Outgoing Shell chief executive Peter Voser says in an interview with the Financial Times his biggest regret during his time at the company is the failure of the company’s huge bet on US shale gas.

The FT says Shell has invested at least $24bn in so-called unconventional oil and gas in North America. But the investment has yet to pay off and in August Shell said it would carry out a ‘strategic review’ of its US shale activities.

‘Unconventionals did not exactly play out as planned,’ Voser is quoted as saying.

He also admitted exploration results in US shale beds had been disappointing. ‘We expected higher flow rates and therefore more scalability for a company like Shell,’ he said.

Speaking to the paper three months before he is due to step down, Voser also described the technical setbacks Shell has suffered in its exploration campaign off the coast of Alaska as one of his greatest disappointments in the job.

Voser said he did not know if Shell would go back into Alaska in 2014 or 2015.