f) Studies in 2000 - present:
St. George and Barakat (2001) in their experiment using sub-bituminous
coal demonstrated how desorption of coal gas affected the coal matrix and
effective stress. The experimental set up was similar to the Harpalani
and Chen (1995) apparatus. Tests were conducted on cylindrical, 54 mm diameter
core coal samples and in four different gas environments. Gases used included
CO2, CH4, N2, and He. They found coal
swelling due to sorption of carbon dioxide was about 12 times greater than
for nitrogen and 8 times more than for methane. They also mentioned that
the swelling due helium was negligible. They postulated that
the strength characteristics of coal could be affected by compressive strains
due to gas pressure reduction and coal matrix shrinkage. Also, in the presence
of carbon dioxide, the coal underwent an initial contraction for a period
of less than 45 seconds due to the hydrostatic pressure, which was then
followed by expansion due to gas sorption. However, based on the experimental
studies, this initial shrinkage time period was found to be shorter than
that reported by St. George and Barakat (2001).
|A later study, Chikatamarla, Xiaojun and Bustin
(2004) examined the shrinkage and swelling of various Canadian coal samples
with
different ranks from sub-bituminous to medium volatile coals. The tests
were made with regard to determining the capacity of the various coals
for sequestration by adsorption. By using various gases, CO2,
CH4, H2S, and N2, they demonstrated that
the volumetric strains are proportionally related to the amount of adsorbed
gas. H2S caused larger volumetric changes than the others, up
to 15 times greater than carbon dioxide, 20 times more than methane and
about 40 to 130 times more than nitrogen. In the second part of their experiments
they compared the swelling and shrinkage of the coal matrix by introducing
various gases to determine the effect on coal permeability. They reported
that, by injecting carbon dioxide into the coal seam, the relative swelling
of coal was markedly greater than the shrinkage of the coal matrix.
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