Study of hydrochemical characteristics of CBM co-produced water of the Shizhuangnan Block in the southern Qinshui Basin, China, on its implication of CBM development

•The influence of fracture fluid on CBM co-produced water was analyzed.•The relationships between major CBM co-produced water ions were analyzed.•The Na and Cl ions in CBM co-produced water are partially from coal complex.•The TDS and HCO3– for gas production CBM wells have lower limit values.

Coalbed methane (CBM) co-produced water is primarily coal seam water. It may be mixed with fracturing fluids, especially during the early drainage stage, or with other aquifer water if the coal seam is connected with an aquifer via artificial or natural fractures/faults. If the impacts of fracturing fluids and other aquifer water can be removed, the chemical characteristics of CBM co-produced water can act as effective indicators for CBM exploration and development. Based on the water chemistry analyses of 119 samples collected from 47 CBM wells over four years (quarterly sampling) in the Shizhuangnan Block, Qinshui Basin, the dynamic characteristics of the CBM co-produced water chemistry that were influenced by fracturing fluids were studied in this study. Relationships between the major CBM co-produced water ions were determined, and the relationship between the CBM co-produced water ions and gas production were investigated. The results suggest that the CBM co-produced water chemical signature of the Shizhuangnan Block is similar to that of other coal seam waters from around the world, being characterized by the sodium-chloride-bicarbonate water types that are depleted in calcium and magnesium ions and exhibit highly lowered sulfate concentrations. Due to the mixing with the fracturing fluids and coal seam water, the TDS, sodium, chloride, and calcium and magnesium ion concentrations decrease during the early drainage stage following a power law function. However, bicarbonate and sulfate concentrations do not significantly vary. The Na and Cl, Na/Cl and alkalinity/Cl, and Na/Cl and Na/alkalinity ratios exhibit a positive linear relationship, strong positive linear relationship and exponentially decaying relationship, respectively. Additionally, 10 meq/L of Cl may serve as a dividing line for the presence or absence of the fracturing fluid affect. Na and Cl may release organic-bound chlorine complexes from the coal during coal metamorphism. The TDS and bicarbonate concentration of the CBM co-produced water produced by the gas production wells are all > 1000 mg/L and 8 meq/L, respectively. The bicarbonate concentration is influenced by both the LCH, which is the height of the liquid level compared to the coal bed surface at the onset of water production at a CBM well, and hydraulic conductivity fault, and its distribution controls the distribution of the high gas yield wells.