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. There are no commercially active sites in operation in the UK at the moment.
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 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 method. 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 surface. These chemicals make up less than 1% of the total fluid used.
Operators should disclose, either on their own website or on third-party websites, the chemicals used in their fracturing fluid. Only substances that have been assessed as being non-hazardous within the specified situation can be used. 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.