Shell Canada says the Quest carbon capture and storage project north of Edmonton has reached the milestone of four million tonnes of stored carbon dioxide, equivalent to the annual emissions of about one million cars.

It says the accomplishment is ahead of schedule and has been attained at a lower cost than expected.

Quest opened in 2015 and cost about $1.35 billion, backed with $745 million from the Alberta government and $120 million from Ottawa.

The project was sold to Calgary-based Canadian Natural Resources Ltd. in 2017, along with most of Shell's Alberta oilsands assets, but is still operated by Shell.

The facility captures and stores underground about one-third of the CO2 emissions from the Shell-operated Scotford Upgrader, which turns oilsands bitumen into synthetic crude that can be refined into fuel and other products.

Shell says Quest has stored more carbon dioxide than any other similar project in the world and is doing it at a higher annual rate.

"Quest continues to show the world that carbon capture and storage is working, its costs are coming down and that Canadians are leaders in CCS," said Shell Canada president Michael Crothers, estimating that building and operating a similar project now would cost 20 to 30 per cent less because of lessons learned at Quest.