overview of possible impacts from coal seam gas
development in Northern Rivers, New South Wales
Project by Elfian Schieren, 2012
3 Coal seam gas extraction process
Functionally coal seam gas is the same as
natural gas which is methane, formed from the decay of organic matter in
anaerobic (low oxygen) environments.
Coal seam gas differs from conventional gas
in extraction method and geology.
Conventional gas is generally found in
sandstone or limestone in pockets or reservoirs from which the gas flows easily
once tapped (Rutovitz et al, 2011). Coal seam gas is found within subterranean
coal seams of hard coal such as anthracite to soft lignite or bituminous coals.
Coal seams are less permeable and require
more invasive processes to extract the methane (Zupanick, 2001).
The general processes include drilling and
dewatering initially and hydraulic fracturing to increase permeability when
these processes are not adequate to create gas flow.
- Drilling and dewatering
Gas wells are drilled using a system of
enclosed fire retardant fluids to reduce friction and keep the drill from
setting the coal seam or other geological layers on fire (Spehe, 1999).
The methane is held tightly in the coal
seam in a saturated environment and requires a dewatering process before the gas
can be released (Rutovitz et al, 2011).
The coal seam is a highly saturated
environment and gas wells often produce water or other liquids such as oil or
These liquids have the potential to inhibit
gas flow from the well and have to be removed via pumping mechanisms (Evans,
The water removed from the coal seam is
called “produced water” and can contain potentially hazardous compounds such as
salts and chemicals so has to be treated and disposed of accordingly (Rutovitz
et al, 2011).
- Hydraulic Fracturing
If the dewatering of the coal seam is
unable to release the gas it is necessary to fracture the seam to increase
Hydraulic fracturing or fraccing involves
pumping large volumes of fraccing fluid at high pressure into the coal seam to
create a fracturing that can extend to around 400m into the coal seam (Puri et
Fraccing fluid is often but not always
water and contains proppants and chemicals to increase fracturing capacity.
Gel like fluids are considered more
effective at transporting the proppants and gel compounds are added to a water
base to thicken it.
Most fraccing fluids contain a mixture of
bactericides, pH control and anti-solidifying chemicals (NTN, 2011).
Very small amounts of the chemical are used
in the fluids but huge volumes of fluid are required to complete the fracturing
process which can result in large amounts of chemicals needing to be disposed of
(Rutovitz et al, 2011).
This method is not always desirable
particularly when the coal bed is thin or near a submerged aquifer as the
fractures can permit transfer of water either into or out of the well bore (Puri
et al, 1991).
- Produced Water
The process of dewatering and fraccing
brings water to the surface that contains chemical characteristics drilling and
fraccing fluids but also of the formation and hydrocarbon being extracted.
This water, known as produced water,
contains large amounts of salts, oils and grease, various inorganic and organic
compounds or chemical additives and naturally occurring radioactive material.
It is estimated that over 71 billion
barrels (1 barrel = 158.76 L) of produced water is generated annually around the
globe from oil and gas production which then needs to be treated and disposed of
or utilised for other purposes (Veil et al., 2004).
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