LONG-TERM MONITORING OF METHANE CONCENTRATIONS IN A SHALE AQUIFER USING TOTAL DISSOLVED GAS PRESSURE (PTDG) PROBES: COMPARISON WITH DATA FROM GROUNDWATER SAMPLES

Monitoring of dissolved methane concentrations in groundwater is required to detect potential impacts from oil and gas development and to understand the range of temporal variability under natural conditions. As required by many jurisdictions, such monitoring is currently performed via periodic groundwater sampling. However, any sampling set-up may induce degassing (hence partial loss of methane) in gas-charged groundwater, which biases measured variations. To investigate the range of natural variations measured in a “pristine” region where no commercial hydrocarbon production has taken place (St-Edouard area, southern Québec, Canada), several monitoring wells were repeatedly sampled between 2012 and 2016 using a meticulous, standardized and consistent methodology. A subset of 3 wells containing methane-rich water was then selected for a subsequent study (2017-2019), where total dissolved gas pressure (P_TDG) probes were used as a continuous monitoring method to infer downhole methane concentrations. The aim was to investigate the extent to which the variation measured in groundwater samples was a result of degassing during sampling. Results from groundwater samples (2012-2016) in the 3 selected wells had shown very little variation in methane isotopic ratios (δ¹³C_CH4), with coefficient of variation (CV) between 2.4 and 3.3% in a given well, suggesting that the source of methane remained the same over time. On the other hand, methane concentrations in sampled groundwater varied widely (CV between 22 and 57%), despite the use of a rigorous sampling method and the absence of hydrocarbon activities or groundwater pumping nearby. P_TDG measurements were then recorded (2017-2019) every 15 to 30 minutes for 2 years, using two types of probes. Results show that methane concentrations inferred from P_TDG were fairly stable over time (CV between 2.3 and 7.4%), which supports the interpretation from earlier isotopic data, and implies that most of the variation noted from groundwater samples was due to degassing. These results have important implications regarding the way monitoring is currently conducted in most regions, and eventual liability issues. Here we discuss the insights gained from using P_TDG probes, limits to their application, and provide recommendations for optimal use and monitoring.