in-place (OGIP) or

  2. Flowing material balance where gas rates and flowing pressures are used to estimate average reservoir pressure.

The flowing material balance concept of Agarwal-Gardner (1999) was extended to dry coalbed methane (CBM) reservoirs by Clarkson et al. (2007a) and Gerami et al. (2007) and to 2-phase (gas and water) CBM wells by Clarkson et al. (2007b). The present study further enhances the flowing material balance for dry CBM reservoirs by presenting a p/z* implementation of the concept.

This application, while accounting for the distinguishing characteristics of a CBM reservoir, uses the industry-standard practice of p/z material balance to calculate original-gas-in-place. As with the Agarwal-Gardner approach, the flowing p/z* method can be applied to variable gas rates and/or flowing pressures conditions.

In the present work, the derivation and iterative procedure of calculations are explained. Several test cases based on dry/immobile water saturation using real and synthetic data were generated. The resulting estimates of OGIP calculated from implementation of flowing p/z* material balance show excellent agreement and the estimated OGIP's are reliable.

Introduction

Material balance is the application of mass balance to a producing reservoir. As gas is produced, the reservoir pressure declines. By monitoring the cumulative gas production and the average reservoir pressure, and using the PVT properties of gas, one can determine OGIP and the remaining gas-in-place.

Material balance analysis, although simple and more reliable than volumetrics calculation, does suffer from a number of shortcomings and limitations. For example, to measure the average reservoir pressure the well has to be shut-in and that means loss of production. Among other complexities are:

  • low permeabilities lead to poor pressure build-ups (long-buildup times required)

  • pressure build-up can be masked in multiple coal seams

  • reservoir can be recharged from aquifers

CBM reservoirs have additional complexities. The gas storage mechanism as well as the compressibility of CBM reservoirs is dominated by adsorption. These and other CBM-specific characteristics have to be accounted for in any material balance calculations.

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