Establishment of the comprehensive material balance equation for coalbed methane reservoirs at the gas desorption stage

Abstract

• A cleat porosity model containing the adsorption hysteresis is established. • A material balance equation for coals with different ranks or depths is built. • Adsorption hysteresis helps improve the accuracy of gas reserve estimation. • The improvement increases with an increase in the adsorption hysteresis degree. • Free gas in the matrix and matrix shrinkage effect should be considered in coals. Evaluating the original gas in place (OGIP) is vital to the development and production of CBM reservoirs. However, most of the existing material balance equations (MBEs) perform well in some CBM reservoirs but have significant errors in the CBM reservoirs with a different coal rank. In addition, the MBEs for deep and ultra-deep coals with different coal ranks are still lacking. Therefore, this paper established a comprehensive MBE at the gas desorption stage for CBM reservoirs, which can be reliably used in different ranked coals and even deep and ultra-deep coals. Based on the validation and factor analysis results, some conclusions are drawn. (1) The cleat porosities calculated by the proposed model containing adsorption hysteresis well match the values from the numerical simulators. (2). The proposed MBE has been proven reliable and robust under validations by two medium-rank CMG models and a high-rank field case with errors of about 0.127%∼0.213%. (3). The adsorption hysteresis has been verified as an essential factor; it can help improve the accuracy of evaluating OGIP for the studied CBM reservoirs by 30%-50%. The improvement goes up with an increase in the adsorption hysteresis degree. (4). The free gas in the matrix and a matrix shrinkage effect have been proven to be necessarily considered in an MBE method to help decrease its relative errors. (5). The proposed comprehensive MBE method has been verified to have higher accuracy and advantages than other compared MBE methods in estimating gas reserves in CBM reservoirs. Overall, this study contributes to knowledge gaps in evaluating OGIP by MBE methods in CBM reservoirs with different coal ranks or depths. And it provides a relatively simple method for engineers to accurately estimate gas reserves, recoverable reserves, and recovery factors and correctly design development strategies to avoid unnecessary waste of resources in the post-adjustment stage.