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, claims a study from Cornell University published on Monday. The study, which looked 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.
Methane gas is often offered as a ‘greener alternative’ to other fossil fuels because, when combusted, it produces about 45% less carbon dioxide than coal in producing the same amount of electricity. In its pure state, however, methane is far more harmful to the environment than CO2.* Studies show that over the period of 25 years, the global warming potential of methane is 72 times that of carbon dioxide.* This is why tracing methane emissions is so important.
As the website Climate Central reports, the new study analysed more than 75,000 publicly available state environmental compliance records for oil and gas wells drilled between 2000 and 2012 across Pennsylvania and found that shale wells – drilled using advanced horizontal drilling and multi-stage fracking procedures – are more likely to leak than older wells which used more traditional methods of extraction. For post-2009 drilled wells, risk of a cement/casing impairment is 1.57-fold higher in an unconventional gas well relative to a conventional well drilled within the same time period.
The study found that methane leaks come from defects in the cement and steel linings of oil and gas well bores, which cause both groundwater contamination and methane leakage into the atmosphere.
Environmental engineering professor Anthony Ingraffea, responsible for the study, said that the cement used to encase a well bore to prevent fluid and gases from leaking into underground aquifers sometimes fail for many reasons.
“Some of them are installation problems,” he said. “Some of them are material problems. Long-term performance problems.”
The cement could have been installed improperly or allowed to set too quickly, he said, or too much water could have been used to mix the concrete.
It also seems that wells built in the north-eastern part of Pennsylvania have higher risk of leakage than wells built elsewhere in the state. The predicted cumulative risk for all wells (unconventional and conventional) in the north-eastern region is 8.5-fold greater than that of wells drilled in the rest of the state. The reason for this discrepancy is not yet known.
Ingraffea’s findings were criticized by Pennsylvania State University geosciences professor Terry Engelder, who said that he doesn’t have full confidence in Ingraffea’s conclusions, particularly his analysis of the wells in north-eastern Pennsylvania.
“Taken at face value, this paper predicts that for every well that has failed to date, four will fail at some unspecified time no more than 300 weeks into the future,” Engelder said, adding that he needs a better understanding of how Ingreaffea’s team estimated the well failure rate in north-east Pennsylvania in order to gain confidence in the study’s conclusions.
Engelder said that the reasons for higher well casing and cementing failure rates in north-eastern Pennsylvania may be more related to geology than problems with the cement jobs used there.
Robert Jackson, a professor of global environmental change at Duke University who has conducted research on methane leakage from oil and gas distribution systems, had a different take.
“Hydraulically fractured shale wells appear to have more problems than conventional wells,” Jackson said. “If so, it’s probably because the wells are longer, must bend to go horizontal and take more water and pressure than in the past. The combination makes well integrity a challenge.
“We can’t tell how much methane is leaking to the air based on this study. That will require more work.”
Another study, carried out by Princeton University found that many of abandoned oil and gas wells in Pennsylvania may be leaking methane. All 19 wells measured in the study showed positive methane leakage. The presence of ethane, propane, and n-butane, along with the methane isotopic composition, indicate that the methane emitted from the wells is predominantly of thermogenic origin, i.e. associated with hydrocarbon production.
Meanwhile, an earlier study prepared jointly by Purdue and Cornell universities, shows that shale gas wells also emit high levels of methane during the drilling process; a stage previously not associated with greenhouse gas emissions.
Paul Shepson, a professor of chemistry and earth atmospheric and planetary sciences at Purdue who co-led the study said that the findings “present a possible weakness in the current methods to inventory methane emissions” by not adopting a “top down” approach that measures methane gas levels in the air above wells.
“This small fraction of the total number of wells was contributing a much larger portion of the total emissions in the area, and the emissions for this stage were not represented in the current inventories.” he added.
*Source: Vikram Rao, Shale Gas: The Promise and The Peril.
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