A Royal Dutch Shell Plc-operated carbon capture and storage project in Canada has hit a milestone of sequestering 4 million tons of carbon dioxide about six months ahead of schedule and at a lower cost than estimated, helped by better-than-expected reliability.

The Quest facility, which sequesters emissions from the Scotford Upgrader near Edmonton, Alberta, started up in November 2015 and has since run ahead of its target of capturing 1 million tons of carbon a year, said Anne Halladay, a...