Trends in water quality variability for coalbed methane produced water

Energy production from unconventional natural gas resources, such as coalbed methane, has the potential to generate significant water quantities for use in water-stressed areas to augment existing water supplies. Coalbed methane (CBM) produced water is generated from shallower formations than traditional oil and gas resources where water quality may be influenced by fresh water supplies in the area. Variability in produced water quality between wells and across geologic basins must be characterized in order to categorize water types appropriate for beneficial use. Principal component analysis (PCA) was applied to a composite geochemical database to identify indicators of variability in water composition and quality. Component analysis revealed that water quality indicators of variability were related to: (i) aquifer recharge that dilutes constituent concentrations (37%), (ii) dissolution of soluble aquifer minerals such as sodium and exchange of calcium and magnesium (13.8%), and (iii) coal depositional environment influence on chloride and trace metal fractions (14% of variability). Ternary relationships between Na-Cl-HCO₃ and Na-Ca-Mg correlate to marine influence in the coal depositional environment and well proximity to recharge, respectively. Relationships identified in this study highlight water quality compositions with opportunities for beneficial use.