Hydrogeochemical constraints on coalbed methane productivity control mechanism: synergistic effects of fracturing fluid contamination identification and reservoir dynamic response

Accurate identification of geochemical features in coalbed methane (CBM) well-produced water is vital for evaluating the gas potential of reservoirs. However, fracturing fluid interference with water chemistry and productivity analysis is a challenge. This study reveals the origin of produced water ions and their control mechanisms on productivity in CBM wells, based on hydrochemical data from 31 CBM wells in the Gujiao Block, Xishan Coalfield, China. To extract original reservoir information, we established quantitative identification criteria for fracturing fluid contamination (Cl⁻ < 70 meq/L and K⁺ + Ca2⁺ + Mg2⁺ < 2.5 meq/L), effectively excluding 29% of contaminated samples. Analysis of the water origin and system conditions indicated that the dominant Cl-Na water results from salt dissolution and cation exchange. Ba2⁺-Sr2⁺ enrichment reflects a marine-terrestrial transitional environment in the Taiyuan Formation, while δD-δ1⁸O indicates slow water circulation, with CBM water mainly sourced from sandstone. The produced water in the study area mainly comes from the Member 3 Formation sandstone aquifer. The synergistic effects of SrSO₄-BaSO₄ precipitation in microfractures, as well as the enrichment and fractionation of hydrogen and oxygen isotopes in high hydrodynamic zones, are the main controlling factors for low productivity. Under weak hydrodynamics, cation exchange, microbial dissolution, sulfate reduction, and CO₂ dissolution enrich Na⁺-HCO₃⁻-TDS-Li⁺, enhancing gas well productivity. Ultimately, by integrating these hydrochemical parameters, the ion evolution pathways in wells with different production capacities were analyzed, and a geochemical mechanism model for CBM water production control was constructed. This research provides a crucial hydrochemical evaluation basis for the efficient development of CBM.