The extensive use of fracking in the US, where it has revolutionised the energy industry, has prompted environmental concerns.
Fracking uses huge amounts of water, which must be transported to the fracking site, at significant environmental cost.
Environmentalists say potentially carcinogenic chemicals used may escape and contaminate groundwater around the fracking site. The industry suggests pollution incidents are the results of bad practice, rather than an inherently risky technique.
There are also worries that the fracking process can cause small earth tremors.
Fracking is the process of drilling down into the earth before a high-pressure water mixture is directed at the rock to release the gas inside. Water, sand and chemicals are injected into the rock at high pressure which allows the gas to flow out to the head of the well.
The process can be carried out vertically or, more commonly, by drilling horizontally to the rock layer and can create new pathways to release gas or can be used to extend existing channels.