Stable isotope characteristics of CBM co-produced water and implications for CBM development: The example of the Shizhuangnan block in the southern Qinshui Basin, China

Coalbed methane (CBM) is a rapid growing industry in the Qinshui Basin, China. However, as a relatively new industry, few studies have examined the co-produced water from CBM in terms of either water quality or stable isotope composition. This study aims to determine the isotopic composition of coal mine roof water and CBM co-produced water, to correlate stable isotope characteristics of the co-produced water with gas and groundwater production and flow pathways, and to identify the origin of the co-produced water of the Shizhuangnan CBM block in the southern Qinshui Basin. Stable isotope analyses were conducted on 58 water samples collected from three coal mines and 29 CBM wells. Well production data including gas and water production rates, and well liquid level were also collected. Ten additional sets of literature data were also used in the analyses from this area. The stable isotope analyses show that both coal seam roof water and CBM co-produced water are predominantly meteoric origin, with a trend towards lower isotopic composition where the aquifer is shallower. It was found that co-produced water samples with δD and δ18O values above the local meteoric water line (LMWL) were associated with higher gas production, whereas those with δD and δ18O values lower than the LMWL were associated with lower gas production. In addition, the co-produced water of highly productive gas wells has positive and relatively higher δ13C-DIC (DIC, dissolved inorganic carbon) values, while water from wells with no gas production has negative δ13C-DIC values. We propose that coal seam water is characterized by “D drift” above the LMWL in the plots of δD vs. δ18O and positive δ13C-DIC values. On the contrary, sandstone and limestone aquifer water is normally characterized by “18O drift” below the LMWL and negative δ13C-DIC values. Therefore, the findings of this study will help to identify the origin of co-produced water, and guide CBM exploration and development. They can also be used in the management of CBM co-produced water.