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A pump jack draws oil from the ground near a hydraulic fracturing operation near Bowden, Alta., Tuesday, Feb. 14, 2012.Jeff McIntosh/The Globe and Mail

With debate over hydraulic fracturing or "fracking" running at a fever pitch, it seems the only thing everyone can agree on is that, for better or worse, there is plenty of natural gas down there for the taking.

Now a provocative analysis of unconventional fuel reserves in the United States aims to slap a big question mark over that assumption. In a comprehensive look at all the major shale gas plays currently being tapped across the U.S., the study focuses on the rapid decline of individual gas wells, along with entire fields, and concludes that optimistic projections for a long-running boom that will unleash cheap gas for decades to come are unwarranted.

"The hype around shale gas is just that," said David Hughes, a geologist and former research manager with the Geological Survey of Canada who authored the study, which was release on Tuesday.

Dr. Hughes is not exactly reporting from neutral ground. The study was sponsored by the Post Carbon Institute, a California-based think tank that promotes sustainable energy. Yet, Dr. Hughes says, the numbers are there in black and white, drawn from a widely used industry database.

It's not that shale gas isn't abundant, Dr. Hughes says, but that fracking only releases gas from a relatively small volume of rock. And apart from a few "sweet spots," much of the overall reserve is of relatively marginal quality. Once the sweet spots are tapped out, continuing to get gas out of the ground at the current rate is going to require a vastly ramped-up effort with attendant environmental costs. The likely outcome, he says, is a decline in production and a rise in price, which means that expectations that gas will replace coal as an economically viable, cleaner burning alternative, or enable the U.S. to become a net exporter of liquid natural gas, are off base.

"Sometimes people get caught up in talking about the size of a resource when what they really need to be thinking about is the net energy yield," Dr. Hughes said.

Starting around 2006, U.S. shale gas production began to climb steeply, thanks to innovations in horizontal drilling and fracking – a technology that uses high-pressure fluids to crack open the shale, allowing gas to seep out.

Shale gas now accounts for roughly 40 per cent of domestic supply in the U.S. – about 9.7 trillion cubic feet per year. A projection released in December by the Energy Information Administration, a branch of the U.S. government, suggests that fraction will grow to 50 per cent of what the country produces by 2040.

But Dr. Hughes say those projections are too rosy, and fail to take proper account of the fact that most wells fall to 15 per cent of initial productivity after just four to six years. Maintaining supply in the face of such decline will require much more fracking, at the rate of more than 70,000 new wells per year by 2040, to meet the EIA projection. Increased costs and growing concerns about environmental impact could make such a level of development untenable.

The assessment is sure to meet with plenty of resistance. Daniel Whitten, a spokesman for America's Natural Gas Alliance, based in Washington, said Dr. Hughes`conclusions run counter to several government and academic studies, "all of which forecast robust supplies of natural gas for many generations to come." Andrew Miall, a professor of geology at the University of Toronto who provided an independent review of Dr. Hughes' report, said that while it may represent a minority opinion, it is a "clear-eyed look" at the geological underpinnings of the shale gas industry that deserves wider notice. "It shows that this isn`t a resource that flows freely," Dr. Miall said.

Dan Arthur, president and chief engineer for ALL Consulting in Tulsa, Okla., which recently completed a report on shale gas for the Petroleum Technology Alliance of Canada, notes that Dr. Hughes is not the first expert to have raised questions about the longevity of the gas boom. However, Mr. Arthur still forecasts a robust industry going forward, buoyed in part by further technical innovation. "I figure I`m going to be busy for quite a while," he said.