The biggest environmental concern with shale gas exploration and production is water contamination. Whether or not hydraulic fracturing causes methane contamination of drinking water remains to be proved (Davies, 2011), although the possibility cannot be totally ruled out. Well integrity failure is a far more likely cause of elevated thermogenic methane concentrations in shallow groundwater and water supplies than pathways induced solely by hydraulic fracturing (Davies et al 2014 and references therein).

Thus, several recent publications deal with statistical analyses of well integrity failures. An overview is given for the Marcellus Shale Formation (table 1).

Tab.1: The authors used the dataset for the Marcellus Shale Formation provided by the Department of Environmental Protection of Pennsylvania, USA

Source                   well barrier leakage (%)    investigated dataset (wells)    time span
Vidic et al (2013)               3.4                                   6466                        2008-2013
Ingraffea (2012)                 6.2                                    3391                       2011-2012
Considine et al (2013)        2.6                                   3533                        2008-2011
Davies et al (2014)             6.3                                   8030                        2005-2013

The research team Davies et al (2014) have assessed all the reliable datasets on well barrier and integrity failure around the world and they give an overview of the global well inventory. The global well inventory accounts for at least 4 million onshore hydrocarbon wells drilled in Australia, Austria, Bahrain, Brazil, Canada, the Netherlands, Poland, the UK and the USA where in the USA alone, at least 2.6 million onshore wells have been drilled since 1949. 

Well integrity failure refers not only to unconventional resources, however; it is also known in conventional oil and gas wells. Ingraffea et al (2014) assessed the casing and cement impairment of conventional and unconventional oil and gas wells. The study analyzed publicly available Pennsylvania Department of Environmental Protection inspection reports on more than 41,000 oil and gas wells drilled from January, 2000 to December, 2012. The study found that well type, location, and when wells were drilled all drive differences in the rates of cement and/or casing failure:

• The statewide failure rate to date for Unconventional (Marcellus Shale) wells is at least 6.2%.
• Marcellus wells drilled after 2009 were found to fail 1.57 times more frequently than conventional wells drilled within the same time period.
• Unconventional gas wells in northeastern Pennsylvania were found to be at a 2.7 times higher risk relative to the conventional wells in the same area.
• The predicted cumulative risk for all wells (unconventional and conventional) in the northeastern region is 8.5 times greater than that of wells drilled in the rest of Pennsylvania.

While their data is unable to identify water contamination events, the article does provide a first step towards understanding the mechanisms, feasibility, and degree to which this is being caused by a gas (and fluid) migration mechanism in Pennsylvania and potentially elsewhere throughout the globe.

For more information regarding well integrity, please read the expert article here.

References and related literature:

Considine, T.J., Watson, R.W., Considine, N.B., Martin, J.P., 2013. Environmental regulation and compliance of Marcellus Shale gas drilling. Environmental Geosciences 20, 1-16.

Davies, R.J., 2011. Methane contamination of drinking water caused by hydraulic fracturing remains unproven. Proceedings of the National Academy of Sciences 108, E871.

• Jackson, R.B., Osborn, S.G., Vengosh, A., Warner, N.R., 2011. Reply to Davies: Hydraulic fracturing remains a possible mechanism for observed methane contamination of drinking water. Proceedings of the National Academy of Sciences 108, E872.

Davies, R.J., Almond, S., Ward, R.S., Jackson, R.B., Adams, C., Worrall, F., Herringshaw, L.G., Gluyas, J.G., Whitehead, M.A., 2014. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. Marine and Petroleum Geology 56, 239-254.

• Thorogood, J.L., Younger, P.L., 2014. Discussion of “Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation" by R. J. Davies, S. Almond, R.S., Ward, R.B. Jackson, C. Adams, F. Worrall, L.G. Herringshaw, J.G. Gluyas and M.A. Whitehead. Marine and Petroleum Geology.
• Davies, R.J., Almond, S., Ward, R., Jackson, R.B., Adams, C., Worrall, F., Herringshaw, L.G., Gluyas, J.G., Whitehead, M.A., 2014. Reply: "Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation”. Marine and Petroleum Geology.

Ingraffea, A., 2012. Fluid Migration Mechanisms Due to Faulty Well Design and/or Construction: an Overview and Recent Experiences in the Pennsylvania and Marcellus Play. www.psehealthyenergy.org/site/view/1057.

Ingraffea, A.R., Wells, M.T., Santoro, R.L., Shonkoff, S.B.C., 2014. Assessment and risk analysis of casing and cement impairment in oil and gas wells in Pennsylvania, 2000–2012. Proceedings of the National Academy of Sciences 111(30), 10955–10960.

Jackson, R.B., 2014. The integrity of oil and gas wells. Proceedings of the National Academy of Sciences 111, 10902-10903.

King, G.E., King, D.E., 2013. Environmental Risk Arising from Well-construction Failure e Differences between Barrier and Well Failure and Estimates of Failure Frequency across Common Well Types, Locations and Well Age. SPE 16142.

Vidic, R.D., Brantley, S.L., Vandenbossche, J.M., Yoxtheimer, D., Abad, J.D., 2013. Impact of shale gas development on regional water quality. Science 340, 6134.