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Thesis Summary for Rocha Medal 2014
1
M.S.A.Perera
Investigation of the Effect of Carbon Dioxide Sequestration on Coal Seams: A Coupled
Hydro-Mechanical behaviour
Abstract
The process of CO2 sequestration in deep coal seam causes both coal seam permeability and
strength to be significantly reduced due to CO2 adsorption-induced coal matrix swelling. In
deep coal seams CO2 exists in its super-critical state, which has quite different properties
compared to sub-critical CO2. The main objective of this study is to understand the effects of
sub-critical and super-critical CO2 injections on coal flow and strength properties.
A high pressure rig was first developed to conduct permeability tests and then used to
conduct permeability tests for naturally-fractured black coal samples to identify the effect of
CO2 injection on coal permeability. According to test results, super-critical CO2 adsorption
creates much greater swelling effects and exhibits lower permeability values compared to
sub-critical CO2. Interestingly, N2 has the potential to reverse CO2-induced swelling to some
extent. Strength tests were then conducted for different rank coals under different saturation
conditions. According to the test results, the strength and Young’s modulus of both types of
coals are reduced due to CO2 saturation and the reduction is higher for super-critical CO2
compared to sub-critical CO2 saturation. For example, the reductions of UCS strength and
Young’s modulus in black coal due to super-critical CO2 adsorption are about 40% and 100%
higher than the sub-critical CO2.
Numerical models play an important role in identifying CO2 flow behaviour in coal
prior to field-work. Therefore, CO2 movements in coal under laboratory conditions were
successfully modelled using the COMET 3 simulator and field-scale models were then
developed using the COMET 3 and COMSOL Multiphysics simulators. According to the
models, CO2 storage capacity in coal increases with increasing injecting pressure and
temperature and decreasing moisture content. Moreover, determination of well arrangements
is crucial for optimisation of CO2 injections Furthermore, CO2 injection causes the coal seam
cap rock to be significantly deformed in an upward direction and may cause the injected CO2
to leak.
Theoretical and empirical models play an important role in CO2 sequestration in deep
coal seams as they speed the identification of coal mass properties. Therefore, this has been
considered in the next stage of the study. First, a theoretical equation for coal cleat
permeability under non-zero lateral strain triaxial test conditions was developed, and then an
empirical relationship for gas adsorption capacity in coal was developed.
Keywords: CO2 sequestration, swelling in coal, permeability reduction, strength reduction
Thesis Summary for Rocha Medal 2014
2
M.S.A.Perera
1 Introduction
Global warming is an extremely important challenge for 21st
century scientists, and numerous
greenhouse gas mitigation and global warming control programs have been initiated during
the last few decades. In addition, in order to address the critical human impact on climate
change, many countries gathered and established the United Nations Framework Convention
on Climate Change (UNFCCC). According to the policies of this gathering, industrial
development should occur in a sustainable manner, and the greenhouse gas content in the
atmosphere should be maintained such that it does not interfere with natural eco-systems.
UNFCCC issued legally-binding warnings to many developed countries including Australia
at the 15th
UNFCCC conference held in Copenhagen in 2009. At this conference and at the
16th
UNFCCC conference held at Cancun in 2010, all the member countries agreed to reduce
greenhouse gas emissions in order that the total average world temperature increment will be
limited to 2 oC in the future. It is therefore vital for Australian scientists to investigate
possible CO2 mitigation methods. Four main techniques have been identified to minimize
atmospheric CO2 levels: 1) less carbon-intensive fuels; 2) more energy-efficient methods; 3)
increased conservation; and 4) carbon sequestration.
After much research on these techniques (Li et al., 2006), CO2 sequestration has been
identified as the most economical and most environmentally attractive method. According to
scientists’ estimations (Schrag, 2007), it is necessary to sequestrate trillions of tons of CO2 by
the end of this century to maintain a safe CO2 level in the atmosphere and therefore, it has
been named as one the “21st Century Engineering Grand challenges”
(http://www.engineeringchallenges.org/cms/8996.aspx). Up to date various CO2 sequestration
methods are being tested: 1) in depleted oil and gas reservoirs, 2) in saline aquifers, 3) in
deep ocean beds, 4) in deep un-mineable coal beds and 5) as mineral carbonates.
Thesis Summary for Rocha Medal 2014
3
M.S.A.Perera
Coal bed CH4
production
Unminable
coal seam
Deep saline
aquifer
Depleted oil
reservoir Oil production
Natural gas
reservoir
Saline aquifer
Injection of CO2
into geologic
reservoirs
Pipeline transporting
CO2 from industrial
sources to injection site
Offshore natural gas production
with CO2 separation and
sequestration
Figure 1. CO2 sequestration in various geological formations (Steadman et al., 2006).
