Jim Wilson/The New York Times
Oil fields in the Permian Basin of Texas and New Mexico, an Apache Energy site where wells were drilled in February last year.
Drilling for shale gas through hydraulic fracturing, or fracking, appears to cause smaller leaks of the greenhouse gas methane than the federal government had estimated, and considerably smaller than some critics of shale gas had feared, according to a peer-reviewed study released on Monday.
The study, conducted by the University of Texas and sponsored by the Environmental Defense Fund and nine petroleum companies, bolsters the contention by advocates of fracking — and some environmental groups as well — that shale gas is cleaner and better than coal, at least until more renewable-energy sources are developed. More than 500 wells were analyzed.
The Texas study concluded that while the total amount of escaped methane from shale-gas operations was substantial — more than one million tons annually — it was probably less than the Environmental Protection Agency estimated in 2011.
In particular, it indicated that containment measures captured 99 percent of methane that escaped from new wells being prepared for production, a process known as completion.
The Environmental Protection Agency has begun to require drillers to control leaks during completions, which are believed to be one of the major sources of methane losses at fracking wells. Although controls will not be required until January 2015, a number of companies already capture escaped gases at wells being prepared for production.
"Can we control it? Thanks to new E.P.A. regulations coming online, the answer to that is good news," Eric Pooley, a senior vice president at the Environmental Defense Fund, said in an interview.
The report was published Monday in The Proceedings of the National Academy of Sciences. With the study, which ran from May through December of last year, the university was the first to conduct detailed examinations of individual drilling sites. It did so with the consent of petroleum companies, which provided about 90 percent of the financing for the study. Previous E.P.A. estimates relied on engineering calculations, and other studies gathered data via aircraft flights over drilling sites.
The study's connection to the petroleum industry — among its sponsors and financiers are Shell, Anadarko Petroleum Corporation, Exxon Mobil and Chevron — may lead some to question its objectivity, some outside experts said. But most said the research and the reputations of the researchers appear solid.
"Previous studies that have gotten a lot of attention have had red flags jumping out all over them. This one didn't," said Michael A. Levi, the director of the program on energy security and climate change at the Council on Foreign Relations. In an e-mailed statement, Shell's president, Marvin Odum, called the study "a prime example of key groups — that may not have the exact same interests — working collaboratively and taking a science-based approach" to the methane problem.
Mr. Odum said that collecting actual emissions data, rather than relying on estimates, would "ensure that both improvement efforts and regulatory changes can be focused on the areas that will have the biggest impact."
The report comes at a time when shale-gas drilling is growing at a breakneck pace — production, now 30 percent of all United States natural gas, is expected to reach 50 percent by 2040 — but also when the industry is beset by controversy.
Citizen groups accuse drillers of despoiling streams and water supplies, and suppliers of wrecking local roads with parades of massive trucks. Environmental groups have split sharply over whether to support shale-gas development as a cleaner fuel that will suffice until wind and solar power assume a greater share of the nation's energy supply.
The Texas study is the most comprehensive look to date at a contentious issue in the debate over fracking: the extent to which methane leaks during drilling and production offset the environmental benefits of the clean-burning natural gas the wells produce.
When burned as fuel, methane — the main component of natural gas — is comparatively clean, producing less carbon dioxide than coal and oil. But vented to the air, it is a short-lived but extremely potent greenhouse gas, trapping heat at a much higher rate than carbon dioxide, the main greenhouse gas.
Such potency means that even a small loss of methane to the air — just 3.4 percent, scientists say — can negate its climate-changing advantage over coal.
Some previous studies suggested that as much as 7.9 percent of all shale gas was lost to the atmosphere, a figure that included every stage of the gas process, from well to stove or furnace burner.
The Texas study did not examine potential losses outside the drilling pad. But its conclusion that losses at the well were comparatively low could remove one issue from the debate. The Environmental Defense Fund is leading other research efforts that should address losses elsewhere, including a look at methane leaks in municipal gas-distribution lines.
The American Petroleum Institute hailed the study's conclusions, saying in a statement that its own efforts to reduce methane emissions "are paying off." But while E.P.A.-mandated measures appear to have reduced emissions during well completions, the study also concluded that leaks elsewhere in the fracking process were higher than the E.P.A. had previously estimated.
Estimates of leaks from chemical pumps, while small, were twice past estimates, while leaks from pneumatic controllers, or valves, were pegged at more than 639,000 tons a year, roughly a third greater. None of those components are currently subject to federal regulation.
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