The project receiving Royal Dutch Shell's largest foreign investment, the Pearl super-clean fuels plant in Qatar, is tough but on schedule, a top Shell executive said on Tuesday.

"It's a very challenging project, but so far so good," Linda Cook, Shell's executive director for gas and power, told reporters on the sidelines of a conference.

Cook reiterated Shell's guidance on the start date for the plant at the end of this decade.

Qatari officials have said the plant, which converts gas to liquid fuels, would start in late 2010.

Spiralling costs have taken the price for Pearl to $18 billion, from an initial budget of $5 billion. It will be the world's largest gas-to-liquids plant, with capacity of 140,000 barrels per day. - Reuters