He explained: “On 25 February, we observed irregular pressure in the annuli on the plugged G4 well on the Elgin field. We very quickly moved to kill the annuli pressure by pumping it full with high density mud. During this process, on 25 March, we observed a sudden pressure increase followed by an escape of mud and then gas. This was released below the well head at deck level on the platform, not subsea.
“The leaking hydrocarbons are believed to be coming from a rock formation above [at a depth of 4,000m below the sea bed]. This tight formation at high pressure is not a producing reservoir in the Elgin field; however, it contains a non-toxic gas which could have migrated to the annuli [the well casing].”
He continued: “This is a gas leak which is happening in annuli which has no seal by design. I would say that the rupture of something in the well has made possible the release of gas in that annuli. It is the last annuli between the casing for a 30in conductor pipe and 20in conductor pipe.”
The well at the centre of the leak is one of ten wells in the Elgin complex. Three of the wells have been plugged and abandoned. The well involved in the leak had been plugged a year ago, but had not been officially abandoned.
Mr Guys said: “In the G4 well, the main producing reservoir situated at around 5,500m depth has been plugged for more than one year.
“From our observations, we believe there has been little change over the five days since the incident and the leak remains ongoing.
“And while we cannot make a direct measurement of the leakage rate, based on recorded data and reservoir modelling, we estimate it to be around 2kg per second [about 200,000 cubic metres of gas per day].
Mr Guys also stressed that there was no evidence that the uncontrolled release of gas had been caused by “human failure”.