Abstract
Migration of stray methane gas near the town of Dimock, Pennsylvania, has been at the center of the debate on the safety of shale gas drilling and hydraulic fracturing in the United States. The presented study relates temporal variations in molecular concentrations and stable isotope compositions of methane and ethane to shale-gas well activity (i.e., vertical/horizontal drilling, hydraulic fracturing and remedial actions). This was accomplished by analyzing data collected, between 2008 and 2012, by state and federal agencies and the gas well operator. In some cases, methane migration started prior to hydraulic fracturing. Methane levels of contaminated water wells sampled were one to several orders of magnitude greater than the concentrations due to natural variation in water wells of the local area. Isotope analyses indicate that all samples had a thermogenic origin at varying maturity levels, but from formations above the hydraulically fractured Marcellus Shale. The results from the initial water well samples were similar to annular gas values, but not those of production gases. This indicates that leakage by casing cement seals most likely caused the impacts, not breaks in the production casing walls. Remediation by squeeze cementing was partially effective in mitigating impacts of gas migration. In several cases where remediation caused a substantial reduction in methane levels, there were also substantial changes in the isotope values, providing evidence of two sources, one natural and the other man-induced. Sampling water wells while venting gas wells appears to be a cost-effective method for determining if methane migration has occurred.
Highlights
Dimock, a small town in Susquehanna County, northeastern Pennsylvania (USA), received national and international attention due to much publicized incidents of methane contamination of water wells, just outside of town in 2009, which were featured in the 2010 Gasland and 2013 Gasland 2 movies on HBO
Isotope analyses were reviewed for samples taken from gas and water wells in the subject area and are shown in a Schoell diagram (Fig. 4) and details are contained in Table S4 of the electronic supplementary material (ESM)
Initial samples were collected by Pennsylvania Department of Environmental Protection (PA DEP) in 2009 from gas well annular spaces and water wells
Summary
A small town in Susquehanna County, northeastern Pennsylvania (USA), received national and international attention due to much publicized incidents of methane contamination of water wells, just outside of town in 2009, which were featured in the 2010 Gasland and 2013 Gasland 2 movies on HBO. The Pennsylvania Department of Environmental Protection (PA DEP) conducted an investigation into the problem and entered into a consent order with the operator in December 2009 that required remediation of certain shale-gas wells and mitigation for the residents having affected water supplies. In December 2010, a Consent Order and Settlement Agreement was reached which included a financial settlement for 19 residents in the Dimock area. In addition to high methane concentrations, there was some evidence of contamination by other chemical compounds. This led the United Sates Environmental Protection Agency (US EPA) to conducted extensive sampling and analysis of water from the affected and other nearby water wells in early 2012 (US EPA 2012)
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