IN WESTERN AUSTRALIA - Department of Mines and Petroleum

36
PWA April Edition - Coal Seam Methane
Other considerations
Figure 2. Rate of methane production against produced water over time,
water production shown in blue, methane in orange.
The processing of CSM is in many cases simpler
than that required of conventional gas. Conventional
gas can have a highly variable composition, from
‘dry’ (where the dominant hydrocarbon in the gas is
methane) to ‘wet’ (where the heavier hydrocarbons
such as butane, ethane and pentane are present in
the gas in addition to methane). The ‘wet’ gases
need to be stripped of the heavier hydrocarbons
(which are sold separately as condensate). In
addition, extraneous gases such as nitrogen, carbon
dioxide and hydrogen sulphide may be present in
the gas and these must be removed during
processing, as well as water vapour, which must be
stripped from the gas. Once the extraneous
components of the conventional gas have been
removed, the gas is then compressed and piped to
users. In CSM, the gas is predominantly methane,
and impurities such as carbon dioxide are generally
localised and comprise a small portion of the gas.
The major steps for processing of coal seam gas
include removal of water vapour from the gas and
compression of the gas to pipeline pressure
conditions. Gas plants for CSM operations therefore
tend to be less complex than those used to process
conventional gas.
To assist the flow of gas from the coal to the
producing well, the coal is hydraulically fractured.
This is accomplished by pumping large volumes of
water and sand at high rates down the well and into
the coal seam. This operation either produces new
fractures or forces the pre-existing cracks and
fractures in the coal seam to enlarge and extend.
The fractures begin at the well bore and then
extend for distances of up to several hundred
metres away from the well. Fractures deep in the
coal seam are less than one centimetre wide, and
have no effect on the ground surface.
Producing coal seams must be isolated from flowing
aquifers in order that they may be successfully
dewatered. This is necessary because coal seam
methane is produced from a seam after the water is
allowed to flow in the well, producing a lowered
pressure regime suitable for desorption of the methane
from the coal. Paradoxically, too high a permeability in
the coal can allow too much water flow, thus inhibiting
the methane desorption and recovery.
In recent years there has been a growing increase
in interest by both the coal industry and the
petroleum industry in CSM resources. There are
considerable commercial advantages offered by
methane, that has been drained from coal seams,
as an energy resource. These advantages include a
relatively low “finding cost”, and, often a convenient
location close to major markets.
Potential sources of CSM in WA
Perth Basin
Cattamarra Coal Measures
Jurassic coals found in the northern Perth Basin in
1961 on the western flank of the Dandaragan
Trough show potential for CSM production. The
extent of high quality coal is estimated at 800 km 2
from Eneabba down to Wongonderrah Spring. A
number of CRAE exploration programmes have
covered the near-surface area of the coals and will
form the basis of future studies by DoIR on the
potential for CSM generation from these coals.
Irwin River Coal Measures
Low-grade Lower Permian coals are found in the
northern Perth Basin, and were the first coal beds to
be recorded in Western Australia in 1846. The coal
measures extend of over 600 km 2 and CRAE
undertook a systematic study of the region during
the late 1980s and early 1990s. Coal resources for
the Irwin River Coal Measures are estimated at
1180 Mt of inferred in situ coal. The coals have an
ash content of 11 to 31%, are sub-bituminous and
generally dull. At a depth of 145 m, the vitrinite
reflectance ranges from 0.5 to 0.6 and the coals
have a medium to low sulphur content.
Sue Coal Measures
Low-moderate grade Lower Permian coals of the
Sue Coal Measures are found in the southern Perth
Basin in the graben between the Darling and
Dunsborough Faults. Up to 17 coal seams with a
maximum recorded seam thickness of 5.5 m are
present, varying in rank from lignitic to bituminous,
with the most economically attractive seams near
the base of the Permian strata.
Collie Basin
The Collie Basin is a sedimentary outlier on the
southwest Yilgarn Craton and hosts Western
Australia’s most significant coalfield. The coal
sequence has a maximum thickness of 700 m and
individual seams vary in thickness, with a maximum
of 13 m recorded. The vitrinite reflectance of coals
mined from the Collie coalfields is approximately
0.65 at 130 m.
Canning Basin
Coal-bearing areas of the Canning Basin are mainly
confined to the Fitzroy Trough and occur in Upper
Permian sediments. The coal is thin and sparse, and
the resource appears to be limited and highly
faulted. Upper Jurassic coals have also been
recorded in the Canning Basin in petroleum wells,
but no specific details on these coals are known at
present.
Carnarvon Basin
Thin Permian coals have been recorded from the
southern Carnarvon Basin, notably within the Byro
and Merlinleigh Sub-basins and along the
Wandagee Ridge. Limited data is available on these
coals, and comes from both petroleum well data
and mineral exploration programmes.
Bremer and Eucla Basins
Cainozoic brown coals are found within the Bremer
and Eucla Basins and the deposits cover a
significant area of the basins including
contemporaneous drainage channels. The coal
generally occurs as a single seam up to 12 m thick
within the Werillup Formation. Exploration drilling of
these coals has occurred on a limited basis, and the
coal has higher ash, salt and sulphur content than
brown coals from Victoria.
Conclusion
The capacity of coal to hold gas is dependent on
porosity, which in turn is related to coal quality, gas
composition and gas pressure. A general
relationship has been established that adsorptive
capacity (and therefore potential total gas content)
increases with rank of the coal and with depth.
Permeability of coal decreases with depth.
Within Western Australia, there does appear to be
some potential for coal seam methane production,
generally confined to the southwestern portion of
the State in the northern and southern Perth Basin.
The subsurface extents of the Lower Permian Irwin
River and Sue Coal Measures and the Lower
Jurassic Cattamarra Coal Measures, where the coal
seams lie between 300 and 1200 m are the areas
that at this stage of the Department’s CSM
prospectivity study appear to show the most
potential. DoIR