This recently published study (weblink) considered the fugitive emissions of methane from former oil and gas exploration and production wells drilled to exploit conventional hydrocarbon reservoirs onshore in the UK. This is important since fugitive emissions to the atmosphere are one of the environmental concerns surrounding the oil and gas industry. 

Davies et al. (2014) suggested that surveying soils above abandoned well sites would be an important step in establishing whether there was a loss of integrity and fluid migration following well abandonment. The aim of this study was therefore to assess whether abandoned, but properly decommissioned, wells represented an ongoing source of CH₄ to the atmosphere.

In most UK cases an abandoned well is defined as one that has been cut-off, sealed and then buried under ~ 2 m of soil. Well integrity failure occurs when cement and/or casing barriers fail, causing a loss of zonal isolation that creates pathways for the migration of fluids, including CH₄, to groundwater, surface water and the atmosphere.

The study selected from the 66% of all onshore wells in the UK which appeared to be properly decommissioned (abandoned) that came from 4 different basins and were between 8 and 79 years old. The soil gas above each well was analysed and assessed relative to a nearby control site of similar land use and soil type. The results showed that of the 102 wells considered 30% had soil gas CH₄ at the soil surface that was significantly greater than their respective control. Conversely, 39% of well sites had significantly lower surface soil gas CH₄ concentrations than their respective control. 

The authors interpret elevated soil gas CH₄ concentrations to be the result of well integrity failure, but  the source of the gas nor the route to the surface is known. Where elevated CH₄ was detected it appears to have occurred within a decade of it being drilled. The flux of CH4 from wells was 364 ± 677 kg CO₂eq/well/year with a 27% chance that the well would have a negative flux to the atmosphere independent of well age. This flux is low relative to the activity commonly used on decommissioned well sites (e.g. sheep grazing), however, fluxes from wells that have not been appropriately decommissioned would be expected to be higher.