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Ionic composition, geological signature and environmental impacts of coalbed methane produced water in China

ABSTRACT Ionic composition data of coalbed methane (CBM) produced water from five CBM blocks in China were used to investigate the variability in and the geological controls on water quality across geologic basins and the resulting environmental impacts. The assessment tools included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO3 ternary diagram. Techniques such as principal component analysis (PCA) and R-mode hierarchical cluster analysis were also applied to the ionic database to reveal the relationships between the ions. The results from this study indicate that three major ions (Na, Cl, and HCO3) and two primary processes (sulfate reduction and mixing of the coal seam water with the fracturing fluid) affect the ionic compositions of CBM produced water. The burial depth of the coal seam, the fresh water influence and the fracturing fluid contamination jointly control the water quality variability of produced water. The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of CBM produced water, such as coal seam water in the partially equilibrated region and exotic water (including the fracturing fluid or shallow cold groundwater) in the immature water region. Most of the CBM produced water in China has high concentrations of Na, HCO3, Cl and TDS compared to the standards for drinking water and irrigation water. The produced waters from deep formations (>1000 m), or those produced during the early drainage periods, are always saline, “Na-Cl” type waters. Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.