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| Titel |
Thermodynamic model for swelling of unconfined coal due to adsorption of mixed gases |
| VerfasserIn |
Jinfeng Liu, Colin Peach, Christopher Spiers |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076219
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| Zusammenfassung |
| Permeability evolution in coal reservoirs during CO2-Enhanced Coalbed Methane (ECBM)
production is strongly influenced by swelling/shrinkage effects related to sorption and
desorption of both CO2 and CH4. Other gases, such as N2, may perhaps also be used in
ECBM operations. Much work has been done on the sorption/swelling response of coal to the
pure gases. However, there is a clear need for an improved understanding of swelling
behaviour of coal matrix material as a result of mixed gas adsorption. We therefore
constructed a thermodynamic model for swelling of unconfined coal due to mixed gases
adsorption, considering the equilibrium state (swelling strain eadseq), focusing initially on a
binary gas mixture. Following Hol et al (2012, IJCG, 93, 1-15), we started with
the following basic assumptions: a) nanoporous coal matrix material only allows
diffusion and adsorption, b) the matrix hosts nsi(i=α, β) localised adsorption sites
for the two gas components α and β, c) the material is homogeneous in structure
and composition but may be anisotropic in properties as appropriate for natural
coal, d) adsorption is allowed to proceed until equilibrium is reached, at which
point the chemical potential of the adsorbed component i is equal to the potential
of the free component phase i, and e) the volume change (strain) associated with
adsorption of one molecule of component i is insensitive to the adsorbed concentration
of either component. Three models were derived corresponding to three possible
interactions:
1) Isolated adsorption sites model. This assumes that each component has its own specific
adsorption sites. Adsorption of α and β accordingly leads to independent swelling responses
that sum to give total volumetric strain.
2) Shared adsorption sites model. This postulates that both gases have full access to all
adsorption sites (nsα = nsβ = ns). This model is thermodynamically equivalent to the
Extended Langmuir model. If the free fluids behave as ideal gases, the adsorbed
concentrations predicted reduce to the Langmuir isotherm for mixed gases.
3) Preferential adsorption sites model. This assumes adsorption sites show selectivity
towards one gas, i.e. nsα adsorption sites can be occupied by both gases (cf. 2) while only
(nsα -nsβ) adsorption sites can be occupied by gas β ( cf. 1). This model can be treated as a
combination of Models 1 and 2.
We compared these models to experimental measurements of swelling of an Australian
sub-bituminous coal exposed to CH4, CO2 and to their mixtures at pressures up to 15 MPa,
performed by Day et al. (2012, IJCG, 93, 40-48). Compared to other two models, the
preferential adsorption sites model describes the experimental results best. This finding
complies with the conventional view that CH4 is displaced by CO2 due to both depletion of
CH4 partial pressure and preferential CO2 adsorption. However, our findings contradict the
proposal by Day et al., based on their experiments, that CH4 and CO2 have access to all
adsorption sites and that swelling solely depends on partial pressures. Additional experiments
on different rank coals and gas species are underway to evaluate our models further. |
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