WHAT IS FRACKING?

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 is carried out vertically or, more commonly, by drilling horizontally to the rock layer. The process can create new pathways to release gas or can be used to extend existing channels.

WHY IS IT CALLED FRACKING?

It is shorthand for hydraulic fracturing and refers to how the rock is fractured apart by the high pressure mixture.

WHY THE CONTROVERSY?

The extensive use of fracking in the US, where it has revolutionised the energy industry, has prompted environmental concerns.

The first is that fracking uses huge amounts of water that must be transported to the fracking site, at significant environmental cost. The second is the worry that potentially carcinogenic chemicals used may escape and contaminate groundwater around the fracking site.

There are also worries that the fracking process can cause small earth tremors.

Environmental campaigners say that fracking is simply distracting energy firms and governments from investing in renewable sources of energy, and encouraging continued reliance on fossil fuels.

WHAT ARE THE ADVANTAGES OF FRACKING?

Fracking allows drilling firms to access difficult-to-reach resources of oil and gas. In the US it boosted domestic oil production and driven down gas prices. It is estimated to have offered gas security to the US and Canada for about 100 years, and has presented an opportunity to generate electricity at half the CO2 emissions of coal.

The industry suggests fracking of shale gas could contribute significantly to the UK's future energy needs. A report by the Energy and Climate Change Committee in April said shale gas in the UK may help to secure energy supplies, but may not bring down gas prices.