This answer was kindly provided by Dr James Verdon, geophysicist at the Bristol University:
“The technology of hydraulic fracturing was developed in the 1940s. Approximately 200 conventional wells in the UK have been fracked. However, like all technologies, it has improved through time. Modern fracking uses more powerful pumps and higher fluid volumes than the original frack-jobs performed in 1940s Kansas.
“A second key technology, which has received less attention, is horizontal drilling. The ability to turn the drill horizontally to drill sideways through a shale formation is just as crucial to shale gas extraction as fracking. This horizontal drilling was developed in the late 1980s, and the Wytch Farm oil field underneath Poole Harbour (Dorset) was a pioneer in this technology.
“Geologists have known for a long time that many shale formations contain huge amounts of gas. However, shale is also impermeable: fluids and/or gas can’t easily flow through it. As a result, it was always thought that it would never be possible to get the gas out economically. A recent House of Lords inquiry found that “There was no commercial interest in shale gas or oil because there was no technology to exploit it effectively.”1 However, in the early 2000s a Texan pioneer, George Mitchell, began experimenting to combine horizontal drilling with fracking in the Barnett Shale formation, Texas. He proved that by combining these methods you could extract gas from shale rocks commercially. The shale gas revolution was borne, and is now spreading around the world. It has also been helped by the gradual decline of many conventional gas fields, reducing the supply and increasing price, encouraging people to look for alternative gas supplies.”
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Other factors which have made shale gas viable include: increases in energy prices; the reduction of gas produced in the North Sea which has made the UK a net importer of gas; the need to reduce CO2 emissions since electricity produced from coal creates more harmful CO2 than electricity produced by gas, and the recent conflicts in the Middle East and Ukraine, which have led to further pressure for this country to develop its own cost-effective energy sources.