The relationship between methane migration and shale-gas well operations near Dimock, Pennsylvania, USA

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.

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

“…The low concentration (<10 mg/L) of the 2012 US EPA samples are generally more isotopically enriched than the 2009 results, while the higher concentration samples are generally more depleted. The 2010 samples have relatively high concentrations and tend to Hammond, 2016) be similar to or more isotopically depleted than the 2009 data. Baldassare et al (2014) analyzed isotopes in samples collected during mud gas logging (MGL) programs in northeastern Pennsylvania.…”

“…These results indicate that the source of methane for the 2009 samples could be the intermediate-depth formations immediately above the Marcellus Shale (either or both of the Middle Devonian units (Mahantango Shale and Tully Limestone). As described by Hammond (2016), samples collected after 2009 and the completion of gas well remediation efforts often showed microbial alteration or evidence of a microbial source as summarized in the ESM. Some of these samples are discussed in the following.…”

“…3. The first samples were collected by the Pennsylvania Department of Environmental Protection (PA DEP) in 2009 from gas well annular spaces and water wells along Carter Road, prior to any remedial actions (Hammond 2016). Follow-on samples were analyzed by the PA DEP and the operator's consultant, Civil and Environmental Consultants, Inc. (CEC), in 2010, and the US EPA in 2012.…”

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.…”