Compared with other CO2 sequestration methods, long-term storage in deep un-
mineable coal seams has been identified as safe, practical and economically attractive (White
et al., 2005). The present study is therefore mainly focused on this particular CO2
sequestration method. Adsorption is the main gas storage mechanism in coal, and according
to existing findings, 98% of CO2 is stored in an adsorbed phase and the rest as free gas inside
the cleats (Shi and Durucan, 2005). Therefore, CO2 exists in a more stable form in coal
seams, which causes the risk of CO2 back-migration into the atmosphere to be greatly
reduced (Gray, 1987). In addition, coal seams contain large amounts of methane (CH4),
formed during the coalification process. Therefore, the ability to offset CO2 sequestration
costs using a valuable energy by-product like methane (CH4) is a unique advantage of this
process. Since coal mass has higher affinity to CO2 compared to CH4, while adsorbing CO2
into the coal matrix the available CH4 is released (Perera et al., 2011a). This phenomenon
raises the concept of enhanced coal-bed methane recovery (ECBM). Moreover, coal seams
have the potential to store a substantial amount of gas in their pore spaces due to the large
surface area associated with the micro-pore structure (Stevens et al., 2000).
However, injection of CO2 into a coal seam causes its physical and chemical structure
to be significantly changed. According to existing findings (White et al., 2005), adsorption of
CO2 into a coal matrix causes strain to be induced between the adsorbing CO2 molecules and
the coal matrix surface, which is commonly known as coal matrix swelling. Although coal
Thesis Summary for Rocha Medal 2014
4
M.S.A.Perera
matrix swelling is small compared to its volume, since coal has very low total porosity
values, this process greatly reduces the pore space available for gas movement in the coal
mass (Romanov et al., 2005). Under high pressure conditions, this swelling process can
create a significant volumetric strain, which greatly reduces the CO2 injection capacity or
permeability of the coal seam. Therefore, the amount of storable CO2 in a coal seam is highly
unpredictable. In addition, potential coal seams for the CO2 sequestration process exist at
extremely deep locations (around 1000m below the surface). Therefore, the pressures and
temperatures at these locations are higher than the critical values of gases such as CO2, and
CH4. For instance, in the case of CO2, the critical temperature is 31.8o
C and the critical
pressure is 7.38 MPa (Fig. 2). Since both of these factors normally exceed their critical values
in deep coal seams, CO2 exists in its super-critical state in these locations.
7.38
0.52
0.01
-56.4 31.8
Super critical
Region
Liquid Solid
Gas Critical point
Temperature (oC)
Pre
ssu
re (
MP
a)
Figure 2. Phase diagram for CO2 (Perera et al., 2011a)
According to Qu et al. (2010), super-critical CO2 has gas-like compressibility and
solubility values and liquid-like density values. Therefore, the density and viscosity of super-
critical CO2 vary greatly with pressure and temperature. In addition, super-critical CO2 has
greater potential to displace existing gases from coal seams (White et al., 2005) and its
adsorption capacity is much higher compared to sub-critical CO2 (Krooss et al., 2002). These
facts suggest that super-critical CO2 behaves quite differently from sub-critical CO2 and
therefore, a completely different behaviour in coal seams can be expected for super-critical
CO2 compared to sub-critical CO2.
In addition to the great reduction of coal seam permeability, CO2 sequestration creates
a significant influence on coal’s overall strength. The strength reduction associated with CO2
adsorption is a great threat in terms of long-term safety, because it enhances the risk of CO2
back-migration into the atmosphere and causes outbursts to occur in coal mining. Therefore,
it is most important to address this issue.
Thesis Summary for Rocha Medal 2014
5
M.S.A.Perera
As the CO2 sequestration process in deep coal seams is a highly time- and money-
consuming process, numerical models play an important role in identifying CO2 flow
behaviour in coal prior to field work. Many field-scale models have been developed to
identify flow phenomena in porous media using different computer software programs
including TOUGH 2 (Carneiro, 2009), COMSOL (Liu and Smirnov, 2009), FEMLAB
(Holzbecher, 2005), COMET 3 (Pekot and Reeves, 2002), MSFLOW (Wu, 2002) and
METSIM2 (Shi et al., 2008). However, less consideration has been given to the application of
these software programs in laboratory-scale studies (Hadia et al., 2007). It is important for
laboratory-scale studies such as triaxial experiments to analyse the experimental data and
extend the findings to field conditions such as extreme pressures and temperatures, which are
normally difficult to achieve in laboratories.
In order to inject the optimum amount of CO2 into a selected coal seam, it is
important to identify the effects of coal mass properties (moisture content, temperature), CO2
pressure and injection and production well operations on its CO2 storage capacity. Therefore,
a comprehensive laboratory-scale study is necessary to understand these effects. In addition,
CO2 sequestration in any deep geological formation is associated with the risk of back-
migration of the CO2 into the atmosphere through its cap rock some time after injection. For
instance, around 1700 people died in Cameroon in 1986 due to a leakage of naturally-
sequestrated CO2 from Lake Nyos (Benson et al., 2007). If the CO2 injection pressure is not
properly controlled, it is highly possible to have this kind of leakage in coal seams through
their cap rock because high injecting pressures may cause fractures in the cap rock.
Therefore, it is important to carry out an adequate field-scale numerical study before starting
field work.
