EPA revised its original proposal, released last year, in response to industry-submittted data indicating that equipment needed for compliance would not be widely available or cost-effective until 2015, she added.

SAIC's Kohlhaas says the equipment for green completions is used commonly in offshore drilling operations. He adds that it has not yet been widely adopted for onshore projects, although some companies in the Barnett shale area have used the technology for years.

Oil and gas industry officials say they think the final regulation is better than EPA's earlier proposal, which did not allow for a three-year transitional period. Howard Feldman, American Petroleum Institute director of regulatory and scientific affairs, says,  "EPA has made some improvements in the rules that allow our companies to continue reducing emissions while producing the oil and natural gas our country needs."

Environmental groups generally were pleased that EPA released a final rule, but some expressed dismay over the extended deadline.

Miriam Rotkin-Ellman, staff scientist at the Natural Resources Defense Council (NRDC), says, "The rapid expansion of oil and natural-gas drilling without modern air-pollution controls has exposed millions of Americans to a toxic brew of cancer-causing, smog-producing and climate-changing air pollutants. … Left to police itself for too long, the oil and gas industry has failed even to adopt pollution controls that pay for themselves."

NRDC says the EPA should set strong standards to curb leakage of methane and other pollutants from existing wells and operations. A NRDC report, released in March, suggested that oil and gas companies can reduce methane waste by 80% using available technlogies and still add $2 billion a year to the industry’s total profits.