Summary
Measurements of sorption isotherms and transport properties of carbon
dioxide (CO2) in coal cores are important for designing enhanced
coalbed-methane/CO2-sequestration field projects. Sorption isotherms
measured in the laboratory can provide the upper limit on the amount of CO2
that might be sorbed in these projects.
Because sequestration sites will most likely be in unmineable coals, many of
the coals will be deep and under considerable lithostatic and hydrostatic
pressures. These lithostatic pressures may reduce the sorption capacities
and/or transport rates significantly. Consequently, we have studied apparent
sorption and diffusion in a coal core under confining pressure. A core from the
important bituminous coal Pittsburgh #8 was kept under a constant, 3D effective
stress; the sample was scanned by X-ray computer tomography (CT) before, then
while, it sorbed CO2. Increases in sample density because of
sorption were calculated from the CT images. Moreover, density distributions
for small volume elements inside the core were calculated and analyzed.
Qualitatively, the CT showed that gas sorption advanced at different rates in
different regions of the core, and that diffusion and sorption progressed
slowly. The amounts of CO2 sorbed were plotted vs. position (at
fixed times) and vs. time (for various locations in the sample). The resulting
sorption isotherms were compared to isotherms obtained from powdered coal from
the same Pittsburgh #8 extended sample.
The results showed that for this single coal at specified times, the
apparent sorption isotherms were dependent on position of the volume element in
the core and the distance from the CO2 source. Also, the calculated
isotherms showed that less CO2 was sorbed than by a powdered (and
unconfined) sample of the coal. Changes in density distributions during the
experiment were also observed. After desorption, the density distribution of
calculated volume elements differed from the initial distribution, suggesting
hysteresis and a possible rearrangement of coal structure because of
CO2 sorption.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
19 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
21 July 2008
- Manuscript approved:
26 July 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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