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 HCO3– 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 δ13CDIC from water can be used to distinguish the environmental redox conditions. Positive δ13CDIC values within a reasonable range suggest reductive conditions suitable for methanogen metabolism and were accompanied by high gas production rates. SO42–, NO3– 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 δ13CDIC and SO42– can be used to evaluate gas production rates and predict potentially beneficial areas. The wells with moderate δ13CDIC and negligible SO42– represent appropriate reductive conditions, as observed in most high and intermediate production wells. Furthermore, the wells with highest δ13CDIC and negligible SO42– exhibit low production rates, as the most reductive environments were too strict to extend pressure drop funnels.
Highlights
Coalbed methane (CBM) is becoming increasingly developed worldwide as an unconventional energy resource, with an increasing number of publications establishing CBM as efficient clean energy source (Baldassare et al, 2014)
These findings suggest that D drift can be regarded as a significant indicator for high gas production rates
The study has assessed the relationship between various geochemical parameters and gas production rates in CBM co-produced water samples from the SZN block
Summary
Coalbed methane (CBM) is becoming increasingly developed worldwide as an unconventional energy resource, with an increasing number of publications establishing CBM as efficient clean energy source (Baldassare et al, 2014). CBM is trapped in micropore structures of coal reservoirs due to formation pressure, while the coalbed is saturated with groundwater. During extraction of CBM, it is essential to remove aquifer water from the coalbed to lower reservoir pressure. Large volume of water is inevitably produced when CBM is formed. A variety of geochemical processes exist in coal reservoir water. CBM co-produced water serves as an indicator of these processes it has been subjected to, which could provide valuable information on its evolution (Moore, 2012). CBM co-produced water may act as a signature to allow an improved understanding of CBM preservation and enrichment, which can be used as a tool to analyze gas production rates in the processes of CBM exploration (Wu et al, 2018)
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