Friday 22 August 2014

Assessing the risks of cement casing failure at oil and gas wells in Pennsylvania.

Like other areas of the US, Pennsylvania has seen an increase in oil and gas production in recent years, driven by the expanded targeting of shale gas deposits using hydraulic fracturing (blasting water and chemicals into shale beds at high pressure to break up the shale and release any trapped gas) and directional drilling (drilling in any other direction than straight down; drill bores into shale gas beds often penetrate horizontally along the deposits for some distance). As in other areas this expansion has met with some concern, with residents of Pennsylvania particularly worried about the leakage of methane gas from wells into water supplies and the atmosphere. Methane is often cited as a more environmentally friendly fuel than coal, since burning it produces less of the greenhouse gas carbon dioxide, however methane is itself a powerful greenhouse gas, and if it leaks from wells into the atmosphere at even low levels then any environmental benefits of burning gas over coal are rapidly lost.

A drill rig at Roullette in Potter County, Pennsylvania, targeting the Marcellus Shale. Laurie Barr/Wikimedia Commons.

Raised methane levels have been found both in aquifers from which drinking water is drawn and in the atmosphere close to gas wells in Pennsylvania, strongly suggesting that there is a link between the methane levels and the drilling. One of the most likely ways in which this can happen is loss of integrity along the well bore due to failure of the cement casing, allowing methane to migrate along the borehole, into other strata or the atmosphere. Current regulations in Pennsylvania allow for low pressure leaks to be monitored and periodically bled off, but require that higher pressure leaks must be repaired, or if this is not possible the well must be permanently plugged; however such plugging, while likely to prevent further loss of gas from the wellhead into the atmosphere, may still allow subsurface migration of methane into other strata (including aquifers).

Clearly this makes the rate of failure in drill well cement casings in Pennsylvania a matter of great interest, however most previous studies of such well casings have concentrated on the structural integrity of offshore wells, with little direct relevance to the wells of Pennsylvania, other than the general observation that wells drilled during periods of rapid expansion in the industry are more likely to have problems, as are wells which are drilled in directions other than vertical. 

Data on the monitoring of structural integrity at wells in Pennsylvania is not publically available, however records Notices of Violations issued by the Pennsylvania Department of Environmental Protection are, and several studies on well integrity in the state have used these notices to try to estimate the rate of drill casing integrity failure there.

In a paper published in the Proceedings of the National Academy of Sciences of the USA on 30 June 2014, Anthony Ingraffea of the School of Civil and Environmental Engineering at Cornell University and Physicians, Scientists, and Engineers for Healthy Energy, Martin Wells of the Department of Statistical Sciences at Cornell University, Renee Santoro, also of Physicians, Scientists, and Engineers for Healthy Energy and Seth Shonkoff, again of Physicians, Scientists, and Engineers for Healthy Energy as well as the Department of Environmental Science, Policy, and Management at the University of California, Berkeley, describe the results of a study of the complete inspection records issued by the Pennsylvania Department of Environmental Protection for wells spudded (the spud date of a well is the day on which drilling begins) between 2000 and 2012.

Ingraffea et al. observe that Notices of Violation do not give a complete record of all leaks at well heads, as the inspection agency may not issue such a notice at a well where cement casing failure has been detected but remedial action is being taken by the drill operator, and that accessing the full inspection records for the wells should help to detect problems not identified by studies of Notices of Violation alone. They also not that problems with older wells are more likely to have been detected than problems with newer wells, as the older wells will have been inspected more times since their spud date, giving a better understanding of the full lifetime history of the well, and that many unconventional wells in Pennsylvania have been dug during a fairly recent boom, leading to a combination of two known high risk factors (wells dug during periods of rapid expansion within the industry and wells which deviate from the vertical) with shorter inspection records where problems are unlikely to have been detected.

Ingraffea et al. found that a total of 1.9% of all wells spudded in Pennsylvania between 2000 and 2012 suffered a loss of integrity. They further found that unconventional wells (i.e. wells targeting shale gas) were roughly six times as likely to suffer problems as conventional wells. The highest incidence of failure occurred in the northeast region of the state (Bradford, Cameron, Clinton, Lycoming, Potter, Sullivan, Susquehanna, Tioga, Wayne, and Wyoming counties) in wells targeting the Marcellus Shale spudded before 2009, with a failure rate of 9.84%. They further noted that wells drilled in the same area since 2009 had a failure rate of 9.18%, despite having a shorter inspection record (although wells spudded in 2012, which had an inspection record of less than 12 months when the study was carried out did have a significantly lower failure rate).

Map showing the extent and thickness of the Marcellus Shale beneath New York, Pennsylvania, Ohio and Virginia. Note the shale is thickest in the northeast. Marcellus Connection

The oldest unconventional well in the study was spudded in 2002, and unconventional wells remained a relatively small part of the industry in Pennsylvania until 2009. The rate of inspection also increased in 2009, with 76% of wells inspected in their first year prior to 2009, and 88.7% of wells inspected in their first year subsequently, which may partly account for the high rate of detected failure in post-2009 wells.

Wells in the northeastern counties were more prone to problems than wells in other areas, with 266 well failures recorded in the region, 52% of the total for the state. Within the northeastern region unconventional wells were 8.5 times more likely to have structural integrity problems than conventional wells. Unconventional wells in the northeastern counties had a 20% chance of problems within the first 3-4 years of operation, rising to 40% by year 7.

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