Summary
The “swelling” of coal by a penetrant refers to an increase in the volume
occupied by the coal as a result of the viscoelastic relaxation of its highly
crosslinked macromolecular structure. Projects relating to CO2
sequestration in coal seams suffer a serious setback in terms of injectivity
loss resulting from the swelling of coal. Volumetric swelling associated with
CO2 sorption on coal has a significant influence on the fracture
porosity and permeability of the coal. Two coal samples differing in rank were
used for volumetric strain measurements. With CO2, the high-rank
Selar Cornish coal showed a maximum volumetric strain of 1.48% corresponding to
an average pore pressure of 13 MPa. A matrix swelling coefficient
(Cm ) of 1.77× 10–4 MPa–1
was calculated for this Selar Cornish coal. The low-rank Warndt Luisenthal coal
exhibited higher strain of 1.6%, and a matrix swelling coefficient
(Cm ) of 8.98×10–5 MPa–1 was
calculated. The rank dependence of swelling holds true in this set of
experiments. Repeat volumetric strain measurement on the same Warndt Luisenthal
coal core shows higher volumetric strain values for all pressure steps. A
volumetric strain of 1.9% corresponding to a mean pore pressure of 14 MPa was
measured. This confirms the process of sequential swelling.
A unique feature of this work is that real-time permeability measurements
were done under unconstrained conditions. Permeabilities were measured,
reducing the pore pressure from 16 to 1 MPa at constant flow rate. Although
measured permeability increased with increasing pore pressure under
unconstrained swelling, in-situ permeability will actually decrease because of
fracture closure in a constrained coal. To validate the permeability swelling
relationship, both permeability measurements under unconstrained conditions and
volumetric strain measurements were used.
Introduction
Maturation of coalbed methane (CBM) production operations in some basins,
the emergence of injection schemes for enhanced coalbed methane (ECBM), and
carbon sequestration of greenhouse gases has led to renewed focus on the
behavior of coalbed reservoir properties under these conditions.
Cleat permeability of coal is the most important parameter for coalbed
methane production. Being normal to the bedding plane and orthogonal to each
other, the face and butt cleats in coal seams are usually subvertically
oriented. Thus, changes in the cleat permeability are primarily controlled by
the prevailing effective horizontal stresses that act across the cleats, rather
than the effective vertical stress, defined as the difference between the
overburden stress and pore pressure (Harpalani and Chen 1997). Coal swelling
accompanying CO2 sorption would decrease the permeability of the
coal as the volume increase is compensated within the fracture porosity.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
25 March 2005
- Revised manuscript received:
24 November 2005
- Manuscript approved:
17 January 2006
- Version of record:
20 September 2006
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