Biogeochemistry and the associated redox signature of co-produced water from coalbed methane wells in the Shizhuangnan block in the southern Qinshui Basin, China

Abstract

To meet the global energy demands, the exploitation of coalbed methane has received increasing attention. Biogeochemical parameters of co-produced water from coalbed methane wells were performed in the No. 3 coal seam in the Shizhuangnan block of the southern Qinshui Basin (China). These biogeochemical parameters were firstly utilized to assess coal reservoir environments and corresponding coalbed methane production. A high level of Na + and HCO 3 – and deuterium drift were found to be accompanied by high gas production rates, but these parameters are unreliable to some extent. Dissolved inorganic carbon (DIC) isotopes δ 13 C DIC from water can be used to distinguish the environmental redox conditions. Positive δ 13 C DIC values within a reasonable range suggest reductive conditions suitable for methanogen metabolism and were accompanied by high gas production rates. SO 4 2– , NO 3 – and related isotopes affected by various bacteria corresponding to various redox conditions are considered effective parameters to identify redox states and gas production rates. Importantly, the combination of δ 13 C DIC and SO 4 2– can be used to evaluate gas production rates and predict potentially beneficial areas. The wells with moderate δ 13 C DIC and negligible SO 4 2– represent appropriate reductive conditions, as observed in most high and intermediate production wells. Furthermore, the wells with highest δ 13 C DIC and negligible SO 4 2– exhibit low production rates, as the most reductive environments were too strict to extend pressure drop funnels.