Continual long-term monitoring of methane in wells above the Utica Shale using total dissolved gas pressure probes

Monitoring of dissolved methane concentrations in groundwater is required to identify impacts from oil and gas development and to understand temporal variability under background conditions. Currently, long-term (i.e., multiyear) monitoring is performed via periodic groundwater sampling; hence, the data are temporally limited and can suffer from degassing losses in-well and at surface for groundwater with high dissolved gas concentrations. The application of total dissolved gas pressure (P-TDG) probes for long-term monitoring of methane-rich groundwater was investigated for >2 years in three monitoring wells in a low-permeability bedrock aquifer above the Utica Shale, Canada. The advantage of these probes is that they allow for continual in situ monitoring. A hydraulic packer was installed in each well, below which P-TDG and water pressure were measured every 15 or 30 min. The major dissolved gas species composition, required to calculate methane concentrations from P-TDG, was determined from groundwater samples collected approximately bimonthly. Methane was the dominant gas in each well (similar to 80-97%), with relatively consistent composition over time, indicating P-TDG provided a reasonable proxy for methane concentrations. All three wells had high P-TDG (reaching 53.0 m H2O), with P-TDG-derived methane concentrations (34-156 mg/L) much higher (3-12 times) and relatively more stable than determined by conventional groundwater analysis. P-TDG monitoring also revealed substantial short-term changes during pumping and between sampling events (up to 4 m H2O), possibly associated with background variability. Limitations and technical remedies are discussed. This study demonstrates that P-TDG probes can be a valuable tool for monitoring methane-rich groundwater.

Automated summary

This study investigates the use of P-TDG probes for long-term monitoring of methane-rich groundwater. The study finds that P-TDG probes provide a reasonable proxy for methane concentrations and reveal both long-term stability and short-term variability in methane concentrations.