Well integrity failure in the UK

Abstract:

The aim of this study was to consider the potential legacy of increased onshore, unconventional gas production by examining the integrity of decommissioned, onshore, oil and gas wells in the UK. In the absence of a history of unconventional hydrocarbon exploitation in the UK, conventional onshore sites were considered and an examination of pollution incidents records had suggested that only a small fraction of operational, onshore wells could show integrity failures. A consideration of groundwater and surface water quality monitoring could find no regional impact of historic or current conventional oil and gas exploitation in the UK. As a more direct measure of well legacy this study considered the fugitive emissions of methane from former oil and gas wells onshore in the UK as a measure of well integrity. The survey considered 102 decommissioned (abandoned) wells from 4 different basins that were between 8 and 78 years old; all but one of these wells would be considered as having been decommissioned properly, i.e. wells cut, sealed and buried by soil cover to the extent that the well sites were being used for agriculture. For each well site the soil gas methane was analysed multiple times and assessed relative to a nearby control site of similar land-use and soil type. The results showed that of the 102 wells surveyed, 30% had soil gas CH₄ at the soil surface that was significantly greater than their respective control. Conversely, 39% of well sites had significant lower surface soil gas CH₄ concentrations than their respective control. 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. Where elevated CH₄ was detected it appears to have occurred within a decade of the well being decommissioned. The flux of CH₄ from wells was 364 ± 677 kg CO_2eq/well/yr with a 27% chance that any well would be a net sink of CH₄ independent of well age. This flux is low relative to the activity commonly used on decommissioned well sites (eg. sheep grazing). The problem of poorly, or inappropriately decommissioned wells and orphaned wells will also be considered.