Permeability: Mineralisation
There are some differences between low permeability
and high permeability coal seams. These differences can be related to
the presence of some specific pore and cleat fillings such as mylonite,
the development of cleats and their mineralisation, and the mode of occurrence
of minerals in coal macerals. Successful drainage and a suitable rate
of gas flow through the coal can be influenced by coal microstructures,
especially micro-cleat openings and mineral matter. In good drainage
and high permeability coal seams, the micro-cleats are mostly empty,
or only partly mineralised.
In coal seams with negligible quantities of mineral matter, the coal-bed
gases flow and initially continue to flow when the pressure is lowered
below the desorption pressure. The coal matrix shrinkage as a result
of gas desorption may cause greater cleat openings than the effective
stress. Titheridge (2004) stated, "In mineralised coal, the presence
of calcite (or other minerals) in cleat or fractures adds an additional
factor to the initial and subsequent drainage process. Mineralisation
blocks cleat and fracture permeability routes that would otherwise transport
gas". Thus, an increased presence of mineral matter in coal would
cause a reduction in coal permeability, and the degree of permeability
will be proportional to the extent of mineralisation. Furthermore, mineral
matter impedes the gases from leaving their place by affecting the desorption
and shrinkage properties of the coal matrix.
Gamson, Beamish and Johnson (1993) study on Australian coals found that
the amount of fracture infilling with minerals was one of the factors
which influenced the effectiveness of methane flow through the coal matrix.
They also noted that mineral matter such as clay, calcite and quartz
block the methane flow path through cleats and interconnected pores by
forming a compact amorphous or crystalline structure. The size of infillings
influences gas diffusion as well as laminar flow in the coal matrix.
Later Gurba (2002b) described her microscopic studies of some Australian
coal samples and found that the Bulli seam had two different sets of
cleats. One set of
cleats is open and the other mineralised. Microscopic
studies on coal samples from West Cliff Colliery showed micro-cleats
totally mineralised by carbonates. Siderite nodules (Iron Carbonate)
in the cleats were observed to cause difficulty in drilling and in drainage.
Mylonite is also present in West Cliff coal samples, and mylonitic type
coal could be prone to outbursts (Gurba, 2002a). Additionally, microscopic
examination of the coal samples from difficult drainage areas has shown
that the presence of mylonite in micro-cleats is also likely to cause
difficulties in gas drainage. As revealed by electron microprobe analysis
the mylonite in micro-cleats is cemented by calcite, dolomite or kaolinite.
In the coal samples from Central Colliery in the Bowen Basin that were
collected from the low permeability area and outburst prone zone, the
cleats were totally filled with calcite. In Appin Colliery coals, carbonates
were present in the cleats as well as mylonite, which was cemented by
carbonates so that there was not much space for gas flow. Titheridge
(2004), who did extensive work on Tahmoor Colliery Bulli coal and its
calcite mineral matter, postulated that high fluid pressure was the major
factor responsible for the fibrous veins in coal (sedimentary rock).
He stated, "The origin of high fluid pressure was primarily due
to the fluctuating NE-SW tensional - compressive stress field that
was present during the burial phase of the Southern Sydney Basin".
Calcite in Tahmoor coal was formed from the combination of CO2 and water,
for which one of the CO2 resources was magmatic.
