PG&E closes gas storage field in delta after finding leaks

PG&E installing a natural gas pipeline in case the main pipeline breaks in a catastrophe. PG&E / Courtesy to The Chronicle Ran on: 06-23-2007 PG&E is installing a natural gas pipeline north of the Empire Cut river channel on McDonald Island, in case the main line breaks. less PG&E installing a natural gas pipeline in case the main pipeline breaks in a catastrophe. PG&E / Courtesy to The Chronicle Ran on: 06-23-2007 PG&E is installing a natural gas pipeline north of the Empire ... more Photo: PG&E, Courtesy To The Chronicle Photo: PG&E, Courtesy To The Chronicle Image 1 of / 1 Caption Close PG&E closes gas storage field in delta after finding leaks 1 / 1 Back to Gallery

Pacific Gas and Electric Co. has temporarily closed down its largest natural gas storage field, located beneath an island in the delta, after finding small leaks of the flammable fuel.

The move comes amid heightened concern about the safety of California’s underground gas storage fields following an immense, four-month leak just north of Los Angeles that forced the evacuation of a nearby neighborhood.

The leaks spotted by PG&E at its McDonald Island facility in the Sacramento-San Joaquin River Delta appear to be far smaller.

According to the state’s oil and gas regulatory agency, the leaks range from 236 to 763 kilograms of gas per hour. Southern California’s massive Aliso Canyon leak, in contrast, peaked at more than 60,000 kilograms per hour.

In a letter to PG&E, Kenneth Harris, supervisor of California’s Division of Oil, Gas and Geothermal Resources, called the amount of gas leaking from McDonald Island “similar to or only slightly above background levels at natural gas storage facilities.”

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But PG&E has had trouble isolating the exact sources of the leaks and closed the McDonald Island facility as a result, said division spokesman Donald Drysdale.

“It’s very small, but it’s tricky — I don’t think they’ve found the source yet,” Drysdale said. “So out of an abundance of caution, they shut it down.”

The storage field, which includes 87 wells on a man-made island, lies several miles from the nearest town. According to PG&E, the leaks pose no danger to public health or the environment.

Utilities use underground rock formations — most often, depleted natural gas fields that have ceased production — to store large quantities of the fuel. The McDonald Island facility can supply nearly 25 percent of Northern California’s daily demand for natural gas, according to PG&E.

The company has enough gas in storage elsewhere to meet its customers’ needs while McDonald Island is closed, said PG&E spokesman Greg Snapper. The utility plans to resume gas withdrawals and injections at the field by September.

“Since we’re able to balance the gas system with supply available today, we don’t expect any change in costs for our customers,” Snapper said.

The Aliso Canyon storage field, run by Southern California Gas Co., remains closed, although the company hopes to return some of its wells to service this summer.

In response to the Aliso Canyon leak, which erupted in October and took until February to stop, California adopted emergency regulations requiring expanded inspections of gas storage fields.

The leaks at McDonald Island were first noticed by inspectors on the ground, then measured in greater precision by aircraft equipped with methane detection gear. The Division of Oil, Gas and Geothermal Resources told PG&E in late June to test all the facility’s wells for leaks.

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF