Fracking is slang for hydraulic fracturing, the commonly used process of pumping fluid underground to release trapped oil or gas.
Fracking (or frac'ing as it's really known in the industry) is a technique that's been used around the world for decades.
It's used in the extraction of oil and gas, and also in some deep geothermal (renewable) energy wells, and works by creating a network of tiny fractures or hairline cracks in the target rock.
Combined with horizontal drilling, fracking has helped to revolutionise energy production by enabling the extraction of oil and gas from shale rock.
Shale rock is porous, like a sponge (capable of containing hydrocarbons like oil and gas) but mostly impermeable - which means that the hydrocarbons it contains aren't capable of flowing through it. For that to happen, the tiny pore spaces need to be connected together to create a pathway for the trapped oil or gas to escape, and fracking is the answer to that.
Water, sand and chemical additives that are non-hazardous to groundwater, are mixed together at the surface and then pumped underground into the shale rock.
Gravity and the weight of all the rock above mean that the shale is under pressure, and so the injected fluid has to be pumped in at a pressure that exceeds the breaking point of the surrounding rock. When this happens, it creates the network of hairline cracks as the fluid is forced inside and the pore spaces become laced together.
The sand that is added to the fracking fluid is used to prop open the hairline cracks while the gas or oil flows out. Without the sand, the cracks would close up too quickly.
The fact that single grains of sand are capable of holding open the fractures that are created shows just how tiny they are - the sand used in fracking typically has a grain size of 0.4 - 0.8 mm. For context, that's roughly 10 times the diameter of a human hair.
Terms like "fracture growth" and "fracture propagation" are used to describe the extent of the network of cracks that are created. The maximum vertical reach has been calculated as 350 metres from a horizontal well, but it's important to understand that these cracks are random, won't all form in a vertical plane, and are very narrow indeed - they are certainly not going to be "fissures."
Fracking for shale gas in the UK will take place at very significant depths - typically more than 2,000 metres. Groundwater aquifers are encountered at about 180 metres below the surface and so you can see that even if fractures can "grow" upwards 350 metres, there will be 1,650 metres of separation between the two (that's 5 times the height of the Shard in London, Britain's tallest building) and virtually no chance of any connectivity occurring.
A well is fracked multiple times in small stages. Cuadrilla, in Lancashire, proposes to conduct 45 frack stages in its test wells, each stage using 765 m3 of fracking fluid.
After the fracking fluid has been pumped underground, in order for the gas or oil to get to the surface, pumping stops and the well is depressurised - this creates a low pressure conduit into which the hydrocarbons will naturally flow, because they can't go anywhere else (they can only flow within and from the fracture network, given that the shale is so impermeable). Think of it a bit like a balloon filled with water almost to bursting point and then encased in concrete - there's nowhere for the water to go so it will sit there indefinitely, but make a hole through the concrete and then insert a drinking straw into the balloon, and the water will squirt up the straw under pressure.
Around half of what's pumped underground flows back as soon as the well is depressurised, along with the oil or gas. The remaining fracking fluid stays trapped in the pore spaces within the shale, where it has displaced a broadly equal volume of hydrocarbons. Gradually, over the producing life of the well, more and more of the fracking fluid will return to the surface for collection, and the rest will stay trapped in the pore spaces in the shale as the fracture network slowly closes under the pressure of the surrounding rock, unable to travel anywhere else.
Fracking certainly isn't the controversial technique that the BBC and others would like you to think it is.
For more on fracking and related topics, check out our Fracking Facts pages and our Fracking FAQ's too.