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Shale gas is natural gas trapped within shale rock formations underground. Advances in hydraulic fracturing or "fracking" – the process of forcing fluids at high pressure into wellbores to fracture the shale rock, allowing the gas to escape – encouraged an investment boom in north America.
That led to a big increase in production which brought US gas prices to a 10 year low in April 2012. The price of gas in North America remains far lower than in Europe or Asia.
US success in securing home-grown energy has attracted interest globally, with many other countries now seeking to exploit their own shale gas reserves.
Environmental activists have opposed the growth in shale gas, arguing that techniques like fracking can contaminate water resources and damage the environment.
Some have questioned the viability of shale gas, arguing that gas flows from such operations decline more quickly than from conventional gas wells, making the investment unprofitable in the long term.
Unlike traditional reservoirs, shale is a low-porosity rock from which oil and gas will not flow freely without additional stimulation from hydraulic fracturing or fracking.
Shale gas production was pioneered in the early 2000s by US independent companies such as Devon Energy and Chesapeake Energy.
FT video on the effects of shale gas on a rural US community