Shell waterflooding in Gulf of Mexico

Shell said injecting filtered and treated sea water into the reservoirs to displace additional oil is expected to extend the life of the Usa-Princess field by 10 years, and it expects to continue waterflooding the site for 30 years.

The first water was injected Thursday, it said.

The area, 100 miles (160 kilometers) off the mouth of the Mississippi River, has produced more than 400 million barrels since Ursa went into production in 1999. The Princess field, which is linked to the same reservoir, was opened in 2003.

"We believe potential exists for continued development of material oil and natural gas within this basin," Shell Vice President Russ Ford said in a statement.

Ursa is in water 3,800 feet (1,160 meters) deep, while the Princess field lies under 3,650 feet (1,100 meters) of water.

Shell said the topside injection system pumps water through two separate flow lines to three underwater sites, two existing well sites and a new site northeast of the Ursa platform.

Shell operates the project with a 45.4 percent stake. Its partners are BP Exploration and Production Inc., Exxon Mobil Corp. and ConacoPhillips Co.