CHEMICAL AND ISOTOPIC CHANGES OVER TIME IN CO-PRODUCED GAS AND WATER FROM UNCONVENTIONAL SHALE DEPOSITS

The Antrim Shale is an organic-rich Devonian black shale in the Michigan Basin that has been actively producing gas over the past ~25 years. The gas produced is mixed microbial/thermogenic and the relative contributions of each are profoundly influenced by well location. For a subset of the over 20,000 wells located in the shale, we have been tracking the geochemical and isotopic evolution of water and gas and have, more recently, been examining changes within the microbial communities present. Interactions between the microbes, rock and production practices all lead to a complex chemical signature that requires careful consideration.

Alkalinity in the water from each well decreased by 10 mM on average while the d¹³C of carbon dioxide increased by nearly 5 per mil. It is possible that both of these changes are due to production, as each well became more hydrologically “open” due to active pumping. The previous extremely high concentrations of alkalinity (~60 mM) are no longer sustainable as saturation states change with decreased downhole pressures. These lower pressures also lead to the increase in mole volume percent carbon dioxide produced over time, and the increase in the carbon isotopic value may reflect desorption fractionation. The d¹³C of methane has also shifted towards lower values (~1-3 per mil) in each of the wells over time. This may represent active microbial generation of gas, or simply the methane being sourced from greater distances. Microorganisms most closely related to CO₂-reducing methanogens were the most abundant group in archaeal clone libraries and fermenters were the most common in bacterial libraries in our original baseline sampling of these wells. In contrast, resampling of wells showed a rise in the sulfate-reducing bacterial community. Our results show that commercial gas production has not only caused chemical and isotopic changes in water and gas in the Antrim Shale but also shifts in the microbial community that inhabits. These changes in microbiology and geochemistry can only be explained by examining a multitude of factors, including natural in situ biogeochemical reactions as well as processes driven by commercial gas production, including groundwater flow, gas desorption, and open-system degassing.