Gas extraction from shale requires hydraulic fracturing of the rock, known as fracking, to release the gas.
The fracking process injects a mixture of water, sand and chemicals into the rock. This requires very high pressure and happens deep below the Earth's surface.
This opens up cracks and fractures within the rock, releasing the gas. The sand grains keep the cracks open.
Each gas well uses a vertical drillhole into the shale rock, with a horizontal well drilled off the main vertical shaft. Each horizontal well is then ‘fracked’ and the gas then flows to the surface.
Figure 3 shows a geological cross section schematic of a shale gas well.