P. 8 ~ Continued - Methane in Pennsylvania water wells unrelated to Marcellus shale fracturing
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Our finding is based upon the limited isotopic data provided in the Duke paper. Future analyses, which may include a more complete suite of isotope data such as δ13C and δ2H values of ethane, propane, and other higher value chain hydrocarbons, will likely provide a more comprehensive characterization of gases from the Marcellus shale and shallower Middle and Upper Devonian deposits in this area.
The Duke researchers discuss three possible mechanisms for thermogenic methane migration into shallow drinking water aquifers: i) the displacement of gas-rich fluids from deep target formations, ii) enhanced migration of gas via newly created fractures, and iii) casing leaks from gas production wells.
The Duke study found no evidence of salinity impacts on the drinking water wells, indicating that fluid migration from the gas production zone had not occurred.
The comparable thermogenic isotopic signature of Upper Devonian gas to that observed in the water wells sampled by the Duke team, in conjunction with the much greater proximity of the Upper Devonian shales to the drinking water aquifers, suggests that the Upper Devonian is a more plausible source of the observed methane than upward migration of deep Marcellus gas via newly created fractures.
Furthermore, our evaluation of over 1,700 methane concentrations in gas production vs. nonproduction areas shows no relationship between dissolved methane and oil and gas activities, which is consistent with the fact that casing leaks, when and if they occur, would have only a localized effect.
Significance of findings
This study demonstrates that elevated methane concentrations in water wells in Susquehanna County are common and are correlated with topography rather than proximity to oil and gas operations, suggesting a geologic origin.
Consideration of regional geology, historical publications, water well completion records, and recent isotopic analyses indicates that naturally occurring methane is either thermogenic, originating from deposits overlying the Marcellus shale, or biogenic, originating from alluvial or glacial drift deposits.
In either case, the assertion by the Duke study that hydraulic fracturing of the Marcellus shale is contributing thermogenic methane to local water wells and shallow regional groundwater is unsubstantiated given the lines of evidence discussed in this paper. Rather, it appears that thermogenic methane encountered in water wells is related to the shallower Upper and-or Middle Devonian gases.
Historic records demonstrate that Upper Devonian thermogenic gas will be encountered by water wells or gas wells penetrating the Catskill formation. Water wells in the State of Pennsylvania currently have no uniform construction standards to prevent gas migration or other water quality issues.
To minimize potential impacts from shale gas wells, the Pennsylvania DEP has recently issued new regulations regarding the sealing, prevention and reporting of overpressurization, and inspection of gas production wells.32
Evaluating methane gas sources in groundwater
The evaluation presented in this article underscores the value of a "multiple-lines-of-evidence" approach for site-specific investigation of stray gas incidents.
Important lines of evidence include knowledge of underlying geologic stratigraphy, structure, and existing fracture systems; construction and completion details for the affected water wells and nearby gas production wells; historic information regarding prior stray gas incidents and evidence of naturally-occurring gas seeps; and prior analyses of the composition and isotopic signatures of gas sources.
As observed in this case, thermogenic gas can be sourced from various formations containing gases with carbon and hydrogen isotopic composition that may exhibit subtle variations. Complementing the isotopic analysis with characterization of the mass ratios of methane to ethane and to other higher chain hydrocarbons, as well as ratios of methane concentrations to those of carbon dioxide, nitrogen, and other gases, may provide greater resolution in some cases.
Careful consideration of the correct source of methane impacts, on a site-specific basis, is important for development of appropriate response actions for protection of water resources.
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