|
Levine (1996) suggested that with gas sorption during the gas drainage
process, the coal matrix shrinks and causes permeability to significantly
increase. Different factors affecting this phenomenon include coal rank,
petrographic composition, mineral matter content and composition of gases.
Different coals exhibit different shrinking behaviour and it is crucial
to note that one of his significant results was that carbon dioxide causes
a greater degree of coal matrix change compared to methane. This is one
of the basic principles of carbon sequestration projects. Exposing coal
to CO2 causes different amounts of strain compared to methane
or helium, and this difference is attributable to the different sorption
capacities of the particular gases.
Levine (1996) also studied the mechanical strength of
coal, which was described as the ability to resist stress and change in
dimensions. The
figure opposite is the schematic representation of effective mechanical
forces model of a coal bed reservoir.
Harpalani and Chen (1997) studied the drainage of coal gas and its impact
on the coal matrix and permeability. Based on their test results, they
presented a relationship between changes in permeability and volumetric
strains in the coal matrix, which was described as:
 = change in permeability
 =
constant which depends on coal type and its characrestics
 =relative matrix
volume change
contd.. |
