A study of decommissioned onshore oil and gas wells in the UK found that 30% were leaking methane. But the average leak produced lower emissions than a breeding dairy cow.
The research, published today in the journal Science of The Total Environment, concluded that the leaks were caused by well integrity failure and that wells were most likely to leak within 10 years of being abandoned.
The industry body, UK Onshore Oil and Gas, said the study should reassure people. But Greenpeace said it raised questions about the development of fracking in the UK.
The study was by the ReFINE research consortium on fracking, led by Newcastle and Durham Universities and funded primarily by Centrica and INEOS.
Researchers selected 102 wells from four onshore UK oil and gas basins in the Weald, North Yorkshire, the East Midlands and Wessex.
The wells were chosen to give a range of conditions and they varied in age from eight to 79 years. They had all been decommissioned in line with best practice in the UK. They had been cut off, sealed and buried in soil to 2m. Another well, drilled in 1917, was included because it had not been decommissioned properly.
The study analysed soil gas above each well and compared it with a nearby control site of similar land use and soil type.
- In 31 out of 102 wells (30%) the soil gas methane was significantly higher than control sites
- In the most extreme example, the methane was 147% greater than the control
- In 39 out of 102 (39%) wells the soil gas methane was significantly lower than controls.
- Leaks from the well that had not been decommissioned properly were more than 10 times higher than the average
In the wells that were leaking, the researchers said:
“The estimated fugitive emissions from decommissioned wells are less than that for the agricultural activities that would take place on the reconstituted land.”
However, the researchers acknowledged that they looked only at vertical emissions from well integrity failure and not at the potential for diffuse leakage into surrounding groundwater or over a broad area.
They estimated emissions to be on average 364+/-677kg CO2 equivalent per well per year. The paper compares this with emissions for dairy cows and ewes.
“For dairy breeding herd the CH4 emissions factor (enteric and manure sources) is 128 kg CH4/head/year (2944 kg CO2eq/head/year) while for a breeding ewe the value is 8.9 kg CH4/head/year (205 kg CO2eq/head/year)”.
A DrillOrDrop reader has calculated that based on the figures in the paper the average leaking well could produce emissions just over 1/10th that of a single cow.
The researchers said wells did not appear to leak more as they got older:
“The relative CH4 [methane] concentration above wells did not significantly increase with the age of the well since drilling and 40% of the most recent wells surveyed showed leaks implying that leaks develop early in the post-production life of a decommissioned well.”
They said lower soil gas methane levels than the control indicated that soils on some decommissioned sites acted as a net methane sink.
Well integrity failure
The industry stresses the importance of well integrity to prevent contamination. At a meeting in North Yorkshire last Friday, Ken Cronin, chief executive of the industry organisation, UK Onshore Oil and Gas, told local councillors:
“The most fundamental and important thing about any oil or gas operation is the integrity of the well. If you get the design and creation of that well correct from the start then you reduce the environmental impact of fracking.”
Well integrity failure happens when barriers of cement or steel casings fail, allowing pathways for fluids and gases to groundwater, surface water and the atmosphere.
In today’s study, the researchers said:
“We interpret elevated soil gas CH4 concentrations to be the result of well integrity failure but do not know the source of the gas nor the route to the surface.”
“Oil and gas wells are typically constructed with multiple barriers to maintain well integrity and prevent leaks, thus well integrity failure is a consequence of complete barrier failure.”
“A loss of well integrity is important because it represents an uncontrolled release of fluids – whether liquid or gas – which could pose a risk to groundwater supplies and air quality.”
The importance of monitoring
Leaks of methane can have important consequences for climate change because the gas has a warming potential 28-36 greater than carbon dioxide over 100 years.
Another ReFine study in 2014 found only two confirmed cases of well integrity failure in the UK in 143 active onshore wells but it revealed that there was no monitoring of abandoned wells. Of these 65% were not visible, while 35% had some evidence at the surface of previous drilling activity.
One of the authors of today’s study, Professor Fred Worrall, told the BBC:
“The point is that even with proper decommissioning you will still have those wells that leak as cement cracks and steel corrodes and so monitoring is important,”.
“Overwhelmingly wells are properly decommissioned and our study shows that when methane does leak the levels are low, for example when compared to methane produced by the agricultural use of the land”.
Ken Cronin, of UKOOG, said:
“What ReFINE has shown is that the public should have no health or environmental concerns about emissions from properly decommissioned wells adhering to current industry standards.”
“Indeed the research has found that in the minority of cases where they have recorded some methane emissions from decommissioned wells, these emissions are typically less than one would get from just a handful of livestock grazing in the same fields.”
But Doug Parr, chief scientist of Greenpeace, said:
“If even an industry-funded study suggests that 30% of conventional wells appear to be leaking, it raises serious questions over the long-term impact of the extensive development of unconventional gas in the UK which is clearly the government’s plan.”
Updated 29/1/16 to include figures for methane emissions and comparison with livestock.
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