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,