A novel geochemical approach for environmental risk assessment of stray gas contamination in shallow aquifers within active and prospective shale gas development zones

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Advances in drilling technologies and production strategies, such as horizontal drilling and hydraulic fracturing, have significantly improved the production of natural gas and oil from unconventional hydrocarbon deposits. While these activities have dramatically changed the energy landscape in the U.S., recent research results suggests the potential for methane contamination in shallow drinking water wells near some active shale gas wells in northeastern Pennsylvania. These findings raise important issues about the potential safety of drilling practices. However, other workers highlight that naturally occurring methane occurs within shallow aquifers that overlie black shale deposits. These competing interpretations provoke debate amongst academic, industry, and regulatory scientists regarding the potential environmental impacts of hydraulic fracturing and horizontal drilling. Stable carbon and hydrogen isotopes of methane are commonly used to determine the source of methane. However, some hydrocarbon producing black shales in the US and Canada have overlapping isotopic ratios that mask potential differences between anthropogenic contamination and the natural methane in the subsurface. Consequently, this controversy highlights the need to develop independent and complementary geochemical tools to constrain the source of natural gas and the mechanism of transport in shallow drinking waters, specifically those potentially contaminated by stray gas migration. This project aims to develop noble gas geochemistry as a new geochemical tool, integrated with traditional chemical and isotopic measurements, for delineating the sources of hydrocarbons in shallow aquifers associated with shale gas exploration in the Appalachian Basin. In particular, the project will establish more robust criteria for distinguishing naturally occurring methane from that sourced by drilling-related contamination. Investigators expect to be able to distinguish between: 1) shallow uncontaminated groundwater; 2) groundwater that was affected by natural hydrocarbon gas migration along natural fractures over geological time; and c) groundwater contaminated by hydrocarbon gas migration out of faulty/insufficiently installed wells. Given the importance of hydraulic fracturing and gas drilling in the U.S., significant public interest is expected from this study. Outcomes from this project will inform policy decisions toward establishing the appropriate balance between economically sustainable shale gas development and the development of adequate safeguard environmental regulations.