Ion and Stable Isotope Geochemistry of Produced Water in Coalbed Methane Wells in the Tiefa Basin

Abstract

The potential geochemical information in the produced water of coalbed methane (CBM) wells is conducive to the exploration and development of CBM in the case that the produced water is primitive formation water. A total of 58 produced water samples collected from 13 CBM wells in the Daxing Mine, Tiefa Basin, were investigated. Ionic composition tests and stable isotope analysis were conducted to explore the geochemical characteristics and sources of produced water as well as the method for determining whether the produced water is primitive formation water. The results suggest that the fracking fluid for CBM stimulations is the main factor affecting the ion change of the produced water in the initial stage of drainage. The concentrations of Cl– and Ca2+ + Mg2+ could be taken as the indices to identify whether the produced water is primitive formation water. When the Cl– concentration is lower than 20 mEq/L and the Ca2+ + Mg2+ concentration is lower than 1 mEq/L, the produced water is close to the pristine formation water. Biogenic methanogenic activity may result in a high δ13CDIC and high concentrations of HCO3 – in the pristine formation water in the Tiefa Basin. The data of δD and δ18O in the study area suggest that the formation water might come from atmospheric precipitation, which is later affected by evaporation and the water–rock reaction. The hydrogen isotope values in the produced water derived from the lower coal group display a substantial elevation compared to those from the upper coal group. This disparity in the hydrogen isotope composition presents an opportunity to utilize δD in produced water as a tool for distinguishing the formation water between these two groups.