'Orphan' oil wells warning for fracking
Plans to expand shale gas "fracking" in the UK must learn from leaks and poor monitoring at existing onshore oil and gas sites, scientists say.
A review of 2,152 wells drilled from 1902-2013 found up to 100 "orphaned" wells for which no firm is responsible.
Only two cases of well "failure" were recorded, but legacy sites are not monitored for leaks, the authors note.
The study led by ReFINE (Researching Fracking in Europe) is published in the journal Marine and Petroleum Geology.
It is perhaps the most comprehensive review yet of Britain's inland oil and gas legacy - pulling together scientific papers, government reports, and industry data.
The long-term stability of wells and the risk of pollution are key considerations if the coalition pushes ahead with plans to expand the shale gas industry as a "driver" for the UK economy.
Lead author Prof Richard Davies, of Durham University and ReFINE, said: "Going forward, we know that shale gas exploration will require a lot of wells if it goes ahead, and it's important we make proper provision [for safety].
"This review is a starting point. We've collated a huge bank of information.
"The findings confirm that well barrier failure and well integrity failure is an issue and that publicly available data in Europe on this seems to be sparse.
"Data from the monitoring of active wells and periodic surveys of abandoned wells would help assess the impact of shale exploitation.
"It is important that the public should have access to this information."
The use of hydraulic fracturing (fracking) to recover shale gas has raised fears of earthquakes and contamination of drinking water by chemicals pumped into wells.
But scientists say there is another pollution risk - from structural failure of the well casing - and this risk is not particular to shale gas. It is common to all hydrocarbon drilling sites.
To investigate the risk of leakages, the ReFINE researchers reviewed the publicly available data on 143 actively producing onshore wells.
Between 2000 and 2013, nine pollution incidents were recorded involving the release of crude oil within 1km of an oil or gas well.
Two of these were well integrity failures, which occurred at Singleton oil field, in West Sussex, when the cement casing around the well was breached.
However, the researchers found no data for more than 2,000 inactive wells, many of which have been "abandoned" (sealed and covered) and in many cases, "orphaned".
Orphaned wells are those where the operator has either gone out of business or is insolvent, meaning the company that operated it is no longer responsible for it.
The researchers found between 50 and 100 wells in the UK without any obvious parent company.
"If those wells were to leak, the question is who is responsible? We don't know - it's not clear," Prof Davies told BBC News.
"We've exposed a legal issue that someone needs to investigate. A thorough look would be fruitful."
ReFINE was launched in 2013 with the aim of providing unbiased scientific research into the risks of shale gas exploitation in the UK and Europe.
The consortium is funded by the Natural Environment Research Council, Shell, Total and Chevron, with the Environment Agency and DECC in an advisory-stakeholder role.
The review compared the UK to worldwide data on more than four million wells - in countries such as Australia, Bahrain, Brazil, Canada, Poland, and the US.
Datasets from eight countries published between 1989 and 2013 revealed highly variable well barrier and well integrity failure rates of 1.9%-75%.
For example, of 8,030 wells targeting the Marcellus shale inspected in Pennsylvania between 2005 and 2013, 6.3% were reported to the authorities for infringements related to well barrier or integrity failure.
Rob Jackson, professor of environmental science at Duke University, and co-author on the study, said:
"The data show enormous variation in the number of wells with integrity issues. Figuring out why - what conditions and practices work best in the field - is really important.
"As a society, we're not good at long-term thinking. We need to plan now for keeping track of wells and making sure there is enough money to plug and monitor them for at least 50 years."
The report's recommendations were welcomed by the UK Onshore Operators Group (UKOOG), the representative body for the UK onshore oil and gas industry.
A UKOOG statement said: "The report highlights the very low rate of issues with respect to operational wells in the UK and this underlines the effectiveness of the current regulatory regime.
"Well integrity is a key plank of our regulatory focus and the report highlights the need to monitor wells during and post operations.
"We are in the process of agreeing with government monitoring procedures and post abandonment procedures for shale wells as recommended."
Ken Cronin, chief executive of UKOOG, added: "It is important to note that ReFINE's research focuses on historical records and studies.
"The industry and its practices are constantly improving with experience and technology as required by regulation."