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

Abstract

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 (PTDG) 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 PTDG and water pressure were measured every 15 or 30 min. The major dissolved gas species composition, required to calculate methane concentrations from PTDG, was determined from groundwater samples collected approximately bimonthly. Methane was the dominant gas in each well (~80–97%), with relatively consistent composition over time, indicating PTDG provided a reasonable proxy for methane concentrations. All three wells had high PTDG (reaching 53.0 m H2O), with PTDG-derived methane concentrations (34–156 mg/L) much higher (3–12 times) and relatively more stable than determined by conventional groundwater analysis. PTDG 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 PTDG probes can be a valuable tool for monitoring methane-rich groundwater.

Importance of Multicomponent Mass Transfer on the Dissolution of Trapped Gas: A Focus on Geo-Energy Applications in the Shallow Subsurface

“…19,20 Water pressure changes induced by water pumping can cause degassing thereby changing dissolved gas concentrations. 21,22 Processes that occur after gas release, such as gas mixing and oxidation can change the composition of a gas. 23 Changes in isotopic signature can also occur due to diffusive isotopic fractionation.…”

“…Both the hydrocarbon composition and isotopic signature associated with gas migration can vary over time and can be impacted by multiple processes. For example, mixing can occur in the wellbore, particularly over long completion intervals. , Water pressure changes induced by water pumping can cause degassing thereby changing dissolved gas concentrations. , Processes that occur after gas release, such as gas mixing and oxidation can change the composition of a gas . Changes in isotopic signature can also occur due to diffusive isotopic fractionation. ,− Differences in solubility and sorption to porous media along advective pathways can increase dryness ratios. , Free-phase gas samples collected at field sites can have different hydrocarbon compositions than groundwater samples collected at the same site, and mixing and oxidation are typically determined to be the cause. − This broad set of processes must be interpreted appropriately with consideration given to the hydrogeological setting of the gas release …”

In‐Well Degassing of Monitoring Wells Completed in Gas‐Charged Aquifers

“…Monitoring of dissolved gases in shallow groundwater is important for a variety of applications in hydrogeological studies, such as determine groundwater residence times and flow paths (Wilson and Mackay 1995 ; Clark et al 2004 ), analyzing subsurface biochemical reactions (Blicher‐Mathiesen et al 1998 ; Ryan et al 2000 ), detecting the presence of free‐phase gas (FPG) in the saturated zone (Manning et al 2003 ; Amos et al 2005 ; Solomon et al 2011 ), and more recently, monitoring the occurrence of fugitive natural gas migration from oil and gas development (Jackson et al 2013 ; Cahill et al 2017 ; Forde et al 2019 ; Roy et al 2022 ) and carbon capture and storage sites (Boot‐Handford et al 2014 ; Lindeberg et al 2017 ).…”

“…The expansion of carbon capture, utilization, and storage studies (CCUS) has also increased the need for long‐term monitoring of fugitive gases in the subsurface (Boot‐Handford et al 2014 ; Lindeberg et al 2017 ). Thus, accurate long‐term monitoring of total dissolved gas pressure (Roy et al 2022 ) and dissolved gases concentrations (Humez et al 2016 ; Campbell et al 2022 ) in shallow groundwater is valuable in hydrogeological studies.…”

“…The accurate sampling and analysis of dissolved gas concentrations are particularly challenging in “gas‐charged” or “gassy” aquifers (i.e., where in situ total dissolved gas pressure is elevated) because these wells are subject to in‐well degassing when the water table is lowered due to pumping during sampling (Roy and Ryan 2010 ; Roy and Ryan 2013 ). An increasing number of studies have demonstrated that the use of total dissolved gas pressure ( P TDG ) sensors (Manning et al 2003 ; Roy and Ryan 2010 , 2013 ; Roy et al 2022 ) combined with passive gas diffusion samplers (PGDs) or downhole samplers can provide a better analytical resolution when estimating in situ gas concentrations in gas‐charged aquifers (Spalding and Watson 2006 , 2008 ; McLeish et al 2007 ; Banks et al 2017 ; Evans 2017 ). Passive gas diffusion samplers (PGDs) consist of a gas void volume adjacent to a gas permeable water barrier (e.g., silicone), which allows dissolved gases in the aqueous phase to equilibrate within the gas void volume after an appropriate amount of time (McLeish et al 2007 ), providing a sample of equilibrated gas species composition.…”