"For comparison, well stimulation offshore typically uses 2 percent of the liquids and 7 percent of the sand that is used routinely for onshore hydraulic fracturing," the BSEE said in a statement.
Oil industry estimates show that at least half of the chemical-laced water used in fracking remains in the environment after an operation. Environmental groups say as much as 80 percent of the fluids can be left behind. The rest gets pumped back up to the oil platform and is piped or barged back to shore for treatment. Companies also can pump the fluids into an old well reservoir to discard it.
DCOR, which did not respond to requests for comment, is not the first company to try to tap more oil from California's offshore reserves, nor is the project the most extensive offshore frack here in recent years.
In January 2010, oil and gas company Venoco Inc. set out to improve the production of one of its old wells with what federal drilling records show was the largest offshore fracking operation attempted in federal waters off California's coast. The target: the Monterey Shale, a vast formation that extends from California's Central Valley farmlands to offshore and could ultimately comprise two-thirds of the nation's shale oil reserves.
Six fracks were completed in the project, during which engineers funneled a mix of about 300,000 pounds of fracking fluids, sand and seawater 4,500 feet beneath the seabed, according to BSEE documents.
Venoco's attempt only mildly increased production, according to the documents.
Despite greenlighting offshore fracking projects for years, federal and state regulators now are trying to learn more about the extent of fracking in the Pacific even as officials and marine scientists scramble to weigh the possible environmental effects.
In January, Jaron Ming, the Pacific regional director of the BSEE, told employees in an email that there had been heightened interest in offshore fracking from within the agency and the public.
"For that reason, I am asking you to pay close attention to any [drilling applications] that we receive and let me know if you believe any of them would be considered a 'frac job.'"
That same month, BSEE estimated in internal emails that only two such jobs had occurred off California in the past two decades. Weeks later, though, as the agency worked to respond to public requests about fracking offshore, emails show that it had found 12 such instances of offshore fracking.
BSEE said it cannot be sure how often fracking has been allowed without going through every single well file.
Brian Segee, a staff attorney at the Environmental Defense Center, said the uncertainty makes him skeptical about the actual number of offshore fracks. The Santa Barbara-based environmental law firm, which formed in the wake of the 1969 oil spill, is calling for a moratorium on future fracking in the Pacific until potential environmental effects are studied.
Most fracking efforts off California have yielded mixed results. The first time Venoco fracked offshore in the 1990s, it had limited success. Chevron's one try failed. Out of Nuevo Energy's nine attempts, only one was considered very successful, according to company and BSEE records.
The practice has been more fruitful in the North Sea and the Gulf of Mexico, where it's more common and the porous nature of the geologic formation makes it easier to extract oil, according to regulators and oil industry experts. Still, oil companies surveyed by federal regulators said they haven't ruled out fracking projects in the Pacific in the future.
As fracking technology evolves and companies seek to wring production from old offshore wells, drilling experts caution that strict safety precautions and planning are needed.
Working in the open ocean, "you have to be a lot more careful to avoid any spillage," said Mukul Sharma, a professor of petroleum engineering at The University of Texas.
David Pritchard, a Texas petroleum engineer who has been working in offshore drilling for 45 years, said offshore fracking "no doubt adds complexity and risk."
One concern is that the high-pressure fracking mixture in some jobs might break the rock seal around an old well bore, allowing oil to escape, added another expert, Tulane University petroleum engineering professor Eric Smith.
"I'd say it [offshore fracking] is safe," Smith said, "but nothing's a sure thing in this world."