Summary
Our study has two features. First, laboratory experiments measured the
change of the permeability of coal samples as a function of pore pressure and
injected-gas composition at constant effective stress. Second,
adsorption-solution theory described adsorption equilibria and aided
interpretation. The gases tested include pure methane (CH4),
nitrogen (N2), and carbon dioxide (CO2), as well as
binary mixtures of N2 and CO2 of different compositions.
The coal pack was initially dry and free of gas, then saturated by each test
gas at a series of increasing pore pressures at a constant effective stress
until steady state was reached. Thus, the amount of adsorption varied, while
the effective stress was held constant. Results show that, (i) permeability
decreases with an increase of pore pressure at fixed injection-gas composition,
and, (ii) permeability change is a function of the injected-gas composition. As
the concentration of CO2 in the injection gas increases, the
permeability of the coal decreases. Pure CO2 leads to the greatest
permeability reduction among all the test gases. However, 10 to 20% by mole of
N2 helps to preserve permeability significantly. According to the
mixed-gas adsorption isotherms, adsorption and the selectivity of a particular
gas species on coal surfaces is a function of pressure and the gas composition.
Therefore, we conclude that loading coal surfaces with adsorbed gas at constant
effective stress causes permeability reduction. Finally, gas adsorption and
permeability of coal are correlated, simply to extend the usefulness of study
results.
Introduction
Coalbed methane (CBM) has grown to supply approximately 10% of US
natural-gas production and is becoming important worldwide as an energy source
(EIA 2006). Conventional CBM-recovery procedures stimulate wells and produce
CH4 by depressurizing the coalbed. A full understanding of the
mechanisms underlying CBM production has yet to be established. Injection of
CO2, N2, or mixtures of the two gases enhances CBM
recovery significantly (Stevens et al. 1998; Stevens 2001). Coalbeds also
present a potential sink for greenhouse gases (GHGs), such as CO2.
One issue of particular interest for CO2 injection, and the subject
of our study, is the sensitivity of coal permeability to the partial pressure
of CO2 in the injection gas.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
30 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
14 March 2008
- Manuscript approved:
19 March 2008
- Version of record:
20 August 2008
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