GEOMECHANICS OF COAL-GAS INTERACTIONS
Y. Peng1, *J. Liu1, and M. Y. Wei2
1
School of Mechanical and Chemical Engineering, The University of Western Australia,35 Stirling
Highway, Perth, WA 6009
(*Corresponding author: Jishan.liu@uwa.edu.au)
2
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil
Mechanics, Chinese Academy of Sciences, Wuhan 430071, China
ABSTRACT
Advances in our understanding on geomechanics of coal-gas interactions have changed the manner in
which we treat coal seam gas: from mitigating its dangers as a mining hazard to developing its potential as
an unconventional gas resource recovered as a useful by-product of CO2 sequestration. When coal seam
gas is recovered, complex coal-gas interactions have strong controlling effects on the extraction efficiency
of coal or gas. These include influences on gas sorption and flow, coal deformation, porosity change and
permeability modification. We define this chain of reactions as “coupled processes” implying that one
physical process affects the initiation and progress of another. Therefore, the inclusion of cross couplings is
the key to rigorously formulate the geomechanics of coal-gas interactions.
Although coal-gas interactions have been comprehensively investigated, all of these prior studies focus on
one or more individual processes. They usually assume that these interactions are under conditions of
invariant total stress where effective stresses scale inversely with applied pore pressures and also under the
assumption of the local equilibrium between matrix pressure and fracture pressure. Through our persistent
efforts of almost a decade, we have removed all of these constrained conditions, and developed new cross
coupling relations between coal porosity and mechanical, hydrological, chemical and thermal volumetric
strains under conditions of variable stress. The cubic relation between porosity and permeability is then
introduced to relate coal storage capability (changing porosity) to coal transport characteristics (changing
permeability) also under variable stress conditions. These two relations are the key cross couplings that
define the geomechanics of coal-gas interactions. We implement these two relations into a sequence of
finite element models to represent the geomechanics of coal-gas interactions including single through dual
poroelastic models. These models couple the transport and sorption of a compressible fluid within a
deformable medium where the effects of deformation are rigorously accommodated. This paper reports our
novel framework on geomechanics of coal-gas interactions and its applications primarily in the field of coal
seam gas extraction. A typical example is illustrated in the following figure.
Comparisons
between
our our
modelled
coal results,
Comparisons
between
modelled
permeability,
experimental
data
(Robertson
&
experimental
data
(Robertson
&
Christiansen,
2006)
and
other
commonly
used
Christiansen, 2006), and commonly used PM
coal permeability models. Typically reductions
model:
typical
reductions
in
coal
in permeability are observed from gas-sorptionpermeability are observed even under the
induced swelling even where effective stresses
condition
of free
This behavior
remain
constant.
This swelling.
behavior remains
remains
enigmatic
as
the
permeability
of the
enigmatic as the permeability of the porous coal
is effective
determined
by the
is porous
determinedcoal
by the
stress only.
Our effective
model
has
replicated
this
apparently
anomalous
stress only. Our model has replicated this
behavior.
apparently anomalous behavior.
KEYWORDS
Geomechanics, Coal-Gas Interactions; Coal Permeability; Coupled Processes