Water Conduction and Production Mechanisms in Coal Measure Strata: Key Insights from a Typical Case Study

In recent years, extensive coalbed methane (CBM) development practices have shown that high-water-production CBM wells commonly suffer from low or even no gas production, which constitutes a major cause of inefficient CBM development. The characterization of cross-formational flow pathways for extraneous water leakage recharge and the analysis of water production mechanisms are critical for the efficient development of coalbed methane (CBM) resources in high-water-yield regions. This study integrates regional geological, geophysical, and CBM well production data sets from a typical high-water-yield district (i.e., Sanjiao coalbed methane field, east Ordos basin, China) to systematically investigate the water production mechanisms of CBM wells. Water-conducting channels between the coal seam and adjacent aquifers were precisely delineated via the coupling of seismic and well-logging data. A novel parameter, "water production per unit liquid level drop", was proposed to quantitatively assess the water production capacity of individual wells. Based on our previous fundamental research about water source identification and aquifer classification, multiple correlation analysis was performed to determine the main water-supplying layers and primary controlling factors of water production for wells targeting the No. 8 + 9 coal seam. The extent of regional structural deformation was quantified through trend surface analysis and stratigraphic attitude variation examination, then utilized in the mapping of water production risk in the study area. Results indicate that the produced water of CBM wells is predominantly derived from the limestone aquifers (i.e., L1, L2, L3, L4, and L5 limestone aquifers) of the Taiyuan Formation. The limestone aquifers exhibit strong heterogeneity in fracture development, with the L2-L4 aquifers demonstrating well vertical connectivity. The L1 and L4 aquifers are identified as the main water-producing layers, and water production is primarily governed by the cumulative width of vertical conductive channels. High water production risk is mainly distributed in areas with intense structural activities and well-developed conductive channels. Based on these findings, an extraneous water recharge model was established, incorporating the identified main water-producing layers and vertical fracture system characteristics. This model clarifies the geological mechanism by which fractures and faults regulate peripheral water intrusion into the coal seam, thereby elucidating the planar distribution pattern of water production in coal-bearing areas. These insights provide valuable theoretical and technical references for understanding high water production mechanisms and optimizing development strategies in this and other CBM blocks with similar geological backgrounds.