Unconventional gas and how hydraulic fracturing (fracking) works

Unconventional gas - shale gas and coal bed methane

What is unconventional gas?

The term ‘unconventional gas’ refers to natural gas which is trapped in deep underground rocks which is hard to reach, such as shale rock or coal beds. ‘Conventional’ gas fields are usually situated in easier to reach layers of rock.

Until recently unconventional gas reserves haven’t been exploited because the cost was too high or technology wasn’t available. Technological advances mean it could be economically viable to extract methane from these sources. Exploration of unconventional gases is at a very early stage in the UK.

Unconventional gas is:

• Shale gas
• Coal bed methane
• Underground coal gasification

The different sources of unconventional gas mean the type of gases extracted will vary. Shale gas and coal bed gas are mainly methane, like conventional natural gas. Underground coal gasification produces a mix that can include hydrogen, carbon monoxide and methane.

Shale gas

Shale gas is a natural gas extracted directly from shale. The gas is held in fractures, pore spaces and adsorbed on to the organic material of shale.
The most common way to extract shale gas is by cracking the rock using hydraulic fracturing, this is often referred to as 'fracking'. Fracturing fluid, a combination of water and chemicals, is pumped at high pressure into the rock to create narrow fractures that allow the gas to flow into the well bore and to the surface.

Once the fractures have been created, small particles, usually sand, are pumped into the fractures to keep them open when the water is taken back up the well.

Coal bed methane (CBM)

CBM is extracted by releasing pressure in coal seams, usually by natural gas production or by pumping water from the coal bed. As the pressure is reduced, gas is released and can be extracted.

Underground coal gasification (UCG)

Underground coal gasification and extraction involves the partial burning of coal underground to produce a mixture of gases.

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 A couple of diagrams to explain the process.

And a diagram illustrating just some of the problems!

Sources Environment Agency, British Geological Survey, Phys.org,