Petroleum Safety Authority Norway reports that wells which are left temporarily abandoned year after year represent a safety challenge on the NCS. The regulations have now been tightened up.
Exploration wells must be permanently plugged after no more than two years, according to new regulatory requirements which came into force this year.
Furthermore, production wells which are not being continuously monitored will now have to be plugged and abandoned within three years, or for longer providing monitoring continues.
“This change in the regulations has been prompted by a desire to improve well safety,” explains Johnny Gundersen, principal engineer in the PSA’s drilling and well technology discipline.
“Our audits of well safety management at the companies in 2010-11 indicated that the barrier status of some temporarily abandoned wells was not well enough known.
“We found, for example, that one well had been temporarily plugged for 30 years. Although this didn’t represent an immediate risk, such a duration can hardly be called ‘temporary’.”
A full overview of temporarily abandoned wells requested from the companies in 2011 identified 193 in 2012, including 17 where barriers failed to comply with the regulations.
The PSA asked the companies for an updated status in late 2013 and for a description of their future plans to permanently plug the abandoned wells.
“This latest follow-up has given us a clear impressions that the companies are taking the challenges of temporarily plugged wells seriously,” says Gundersen.
“Experience shows that the cost of permanent plugging rises the older the well becomes. The companies now appear to have become more conscious of this financial aspect.
“Combined with the regulatory requirements, this has given them more reasons to prioritise work on shutting down their abandoned wells permanently.
“That in turn means we now expect the environmental risk of the temporarily plugged wells to be substantially reduced.”
Drilling a well on the NCS is expensive, so deciding whether to leave it for good or preserve it for possible later use can be difficult for the operator. But operational considerations can also justify a temporary abandonment. Producer wells are often drilled before their production facility is put in place, for example.
An exploration well could be relevant for future injection, too, or a company could want to use it to observe developments in a producing reservoir.
The regulations require that a well left for good must be secured in such a way that it does not interfere with other activities and that hydrocarbons do not leak out into the sea.
This means that conductors must be cut off at least five metres below the seabed and non-degradable barriers put in place – in practice, cement plugs.
The latter must be positioned so that they prevent possible hydrocarbons from subsurface structures escaping to the sea or to other formations.
Once a well has been dealt with in this way, re-opening it will in practice be neither feasible nor economic. It also costs money for a company to keep a well in a temporarily abandoned state, and it must also be safeguarded against potential leaks.
However, the barriers used are of a type which can be removed relatively easily if the well is to be taken back into use. That means the well must be monitored to detect possible plug failure.
A subsea well which has been temporarily abandoned must also be equipped with a protective structure which prevents trawls and similar equipment getting entangled in the wellhead.