Coupling effect of water-rock in the process of coalbed methane extraction and its response to production capacity-Fanzhuang block in qinshui basin as an example

To clarify the water-rock coupling mechanism and its influence on productivity during Coalbed Methane (CBM) extraction, a series of hydraulic-flushing-based simulation experiments were conducted on different types of production wells following similarity principles. The variations in ionic composition of produced water before and after the experiments were systematically analyzed. By integrating field production data, the response relationship between the geochemical characteristics of produced water and CBM productivity was elucidated. The key findings are as follows: The extent of water-rock interactions varies under different salinity levels and flow rates. Higher salinity and lower flow rates enhance water-rock reactions, leading to increased salinity and pH in the effluent. The maximum salinity increment was observed at 2 000 mg/L and 0.3 mL/min. Overall, Na⁺+K⁺ and Cl^– concentrations increase with rising initial salinity. Specific salinity and flow rate ranges promote the release of Ca²⁺ and Mg²⁺, whereas elevated salinity and flow rates suppress \mathrmHCO_3^- generation. When salinity and flow rate exceed a critical threshold, \mathrmSO_4^2- becomes significantly enriched. The high productivity of CBM wells is co-controlled by the coupling effect of initial salinity and drainage conditions. In the Fanzhuang block, high-yield wells correspond to a salinity range of 1 800～2 200 mg/L, mostly exhibiting medium-to-low water production rates, under which the simulated experimental conditions demonstrated the most intensive water-rock interactions. A strong positive correlation exists between Ca²⁺ concentration in produced water and gas production in the No. 3 coal seam. However, the productivity response mechanism of Ca²⁺ differs under varying water-gas production regimes. In high-yield water and medium gas production mode, Ca²⁺ release is predominantly governed by water-rock interactions in the underlying sandstone aquifer. Conversely, in low-yield water and high gas production mode, Ca²⁺ mobilization is primarily driven by calcite dissolution within the coal matrix.