7 - Material balance equation for CBM reservoirs

Abstract

The material balance equation (MBE) is a powerful reservoir engineering tool, which is used for evaluation of the initial gas-in-place (IGIP) and prediction of the production performance of the gas reservoirs. In a CBM reservoir the free gas, if present, only accounts for a very small fraction of the gas in reservoir and the vast majority of the gas is stored within the coal matrix by adsorption. Therefore, the MBE for CBM reservoir must account for the gas desorption as the reservoir decreases. A comprehensive MBE for CBM reservoirs has been developed that accounts for the gas desorption, water influx, water production, and water and formation compressibility. The MBE takes advantage of the practical and simple straight-line graphical method by introducing the unconventional gas reservoir deviation factor, z*. However, the determination of IGIP by this method requires iterative calculations. A number of modifications to this MBE have been proposed to account for moisture expansion, solubility of methane in water, and the porosity variation due to matrix shrinkage. However, these modifications do not have a significant impact on the accuracy of the estimated IGIP for most CBM reservoirs.