Image caption The plant in Louisiana was built to receive imports, but will now reverse the process

US gas produced by the controversial technique of "fracking" is due to be exported for the first time.

A $20bn project to prepare an export terminal is under way in Louisiana.

The huge facility on the Gulf of Mexico was originally designed to import natural gas to the US.

But within two years of opening, the owners decided to reverse the process.

In that time, American shale gas has become abundant and relatively cheap.

One of the first contracts will see shale gas shipped to Britain under a contract with Centrica.

Media playback is unsupported on your device Media caption The BBC's David Shukman explains how fracking works

The decision to sell gas abroad illustrates the extraordinary scale of the shale revolution in the United States with a further huge expansion forecast.

A combination of new seismic imaging techniques, the ability to drill horizontally and the process of fracturing the shale rock itself has transformed the energy scene in the United States.

The plant at Sabine Pass, owned by Cheniere Energy, was constructed to receive shipments of liquefied natural gas and a handful of deliveries was made.

But now the company, banking on growing global demand for cheap American gas, is investing in four massive systems, known as "trains", to liquefy gas ready for export.

When completed, the terminal is planned to export nearly 20 million tonnes of LNG a year.

The first shipments are scheduled for 2015.

According to Shell, one of the major producers of shale gas in the US, prospects have gone through a surprisingly rapid transformation.

Peter Brett, manager of Shell's onshore well operations in the US, told BBC News that there is "massive potential".

Future Fuel Future Fuel is a series looking at the changes in energy demand and use

"It's huge - just five years ago we were talking about importing LNG and bringing that in from overseas and now we're looking at self sufficiency for the next 100 years in natural gas.

"We're taking a long term view - there's going to be an ever increasing energy demand, we're going to need all energy sources and shale gas will be a big part of that."

For years, reservoirs of oil and gas in upper layers of rock have been extracted but the layer of shale below - the "source rock" in which the hydrocarbons were formed - was deemed too difficult to exploit.

One major new prospect - or "play", as the industry calls new fields - is the Eagle Ford shale in southern Texas.

Near the Mexican border at Carrizo Springs, this arid scrubland is now the scene of frenetic activity as several major energy companies have moved in.

At dozens of sites, drilling rigs are used to reach at least a mile deep into the shale, and then at least a mile horizontally through the rock formation. Typically, a dozen wells can be drilled from the same site.

The fracking process involves a fleet of trucks carrying pumps to drive a combination of water, chemicals and "proppants" - tiny grains like sand - under high pressure into the rock.

This fractures the shale, forcing open tiny fissures which the proppants then hold open, allowing the gas to flow out.

The technique is controversial for several reasons.

First, there are concerns about the risk of earth tremors but these have been extremely rare despite more than a million "frack jobs" in the US.

Second, as the well passes through water-bearing layers of rock, there is a risk of the contamination of drinking water supplies.

A double barrier - a cement lining of the well and a steel pipe - is designed to prevent any gas reaching aquifers.

John Bickley, who manages technology for Shell's shale operations, said that if drilling and fracking are properly handled, the "possibility of contamination is very small".

If methane is found in drinking water, there are other explanations for how it might have got there, he said.

"There could be old well bores that leaked in the past. You could have methane that naturally migrates up."

He conceded that the likelihood of shale gas being exploited in other countries beyond the US partly depends on public reaction.

"Until we get people to understand that we're doing this in an environmentally friendly manner that could delay or even cancel operations in different countries."

In the big country of south Texas, with a sparse population, the process is inevitably less controversial than in more crowded areas.

In central California, near the town of Shafter, we found that fracking operations had taken place almost on the doorstep of a retired businessman, Walt Desatoff.

"They drilled just across street - they had three giant generators, they just started up. There was lighting, 24/7 activity, a huge waste pit that was of concern

"It was major inconvenience - the activity, the smells, the dust from the trucks and the noise.

"They did allocate us some money for a hotel - we intermittently left for a few days when we couldn't take it."

Image caption Industry says that if drilling and fracking are properly handled the risk of contamination is low

The fracking operation close to Mr Desatoff's home was to produce oil not gas but the technique is the same. We approached the company involved but it said it never commented on individual cases.

For environmentalists, a major concern is the quality of regulation and the ability of the authorities to enforce it.

Jen Powis of the Sierra Club said there was insufficient control over the dumping of "water laced with chemicals", emissions from compressor stations and the impact of pipeline development.

She also said that the plan to export US shale gas carried risks.

"There are 17 proposals to export natural gas and no one has looked at them as a whole.

"What about the pipelines and the wetlands and the ships coming in and out? What about the water with this increased traffic? And to get to that export, what about the risks upstream?"

But for the moment, a shale gas boom, made possible by fracking, is under way in the US with every prospect of it growing for decades to come.

Watch David Shukman's report on BBC One's BBC News at Ten on Tuesday, 16 July at 22:00 BST (UK only).