Major Ion Geochemistry of Produced Water from Coalbed Methane Wells in the Gujiao Block and Its Relationship to Well Productivity

To elucidate the geochemical features of produced water in coalbed methane (CBM) wells of the Gujiao Block and their indicative significance for production, systematic testing of ion composition and total dissolved solids (TDS) of produced water from ten CBM wells was conducted through five discrete sampling campaigns over an 18-month period. Combined with production performance data, the spatiotemporal evolution patterns, controlling factors, and the response relationship with productivity were analyzed. The results show that the water chemistry type of produced water in the study area is mainly identified as the Na-HCO3 type. The TDS averages 1716.62 mg/L. The hydrochemical characteristics are primarily controlled by water/rock interactions, with Na+ and K+ mainly derived from silicate mineral weathering and dissolution, coupled with cation exchange processes. The Na/Cl ratio suggests that halite dissolution contributes to both Na+ and Cl−, whereas the excess Na+ relative to Cl− likely reflects cation exchange or dissolution of Na-bearing silicate minerals. As drainage proceeded, Na+ and K+ concentrations increased, Ca2+ decreased, Cl− increased, and SO42− first increased and then decreased. Spatially, TDS increases from north to south, with the central-southern region representing a stagnant groundwater zone. Productivity response analysis reveals that Na+, HCO3−, and TDS all show a trend of initially slow increase followed by rapid increase with increasing gas production. A negative trend is observed between gas production and the concentrations of Cl−, Ca2+, Mg2+, and SO42−. The productivity response index for the Gujiao Block ranges from 3.75 to 42.43, with an average of 17.78. As the productivity response index increases, gas production initially decreases and then increases. The findings clarify the geochemical evolution mechanisms of produced water in the Gujiao Block, providing a scientific basis for productivity evaluation of CBM wells.