It is an existential question in France: When is fracking not fracking? The country is pushing ahead with plans to harness geothermal energy from smouldering rock deep in the Earth's crust using drilling methods the oil industry says are like hydraulic fracturing, or fracking, which France outlawed in 2011.
Environment Minister Delphine Batho awarded two geothermal exploration licences in February and said 18 more were in review. Some will tap into volcanic heat by permeating rock in a process called "stimulation" that blasts acid and water into fissures to release heat. That may be seen as similar to how US explorers fracture fossil fuels from shale rock with chemical cocktails.
France's socialist-led government grew enthusiastic about generating power from underground heat reservoirs as President Francois Hollande pledged to lower dependence on atomic power. The country banned fracking for its "serious health and environmental risks", and cancelled shale exploration licences held by Total SA, its biggest oil company, and US-based Schuepbach Energy.
However, the French oil industry lobby disagrees.
"Granting geothermal exploration permits is creating a double standard," said Jean-Louis Schilansky, head of Union Francaise des Industries Petrolieres, which is pushing for a review of French shale energy restrictions. "The drilling methods are similar to fracking."
On Thursday, Pierre-Marie Abadie, who heads the energy division in the ministry, told an oil conference in Paris that geothermal fracking "would continue to be allowed. The law hasn't banned it."
At the heart of the dispute is whether French government authorisation for the "stimulation" of rock fissures with water and acid to access geothermal heat is comparable with oil and gas fracking, in which shale is shattered using high-pressure water, sand and chemicals to release hydrocarbons.
France "is fracking for geothermal," executive director of the International Energy Agency Maria van der Hoeven said at the conference. She called on France to review its stance on fracking in light of its "high import bills" for natural gas.
French geothermal permits granted so far are in the Massif Central mountains to Electerre de France SAS, a company backed by investor Charles Beigbeder, and in the Pyrenees to Groupe Fonroche Energie, which installs solar panels.
French geothermal backers say drilling methods are not the same as for shale energy.
The projects "won't require fracking," Elsa Demangeon, project manager at the renewable energy lobby Syndicat des Energies Renouvelables, said. "Some rock stimulation may be needed at the start but this is to reopen existing fissures."
She acknowledged projects were being approved despite a current "legal void" on the difference.
The world had about 11,000 megawatts of installed geothermal power-generating capacity in 2012, more than a quarter of which is in the US, according to a market update published by the Washington-based Geothermal Energy Association. Italy has more than half of Europe's capacity.
In France, a European research project that began in 1987 at Soultz-sous-Forets in Alsace remains the mainland's only deep geothermal site. With a power-generating capacity of 1.5 megawatts, it relied on "mini hydraulic fracturing" and rock "stimulation" for its development, according to a report by French mining authority Bureau de Recherches Geologiques et Minieres, a government advisory group that oversees the installation.
Now, as the country seeks geothermal energy on a bigger scale, it will allow for geology to be "stimulated".
The projects "won't use fracking in the strict sense of the term because there won't be fracturing," head of BRGM's geothermal division Romain Vernier said. "Hydraulic stimulation at the onset will reopen existing fissures blocked by mineral deposits and then it won't have to be repeated."
Acid will be used in geothermal rock cracks as opposed to the chemicals and sand used by the oil and gas industry, while hydraulic pressure will be lower, according to Mr Vernier. Nevertheless, this "stimulation" could create seismic events like one caused by the Soultz-sous-Forets project, which had a magnitude of 2.9 and was felt on the ground, he said.
A project in Basel, Switzerland, was shut down in 2006 after causing an earthquake of magnitude 3.4.
The hazards posed by the type of drilling required for the French geothermal projects are "likely to be lower than for gas fracking," said Stuart Haszeldine, a geologist at the University of Edinburgh. "There still are risks."
Rock that is as many as six kilometres underground is under "critical stress" that could be perturbed by acid or cooling, creating tremors or cracks, he said. Pressure on borehole casings could also cause leaks and water contamination.