Pressure Transient Testing of Methane Producing Coalbeds

Abstract

SUMMARY A new analysis method for pressure transient testing in coal degasification projects is presented. The results from this method show that transient testing can be used to characterize methane producing coalbeds. Formation permeability, reservoir pressure, and wellbore skin can be calculated using the proposed method. Use of conventional single phase analysis produces different results. The current practice calls for testing coalbeds by a series of water pump-in and falloff tests upon completing the wells to ensure a single phase system. Once gas production starts, a multi-phase system raises serious concerns about the validity of conventional pressure transient analysis in coal degasification projects. However, testing of methane producing coalbeds is important because in addition to the need of monitoring the pressure and wellbore conditions, the cleat system permeability may change as the pressure is decreased. Therefore, a new analysis method has been developed for these systems. The method is based on a pseudo pressure function that uses the adsorption isotherm and relative permeability relations of the reservoir. The method enables the interpretation engineer to calculate the absolute permeability at any time even when both gas and water are being produced. The method was verified by successfully analyzing simulated tests from a rigorous coal degasification numerical simulator. The method was then used on field buildup data taken shortly after gas production began from a well. The same absolute permeability was calculated as that from the water pump-in and falloff tests which were run in the same well upon its completion. Results from field tests on another well also are included in the paper.