Assessment and risk analysis of casing and cement impairment in oil and gas wells in Pennsylvania, 2000–2012
Anthony R. Ingraffea, Martin T. Wells, Renee L. Santoro and Seth B. C. Shonkoff
Abstract
Casing and cement impairment in oil and gas wells can lead to methane
migration into the atmosphere and/or into underground
sources of drinking water. An analysis of
75,505 compliance reports for 41,381 conventional and unconventional oil
and gas
wells in Pennsylvania drilled from January
1, 2000–December 31, 2012, was performed with the objective of
determining complete
and accurate statistics of casing and
cement impairment. Statewide data show a sixfold higher incidence of
cement and/or casing
issues for shale gas wells relative to
conventional wells. The Cox proportional hazards model was used to
estimate risk of
impairment based on existing data. The
model identified both temporal and geographic differences in risk. For
post-2009 drilled
wells, risk of a cement/casing impairment
is 1.57-fold [95% confidence interval (CI) (1.45, 1.67); P <
0.0001] higher in an unconventional gas well relative to a conventional
well drilled within the same time period. Temporal
differences between well types were also
observed and may reflect more thorough inspections and greater emphasis
on finding
well leaks, more detailed note taking in
the available inspection reports, or real changes in rates of structural
integrity
loss due to rushed development or other
unknown factors. Unconventional gas wells in northeastern (NE)
Pennsylvania are at
a 2.7-fold higher risk relative to the
conventional wells in the same area. The predicted cumulative risk for
all wells (unconventional
and conventional) in the NE region is
8.5-fold [95% CI (7.16, 10.18); P < 0.0001] greater than that of wells drilled in the rest of the state.
Significance
Previous research has
demonstrated that proximity to unconventional gas development is
associated with elevated concentrations
of methane in groundwater aquifers in
Pennsylvania. To date, the mechanism of this migration is poorly
understood. Our study,
which looks at more than 41,000
conventional and unconventional oil and gas wells, helps to explain one
possible mechanism
of methane migration: compromised
structural integrity of casing and cement in oil and gas wells.
Additionally, methane, being
the primary constituent of natural gas, is
a strong greenhouse gas. The identification of mechanisms through which
methane
may migrate to the atmosphere as fugitive
emissions is important to understand the climate dimensions of oil and
gas development.
The complete article is available for free from the National Academy of Sciences as a pdf file:
http://www.pnas.org/content/early/2014/06/25/1323422111.full.pdf
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This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1323422111/-/DCSupplemental.
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