Evaluation of development effectiveness of deep coalbed methane based on flowback characteristics-a case study of the Daning-Jixian block

The efficient development of deep coal-rock gas resources, exemplified by the pioneering No. 8 coal seam (buried >2000 m) in the Daning-Jixian Block, Ordos Basin, is of strategic importance. This study investigates the critical role of fracturing fluid flowback behavior in determining post-fracturing productivity. Based on systematic analysis of flowback parameters and dynamic data, we evaluate the impacts of fluid-reservoir contact time and flowback chloride ion concentration on well performance. Results reveal a critical contact time of 20 days for gas appearance: while extension within this period facilitates reduction in time to gas appearance, further extension shows diminishing returns. Under current operational conditions, contact time demonstrates limited influence on long-term development effectiveness. The chloride ion concentration evolution follows a distinct tri-phasic pattern: rapid rise, slow rise, and plateau phase. Adjacent wells under identical geological and engineering conditions exhibit remarkable consistency in chloride trends, whereas different flowback regimes lead to significant variations—shorter shut-in times correspond to earlier chloride stabilization and higher corresponding flowback rates, though with minimal difference in final stabilized concentrations. Furthermore, lower flowback rates during chloride rise phases under comparable completion conditions indicate more complex fracture networks and superior stimulation effectiveness. These findings establish a chloride trend-based diagnostic method for early-stage development evaluation, providing crucial scientific guidance for optimizing flowback strategies and fracturing effectiveness assessment in deep coal-rock gas reservoirs.