Establishment and application of material balance equations for coalbed methane reservoirs considering adsorption hysteresis effect

Previous material balance equations (MBEs) were established under the assumption that the adsorption and desorption process of coalbed methane (CBM) is completely reversible and the adsorption hysteresis phenomenon ob-served in CBM adsorption / desorption experiment was neglected,which is obviously not consistent with the actual de-pressurization process of CBM reservoirs by water drainage. This paper proposed the concept of adsorption hysteresis effect based on laboratory observation and analysis of the potential mechanisms. Furthermore,an improved MBE model of CBM reservoirs considering the adsorption hysteresis effect was established,and it was applied in predicting the original gas in place (OGIP),recoverable reserves and recovery factor of some actual CBM reservoir. The results show that MBEs may significantly underestimate the OGIP while overestimating the recovery factor if the adsorption hystere-sis effect is not included in the calculations. Conversely,the calculated OGIP and average water saturation are in good agreement with the actual values using the new MBE model that incorporates the adsorption hysteresis effect. There-fore,the validity and accuracy of the new MBE model is proved,and then the proposed material balance method can be potentially applied in evaluating the recoverable reserves and recovery factor of CBM reservoirs,which would also play a major guiding role in the subsequent development planning and adjustment of CBM reservoirs.