Research and analysis

Integrity of decommissioned wells: studies using integrity-related factors and soil gas monitoring - summary

Published 17 December 2021

Applies to England

1. Chief Scientist’s Group report summary

This project used 2 approaches to investigate the long-term integrity of decommissioned onshore oil and gas wells in England. Firstly, factors related to integrity were used to categorise wells according to their long-term potential to stay sealed or to leak pollutants like methane (report SC190005/R1). Secondly, soil gas was monitored at 6 selected wells to check for any evidence of leaking methane, which might indicate integrity failure (report SC190005/R2).

1.1 Factors related to integrity

There are about 2,150 decommissioned wells in England that date back over the past 100 years or so. The Environment Agency is responsible for protecting the environment if the wells’ seals degrade and leak. In this study each well is assessed for its potential to leak based on 5 integrity-related factors, including the purpose of the well and the standards for construction and sealing. The factors are used to develop a scoring scheme that defines 6 ‘tiers’ of well integrity. Tier 1 contains situations with the greatest potential integrity (the lowest potential to leak) and tier 6 contains situations with the least potential integrity (the highest potential to leak).

About two-thirds of the 2,150 wells are in the lowest 3 tiers, where potential integrity is greater; specifically, there are 4%, 23% and 40% of wells in tiers 1, 2 and 3 respectively. The remaining third of wells are in tiers 4 and 5, which contain about 23% and 9%, respectively.

1.2 Soil gas monitoring

In 2015 an independent academic study measured soil gas at 102 decommissioned wells in England and identified a small number of wells with distinctly elevated concentrations of soil methane. One interpretation is that these may have indicated leaking methane due to well integrity failure. To check this, the present study re-monitored soil gas at 4 of the sites with elevated concentrations, and at 2 other sites. All 6 sites were selected from the tier of the scoring scheme that contained wells with the highest potential to leak; this was tier 5, as there were no wells in tier 6. The re-monitoring used a wider range of methods than the previous study.

The re-monitoring of methane did not identify any measurements above typical natural baseline concentrations at the ground surface or in shallow soils around the 6 decommissioned wells.

1.3 Conclusions

Our results show that decommissioned wells can be placed in tiers, based on their potential to leak methane. However, based on new soil gas measurements, 6 wells that were selected for having a high potential to leak did not show any evidence of leaking. The differences between the new and previous monitoring results suggest it would be useful to develop a protocol for more robust soil gas monitoring at decommissioned wells.

1.4 Publication details

This summary relates to information from project SC190005, reported in detail in the following outputs:

• Report: SC190005/R1 Title: Decommissioned wells: using factors associated with integrity to prioritise stewardship.
• Report: SC190005/R2 Title: Field assessment of decommissioned well integrity in England.

Project manager: Professor Roger Timmis, Lead Scientist for Air Quality and Radioactive Substances Research, Chief Scientist’s Group.

Research partner: Dr Aaron Cahill, Heriot Watt University.

The Environment Agency’s Chief Scientist’s Group provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible. This project was delivered in partnership with Heriot Watt University.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency