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| Titel |
Permeability of coal to CH4 under fixed volume boundary conditions: the effect of stress-strain-sorption behaviour |
| VerfasserIn |
Jinfeng Liu, Peter Fokker, Christopher Spiers |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132863
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| Publikation (Nr.) |
 EGU/EGU2016-13409.pdf |
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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 CO2 and CH4, respectively. Numerous permeability models, coupling the
swelling response of coal to gas sorption, have been developed to predict in-situ coal seam
permeability evolution during (E)CBM. However, experimental studies, aimed at testing such
models, have mainly focused on the permeability changes occurring under constant lateral
stress conditions, which are inconsistent with the in-situ boundary condition of (near) zero
lateral strain. We performed CH4 permeability measurements, using the steady-state method,
on a cylindrical sample of high volatile bituminous coal (25mm in diameter), under (near)
fixed volume versus fixed stress conditions. The sample possessed a clearly visible
cleat system. To isolate the effect of sorption on permeability evolution, helium
(non-sorbing gas) was used as a control fluid. The bulk sample permeability to
helium, under stress control conditions, changed from 4.07×10−17to 7.5×10−18m2,
when the effective stress increased from 19.1 to 35.2MPa. Sorption of CH4 at a
constant pressure of 10MPa, under fixed volume boundary conditions, resulted in a
confining pressure increase from a poroelastically supported value of 29.3MPa to a
near-equilibrium value of 38.6MPa over 171 hours. This is caused by the combined
effect of the sorption-induced swelling and the self-compression of the sample. The
concentration of CH4 adsorbed by the sample was 0.113 mmol/gcoal. During the adsorption
process, the permeability to CH4 also decreased from 2.38×10−17 to 4.91×10−18m2,
proving a strong influence of stress-strain-sorption behavior (c.f. Hol et al., 2012) on
fracture permeability evolution. The CH4 permeability subsequently measured under
stress controlled conditions varied from 1.37×10−17 to 4.33×10−18m2, for same
change in confining pressure, i.e. 28.9 to 39MPa. This difference between fixed
volume and fixed stress boundary conditions likely reflects difference in the degree of
equilibration. The difference in sample permeability measured with helium versus CH4
suggests an additional effect of sorption on transport paths, independently of the
poro-elastic effects. The permeability change with respect to the effective stress for both
CH4 and He are well fitted by the permeability model developed by Walsh (1981),
which considers the effect of contact asperity deformation on effective fracture
aperture and hence permeability. Physically, the model parameters reflect the apparent
specific stiffness of the fracture. Using appropriate parameter values, the Walsh
model therefore offers a promising basis for predicting permeability evolution from
in-situ stress evolution. To predict the effective stress development with time in
situ, i.e. under fixed volume boundary conditions, a simple model, coupling 1D
diffusion model, 2D constitutive model for stress-strain-sorption behaviour, and the
relation for joint closure with the normal effective stress, is presently being developed. |
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