(Bloomberg) -- Call it the invisible spill.

You can’t see it, but it’s there—a steady stream of natural gasseeping out of the pipe casing in a well in Southern Californiathat may spew as much greenhouse gas into the air as a half-millioncars do in a year. Pipeline operator Sempra Energy says itmay take three to four months to plug.

Using the same tactic that eventually ended the giant 2010 oilspill in the Gulf of Mexico, the company is boring a well that’llintercept the damaged one to stop the seepage. Meanwhile, it’smoving hundreds of people into temporary housing and faces as muchas $900 million in costs including relocation and legal expenses,based on Bloomberg Intelligence and government data estimates.The 8,700-foot-deep (2,650-meter) well has leaked 800,000metric tons of gases contributing to global warming over the firstmonth, about a quarter of California emissions by state estimatesover the same period.

“You have this huge volcano of methane pollution and except forthe fact that a lot of people are getting sick, it hasn’tengendered a lot of attention,” said Tim O’Connor, California oiland gas director for the advocacy group Environmental Defense Fund. “When you think about anoil spill, you have birds and fish that are damaged. Here you havefuture generations, children and people across the world that arefeeling the effects.”

Spill costs

The $900 million cost estimate is based on a worst-case scenariothat assumes maximum injuries and includes relocation, response,litigation and the loss of natural gas expenses, according to theanalysis by Bloomberg Intelligence analyst Brandon Barnes and datafrom state officials. It doesn’t include potential fines andpenalties.

Javier Mendoza, a spokesman for Sempra’s SouthernCalifornia Gas Co. utility, said the company has “no idea howan estimate of that amount could be reached.”

“It is much too early to speculate on any financial implicationsfrom the situation at the Aliso Canyon facility,” he said bye-mail. “We will not be speculating on ‘what if’ scenarios.” Thecompany has insurance for its storage business, he said.

Sempra fell as much as 4.7% on Monday. Shares were down 2.7% to$95.00 at the close in New York.

Much as with the crude spill from BP Plc’s Macondo well fiveyears ago, regulators have struggled to get a handle on themagnitude of the leak blowing gas into the Porter Ranchneighborhood. Sempra won’t be able to tell how much gas has escapeduntil the flow’s been stopped, Mendoza said.

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Sempra and its Southern California Gas Co. unit face at leasttwo lawsuits by Porter Ranch residents who claim to have beensickened by the leaked gas. The city of Los Angeles filed suitagainst Southern California Gas over the incident, according to ane-mail statement Monday from city attorney Mike Feuer.

"The only environmental effect is the additional greenhousegas," said George Hirasaki, a Rice University engineering professorand former Shell Oil Co. official who was involved in theresponse to an oil spill off Louisiana’s coast in the early1970’s. “But it is not going to dirty the environment the way anoil spill would.”

Rapid expansion

The incident highlights the risks associated with the rapidtransformation of America’s natural gas pipeline network. Cheapsupplies surging out of shale formations in the past decade arealtering pipe flows and increasing the country’s dependence on theheating and power-plant fuel.

Workers for Southern California Gas discovered the leak on Oct.23 at a well in the mountainous Aliso Canyon storage field, about 30 miles (48kilometers) northwest of Los Angeles. Efforts to stop the flow ofgas by pumping fluids straight into the well have failed. Theutility started drilling the intercepting well on Dec. 4, thecompany said in a statement on Monday. The utility continues towithdraw gas from the storage facility to serve customers, thestatement said.

Southern California Gas CEO Dennis Arriola has apologized andsaid in a letter to Porter Ranch residents that the gas utility isdoing everything it can to fix the leak. The county healthdepartment said those exposed to it may experience short-termsymptoms including nausea, coughing and headaches.

Southern California Gas is trying to determine the best way tocap emissions from the site, the utility said in its statement.

60,000 kilograms of natural gas leaking perhour

As much as 60,000 kilograms of natural gas an hour has beenleaking, according to David Clegern, a spokesman for the California AirResources Board, a state agency that regulates emissions. Thattranslates to about 800,000 metric tons of greenhouse gas that hasalready seeped out of the well, he said.

About $12.2 million worth of gas may be lost should it takeSempra three months to fix the leak, said James Williams, presidentof energy consultancy WTRG Economics, basing his estimates on thestate data and natural gas prices last week. The leak hasn’taffected regional gas or power markets, said Chris DaCosta, aBoston-based analyst for energy data provider Genscape Inc.

A handful of state agencies are coordinating a joint response tothe leak, said Don Drysdale, a spokesman for California’s Division ofOil, Gas and Geothermal Resources. There will probably be athird-party investigation once it’s plugged, he said.

"Right now,” Drysdale said, “the emphasis is on getting thisthing fixed.”

--With assistance from Edvard Pettersson.

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