IN WESTERN AUSTRALIA - Department of Mines and Petroleum

y introducing appropriate organisms into suitable
coals. This process of in-situ biogasification is at the
conceptual stage at present, but has important longterm
implications. Further refinement of this process
may be provided by the use of genetic engineering
to optimise the organisms for gas generation.
There is some difference of opinion regarding the
nature of biogenically generated gas in coal seams
- some authorities consider that there is little, if any,
adsorbed gas, and that the gas is present in a ‘free’
form in the cleats (small fractures in the coal) and
other openings, as well as in formation waters. It is
probable that both biogenic and thermogenic
scenarios embrace adsorbed, ‘free’ and dissolved
gas, but in different proportions. In both scenarios,
the retention of gas is almost totally dependant
upon hydrostatic pressure and commercial
production, thus involving the production of
significant water to lower the hydrostatic pressure to
permit the gas to flow.
Coal quality
Increasing ash content causes coal strength to
increase, thereby decreasing the potential for
fracturing/cleating. Coals with lesser amounts of ash
are, therefore, the most likely to have the greatest
cleat development - and thus the highest
permeabilities. Also, as the ash component of coal
cannot absorb methane, it reduces the volume of
gas that can be contained in a unit volume of coal.
The maximum ash content before a coal becomes
non-commercial has not been determined, but
probably varies according to other parameters such
as maturation level. Additionally, the cleat density is
greater in the brighter coal types - such as vitrain
and bright clarain and substantially less in the dull
coals - like durain.
The process of gas flow from the solid coal to the
cleats is one of diffusion. Usually the cleats are filled
with water and the desorption of gas within the cleats
leads to the two phases existing within the cleats. If a
secondary major fracture system exists, flow may
then take place from the cleats to the major fractures
(Figure 1). Both the cleats and major fractures exhibit
their own phase dependant permeabilities.
Diagenesis may cause deposition of mineral matter
in the coal cleats, significantly reducing coal
permeability. Carbonate infilling is the most
common form of diagenesis in coals, but silica,
pyrite, illite, smectite, kaolinite and other clays have
also been observed as cleat infillings. Prospective
gas coals should thus be relatively free of such
cleat-filling substances.
The gas content of any prospective coal should be
greater than 8.5 cc/g (300 scf/t). The coals also
need to reach a certain level of thermal maturity in
order to generate gas and to produce the structure
and chemistry necessary for storing commercial
quantities of methane within the coal. The vitrinite
reflectance should be in the range of R O of 0.7% to
R O of 2.0%. In general, the higher the maturity, the
greater the adsorption capability of any coal. From
looking at coal seam methane operations in the US,
where the industry is approaching a mature stage, it
appears that the minimum thickness of coal
required to produce commercial quantities of gas is
5 to 6 metres in no more than 3 or 4 seams.
The gas content of coal is usually determined by
gas desorption procedures, which means gas is
desorbed from coal by placing a sample of the coal
(usually from drillcore) in a sealed container, and
measuring the amount of released gas over periods
which may range from days to months. This
procedure requires the gas to be desorbed from the
coal’s micropore structure (ie the thermogenic
scenario); however, the predominantly ‘free’ gas of
the biogenic scenario (or a significant component of
it) may not be detected by this desorption method
and, hence, the resulting low to non-existent gas
contents may give a quite false portrayal of gas
producibility from a formation.
Pressure and permeability
In thermogenic situations, gas is adsorbed onto the
coal’s micropore surfaces and held in place by the
reservoir (water) pressure. The methane within a
coal seam is released when the hydrostatic
pressure is reduced, allowing the cleats in the coal
to expand, increasing permeability and commencing
the process of desorption of the methane gas from
the coal. Most coals show a significant relationship
between effective stress (total stress minus
hydrostatic pressure) and permeability. The cleats
being closed by increasing effective stress cause a
reduction in permeability. If fluid pressure is high, it
tends to open the cleats and as pressure decreases
with fluid withdrawal the cleats close. The reduction
in permeability may be of an order of magnitude for
anything between 2 and 10 MPa of increasing
effective stress. The softer the coal the more
pronounced this effect is.
As water and gas are produced from the seam, the
effective stress increases leading generally to a
reduction in permeability. Many coals, however,
exhibit an increase in permeability with production.
Figure 1. Flow through cleats in coal
PWA April Edition - Coal Seam Methane 35
This is caused by an effect that tends to de-stress
the seam. This de-stressing is a result of the fact
that most coals shrink as gas is desorbed. The
shrinkage reduces the lateral stress on the seam
and shifts that stress into the surrounding rocks.
These two opposing effects on the effective stress
mean that the permeability of the seam may either
decrease or increase with the removal of gas and
water from the seam, depending on the
characteristics of the coal and associated gas.
Frequently both these effects are present, with an
initial permeability decrease in the reservoir
pressure around the producing well, followed by an
increase as significant desorption-induced
shrinkage occurs within the coal.
Depending on the nature of the coal and depth of
burial, this release of methane varies from negligible
gas flow to commercial rates of gas flow, although
in all cases a significant amount of time is required
to dewater the coal bed before any gas is
recovered. Timing of water handling is one of the
major differences between CSM and convention
gas. With conventional gas, gas is trapped under
pressure and overlies water. This pressure is then
used to allow the gas to flow to surface through the
well casing until the rising water level in the trap
increases to the point where the amount of gas
produced relative to water becomes uneconomic
because the rate of gas production is so low. In
CSM operations, the water, which is holding the gas
to the coal through hydrostatic pressure, is drained
first and as the water pressure (measured in terms
of hydrostatic pressure on the gasfield) decreases,
adsorbed gas is released from the coal seam and
then produced through the well bore over a long
period (Figure 2). Over time, the rate of gas
desorption decreases until the flow rates become
uneconomic to continue to produce from the bore.
The water, which is commonly saline but in some
areas can be potable, must be disposed of in an
environmentally acceptable manner. Surface
disposal of large volumes of potable water can
affect streams and other habitats, and subsurface
reinjection makes production more costly.