Studies of Methane Leaks in UK Decommissioned Wells Contrast Methods and Results

A study of a limited number of decommissioned oil and gas wells in England found no evidence of methane leaks, including four wells found to be leaking in an earlier study.

In a press release, coauthor Aaron Cahill from Heriot-Watt's Lyell Center, a partnership between Heriot-Watt and the British Geological Survey, said the findings, published in the International Journal of Greenhouse Gas Control, should be "reassuring" to the public and regulators.

There are more than 2,000 onshore oil and gas wells in England, about 1,700 of which have been decommissioned. When decommissioned, the wells are plugged with cement, then cut and capped 2 m below the ground and buried. The land is returned to use for grazing or farming. They are not typically checked afterward for leakage, according to the press release.

A 2016 study by Durham University found that of the more than 100 decommissioned wells in England surveyed, 30% were leaking methane into their surrounding environment.

Cahill, assistant professor, School of Energy, Geoscience, Infrastructure, and Society, said none of the six wells examined in the 2022 study was leaking methane. Four of the wells found to be leaking in the 2016 study were revisited; no leaks were detected.

The 2016 study took measurements at the surface; the 2022 study drilled 1- to 2-m holes to get closer to the wellhead and used soil-gas samples and a flux chamber to detect methane and how much might be flowing into the soils and air.

How the Studies Differed

The 2016 study visited 102 wells, selected from the 66% of all onshore wells in the UK which appeared to be properly decommissioned (abandoned) that came from four different basins and were between 8 and 79 years old. The soil gas above each well was analyzed and assessed relative to a nearby control site of similar land use and soil type. The wells within each basin were chosen to give a range of conditions and to span the range of possible well ages to include the oldest as well as the youngest available.

Surficial methane concentrations from the air adjacent to the soil surface were measured using a portable tunable diode laser and sampling system positioned on top of the soil surface. At each well the air temperature, relative humidity, and air pressure were measured at the same time as the methane concentration using a Commeter C4141 digital thermo-hygro-barometer

The results showed that of the 102 wells considered 30% had soil-gas methane at the soil surface that was significantly greater than their respective control. 39% of wellsites had significantly lower surface soil-gas methane concentrations than their respective control. In contrast, 39 out of 102 wells showed significantly lower soil-gas methane than their respective controls indicating that soils on some decommissioned sites would act as a net methane sink.

The coauthors interpreted the elevated soil-gas methane concentrations to be the result of well integrity failure but added they did not know the source of the gas nor the route to the surface. The relative 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. Where elevated methane was detected, it appeared to have occurred within a decade of the well being drilled.

The 2022 study visited six onshore decommissioned wellsites in the East Midlands, England, including four of the wells found to be leaking in the 2016 survey.

The method of survey included surficial methane measurements, hand-auger drilling, subsurface soil-gas and sediment sampling (with subsequent laboratory testing), and the measurement of methane to atmosphere with a dynamic flux chamber. Soil properties were examined that would influence fugitive gas migration away from the structure and determine the expression of gas seen at surface.

While no direct measurements of methane efflux to atmosphere were attained, Fick's First Law of Diffusion was used to estimate the flux of methane from the soil surface at each tested site.

Field measurements of surface and soil-gas methane and efflux of methane to atmosphere at the six wellsites were close to atmospheric levels and/or within typical natural ranges: about 2 ppm (i.e., mean atmospheric concentration) or up to 0.0007 µmol CH₄/m²/s² for mineral soils and up to 0.0274 µmol CH₄/m²/s² for organic-rich soils. Only one sampling location produced soil-gas samples which slightly exceeded baseline atmospheric methane levels (i.e., a measurement of 6 ppm of methane observed in the field, confirmed at about 10 ppm in the laboratory).

The authors noted such soil-gas methane concentrations of 6–10 ppm with an estimated flux of 1.8E-4 µmol CH₄/m²/s) are still close to or within natural soil-gas concentrations and emission ranges, where reported concentrations and fluxes can reach up to 0.0063 µmol/m²/s. They concluded the marginally elevated samples from one location are likely to be a result of natural organic soil sources, land use such as animal grazing, and/or a temporal concentration spike and “is likely of no significance with respect to decommissioned well integrity.”

The differences in the two studies were explained by the coauthors of the 2022 study as due to:

• The spatiotemporal variability associated with subsurface fugitive-gas migration.
• False positives associated with land use and/or limitations of the methods previously used.
• The six wells studied were in low-permeability clayey surficial soils, which could act as capillary flow barriers and limit, if not totally inhibit, any gas migration from the wellbore toward the surface and the atmosphere. Therefore, using shallow or minimally intrusive methods may fail to detect leakage.

For future studies, the authors suggest analyzing a larger number of wells with greater durations and scales of monitoring to obtain more-conclusive data.

Constraining Well Integrity and Propensity for Fugitive Gas Migration in Surficial Soils at Onshore Decommissioned Oil and Gas Well Sites in England by P.S. Gonzalez Samanoa and A.G. Cahill, Heriot-Watt University; R.Timmis, Environment Agency; and A. Busch, Heriot-Watt University.

Fugitive Emissions of Methane From Abandoned, Decommissioned Oil and Gas Wells by I.M. Boothroyd, S. Almond, S.M. Qassim, and F. Worrall, Durham University; and R.J. Davies, Newcastle University.