Prioritizing stewardship of decommissioned onshore oil and gas wells in the United Kingdom based on risk factors associated with potential long-term integrity

A portion of decommissioned oil and gas wells develop integrity failure resulting in release of methane, a potent greenhouse gas, into the surrounding soils and atmosphere. As the number of decommissioned wells grows during our transition to NetZero and technologies such as carbon capture and geological storage are implemented, it is essential that strategies for stewardship of this legacy subsurface infrastructure are developed. To formulate an abductive heuristic strategy for ongoing stewardship of onshore legacy wells in the United Kingdom (UK), we reviewed readily available data and identified five risk factors including regulatory frameworks, technologic aspects and construction characteristics, likely to influence long-term integrity. Subsequently we developed a prioritization methodology to segregate wells by ascending Tiers of decreasing potential long-term integrity. The prioritization method, which is supported by independent field observations, identifies 4% (n = 84), 23% (n = 501), 40% (n = 867), 23% (n = 497) and 9% (n = 200) of decommissioned wells in the UK as Tiers 1 (i.e., greatest relative integrity) to 5 respectively, while none were assigned as Tier 6 (i.e., lowest relative integrity). Tier 5 wells, which are generally characterized as production wells completed before 1953 and either deviated or completed during a year of intense drilling activity, are found clustered in several locations in England. Overall, we infer that not all decommissioned onshore wells in the UK are equally likely to suffer integrity failure. Consequently, they can be differentiated into groups of varying potential risk in an abductive heuristic manner using basic well data, thereby facilitating effective and efficient stewardship.