IN WESTERN AUSTRALIA - Department of Mines and Petroleum

34
PWA April Edition - Coal Seam Methane
Darren Ferdinando
Research Geologist, Resources Branch
Coal seam methane (CSM; also known as coal
bed methane - CBM, and coal seam gas - CSG),
is a naturally occurring hydrocarbon that is
generated and reservoired within coal seams. The
methane gas is generated either through biogenic
activity in near-surface coals or through
thermogenic activity for deeper coal bodies. The
generated methane is held within the coal by
burial and hydrostatic pressure in a process
known as adsorption.
Coal seam methane over the last ten years has
become a significant source of sales gas across
eastern Australia. In Queensland for example, nearly
80% of the petroleum wells drilled last year were
for CSM operations. Total Queensland CSM
production in 2002/03 is estimated at 25 PJ, which
equates to almost 25% of Queensland’s current gas
demand - this is an increase from 2 PJ in 1998 and
11 PJ in 2001. In New South Wales, CSM
operations accounted for all the petroleum wells
drilled in the State. In 1999/00 the gross value of
CSM operations in NSW was reported by the NSW
Department of Mineral Resources at $18.56 million
and the value of production is expected to increase
to increase by at least 5% annually for the next ten
years, with a new CSM operation well at Camden
due to be commissioned soon. By 2020 it is
estimated that CSM will account for 100 PJ/annum
of energy production in eastern Australia, with
Queensland accounting for 60% of this and New
South Wales the remaining 40%. ABARE has
estimated a 3% annual growth in natural gas
consumption until 2020, increasing from 521 PJ
(17% of Australia’s total energy consumption) to
974 PJ in 2020 equating to nearly 20% of
Australia’s energy consumption. CSM operations
have the potential to contribute to an increasing
proportion of this natural gas consumption.
Coal Seam Methane - what’s the gas?
At present there are no commercial CSM operations
in Western Australia; however, the level of CSM
exploration in the State has increased over the last
two years. In response to this, the Petroleum and
Royalties Division of DoIR is commencing a study
into the CSM potential of Western Australia to assist
explorers in this field find appropriate acreage in the
State. This article intends to provide a broad
overview of CSM operations and the regions of the
State that may be prospective for CSM.
Differences between conventional gas and CSM
All the gas currently produced in Western Australia
comes from ‘conventional’ gas plays where gas has
been generated at depth in organic-rich claystones
or shales and migrated along permeable rock beds
into an area that has effectively trapped the gas; for
example, through sealing the permeable rock
against impermeable rock by faulting; or in creation
of domal structures through folding of the
permeable rocks that the gas is then trapped in.
Coal seam methane operations involve extracting
methane gas from subsurface coal accumulations.
While it is possible to extract methane from shallow
coals, such as those mined from the Collie coalfields
in the southwest of Western Australia, the production
rates from these deposits tend to be noncommercial.
For optimal methane production rates,
coal seams generally need to be at a depth of
between 500 to 1200 m. The maximum depth of
burial for coal seam methane production (at
commercial rates) appears to be about 1200 to 1500
m, although some wells produce methane at greater
depths. The minimum depth of burial is about 200 to
300 m, depending on the sealing efficiency of the
overburden. These are depths at which coal mining is
uneconomic – thus these two uses of coal for energy
are not competing for the same resource.
Coal has an extremely large internal surface area
due to its enormous micropore surface area, and as
such it can store surprisingly large volumes of
methane-rich gas; six or seven times as much gas
as a conventional natural gas reservoir of equal rock
volume can hold.
One of the greatest advantages associated with the
coal seam methane resource relative to
conventional gas is that the size and extent of the
coal deposits, along with the gas content of the
coal, can be estimated with a reasonable degree of
accuracy before major investments are made.
Technical issues for CSM exploration and
production
The technical considerations in determining whether
a CSM prospect will be commercially viable are
quite different than those used in making such
determination for conventional gas prospects. Key
factors that affect gas flow rates in coal seam
methane projects include the absorption properties
of the gas, presence of fracturing and permeability.
Biogenic vs thermogenic methane
Coal seam methane can be generated through two
distinct mechanisms. The first is generation of
methane through thermogenic breakdown of the
carbon-rich material in the coal due to burial of the
coal seam. Thermogenic methane generation
usually occurs at depths of greater than 300 m. The
second mechanism for methane generation from
coal seams relies on methane being generated from
bacteria found within the coal at generally shallow
depths (up to 500 m). Research has indicated that
biological methane generation can be very rapid in
low-rank coals such as those found in Western
Australia. This biological gas generation is so rapid
that it is technically feasible to generate usable gas