Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells

Abstract

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42− concentrations dropped. These observations are attributed to iron and SO42− reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42− that could distinguish newly leaked methane from older methane sources in aquifers.

Gas well integrity and methane migration: evaluation of published evidence during shale-gas development in the USA

“…The results of an additional case in Sugar Run in Bradford County, PA, will also be presented because of the large dataset available for that site. (Note: This site is not the Sugar Run in Lycoming County recently discussed by Woda et al 2018). Figure 1 is a map of the United States showing the locations of these sites.…”

“…On the same dates as the % LEL levels rose above predrill levels, the Fe concentrations (6.9 and 3.88 mg/L) were elevated in both wells relative to pre-drill concentrations (0.011 and 0.03 mg/L). Such concentrations can result from the reduction of Fe compounds by bacteria using methane as an electron source and typically last for a limited time period (Woda et al 2018); they may provide evidence that methane had migrated prior to hydraulic fracturing of GW-1. GW-1 was then squeezed at 2,091 m, 1.2 m below the top of the Upper Marcellus Shale on October 22, 2008, producing a secondary TOC, above the primary TOC, at 1,673 m. No data indicating that the squeeze was successful were reported.…”

“…Migration of stray methane gas has been at the center of the debate on the impacts caused by shale gas drilling and hydraulic fracturing activities on the quality of shallow drinking water resources. While not a regulated contaminant, methane can accumulate to explosive levels and cause asphyxiation in confined spaces, as well as initiate adverse chemical reactions such as the reduction of iron and manganese compounds in well water (Gorody 2012;Woda et al 2018). In the late 1940's, hydraulic fracturing was developed to increase production from conventional oil and gas wells.…”

Advancing knowledge of gas migration and fugitive gas from energy wells in northeast British Columbia, Canada

“…GM and FG can be caused by energy wellbore integrity failure or mobilization of intermediate depth gas pockets along an energy wellbore during and / or after drilling . Regardless of source or cause, GM results in the flow of both free‐phase and dissolved natural gas in the subsurface, resulting in various cases of groundwater contamination . Once dissolved in groundwater, natural‐gas constituents will migrate via advection and dispersion, where they may be attenuated by microbes (based on availability of electron acceptors), potentially generate undesirable byproducts (e.g.…”

“…9,13 Regardless of source or cause, GM results in the flow of both free-phase and dissolved natural gas in the subsurface, 14 resulting in various cases of groundwater contamination. [15][16][17][18][19] Once dissolved in groundwater, natural-gas constituents will migrate via advection and dispersion, where they may be attenuated by microbes (based on availability of electron acceptors), potentially generate undesirable byproducts (e.g. H 2 S, Fe 2+ , Mn 2+ ), [20][21][22][23][24] and / or potentially induce release of trace metals (e.g.…”