HOUSTON — Only five years ago, several giant natural gas import terminals were built to satisfy the energy needs of a country hungry for fuels. But the billion-dollar terminals were obsolete even before the concrete was dry as an unexpected drilling boom in new shale fields from Pennsylvania to Texas produced a glut of cheap domestic natural gas.

Now, the same companies that had such high hopes for imports are proposing to salvage those white elephants by spending billions more to convert them into terminals to export some of the nation’s extra gas to Asia and Europe, where gas is roughly triple the American price.

Just like last time, some of the costly ventures could turn out to be poor investments.

Countries around the world are importing drilling expertise and equipment in hopes of cracking open their own gas reserves through the same techniques of hydraulic fracturing and horizontal drilling that unleashed shale gas production in the United States. Demand for American gas — which would be shipped in a condensed form called liquefied natural gas, or L.N.G. — could easily taper off by the time the new export terminals really get going, some energy specialists say.

“It will be easier to export the technology for extracting shale gas than exporting actual gas,” said Jay Hakes, former administrator of the Energy Department’s Energy Information Administration. “I know the pitch about our price differentials will justify the high costs of L.N.G. We will see. Gas by pipeline is a good deal. L.N.G.? Not so clear.”