Theoretical and analytical models play an important role in CO2 sequestration in deep
coal seams as they obviate laboratory experimentation and speed the identification of coal
mass properties. Coal mass has a natural cleat system and therefore estimation of cleat
permeability is of utmost importance for CO2 sequestration in coal seams. All the existing
cleat permeability models have been developed on the basis of the basic assumption that
lateral strain is zero. Although this is applicable under the in situ stress conditions in deep
coal seams, it is not applicable for laboratory experiments such as triaxial tests as they clearly
induce considerable lateral strain (Wang et al., 2007). Since triaxial experiments have been
identified as the most suitable for permeability testing, it is important to develop a theoretical
model to investigate cleat permeability variation in coal mass under triaxial test conditions. In
addition to the permeability, the correct estimation of the gas adsorption capacity is also
Thesis Summary for Rocha Medal 2014
6
M.S.A.Perera
important in the CO2 sequestration process in coal. However, this is a difficult task using the
existing gas adsorption models, as it is necessary to provide experimentally-evaluated
parameters to use these models. For instance, although the Dubnin-Radushkevich (D-R)
model is one of the widely-used models to estimate the gas adsorption capacity in coal,
experimental pre-estimation of micro-pore capacity is necessary to use this model. Therefore,
the present study has been structured to provide solutions to these issues.
The main objectives of the thesis can be outlined as follows:
1. To conduct a comprehensive experimental study
a) to identify the sub-critical and super-critical CO2 adsorption-induced swelling
effects on the permeability of coal.
b) to distinguish the permeability behaviours of coal under sub-critical and super-
critical CO2 flow conditions.
c) to investigate the sub-critical and super-critical CO2 adsorption effects on the
strength of low rank and high rank coals.
2. To conduct a complete numerical study
a) to develop a laboratory-scale numerical model using a suitable field-scale
simulator to model CO2 flow behaviour in coal under triaxial experimental
conditions.
b) to develop a field-scale numerical model using a user-friendly simulator to
study the effect of coal seam physical properties, CO2 injecting pressure and
injection and production well operations on the CO2 storage capacity of a
selected coal seam, to be used as a base model for field work.
c) to develop a field-scale numerical model using an appropriate and advanced
simulator to study the risk of CO2 back-migration into the atmosphere from a
selected coal seam through the cap rock.
3. To develop the necessary theoretical and analytical studies
a) to formulate an equation to predict cleat permeability in coal for CO2
movement under non-zero lateral strain condition.
b) to develop an appropriate and user-friendly model for gas adsorption capacity
in coal.
Thesis Summary for Rocha Medal 2014
7
M.S.A.Perera
2. Experimental investigation of CO2 sequestration effects on coal hydro-mechanical
properties
Before starting the CO2 sequestration study it was necessary to design and construct a new rig
to conduct permeability and strength tests for coal under in situ stress and temperature
conditions. A high-pressure rig was therefore designed, constructed and certified for the
required Australian standards. Since this is the first rig constructed in the Monash Civil
Engineering Department for such high gas injection pressure conditions, close attention was
given to avoid any accidents, particularly during the initial tests. The set-up is capable of
delivering gas (N2 and CO2) to the sample at injection pressures of up to 50 MPa, confining
pressures of up to 70 MPa to simulate ground pressures to depths in excess of 2.5 km, and
temperatures of up to 50 oC. In addition, up to 10 tonnes axial load can be applied to the
sample. The newly-developed set-up is shown in Fig. 3. It consists of five main units: 1)
confining unit, 2) gas injecting unit, 3) water injecting unit, 4) loading unit and 5)
temperature control unit.
Thesis Summary for Rocha Medal 2014
8
M.S.A.Perera
Figure 3. The newly-developed high pressure rig in the Civil Engineering Laboratory (Ranjith and Perera, 2011)
Pressure
cell
Gas bottles
Hand pump
(to apply
confining pressure)
Data
Acquisition
System
Gas outlet
(Deshler bottle
and flow meter)
Load cell
Load control
box
Gas booster
Thesis Summary for Rocha Medal 2014
9
M.S.A.Perera
The developed rig was then used to conduct permeability and strength tests to identify
the effects of sub-critical and super-critical CO2 injections on coal permeability and strength.
Two types of coal were considered for the flow and strength experiments; 1) Southern
Sydney Basin coal (from the Appin coal mine in the Bulli coal seam) and 2) Victorian brown
coal (from the Latrobe Valley coal mine in the Yallourn coal seam). However, of these two
types, priority has been given to the Southern Sydney Basin coal due to the lack of data
available on CO2 sequestration effects on high rank coal flow and strength properties
compared to low rank coal. In particular, the effect of CO2 sequestration on coal permeability
was examined only for the Southern Sydney basin coal because some studies have already
been conducted on this aspect for low rank coal (Victorian brown coal) for low gas injection
pressures (Jasinge et al., 2011). Furthermore, it is difficult to conduct permeability testing
under high pressures for this particular type of coal due to its low density and strength values.
In addition, permeability tests were conducted for naturally-fractured Southern Sydney Basin
high rank coal samples as the intact permeability of black coal is quite low (Siriwardane et
al., 2009).
First, a permeability study was conducted to compare the sub-critical and super-
critical CO2 adsorption-induced swelling effects on the permeability of naturally-fractured
black coal. A series of permeability tests was conducted using the newly-developed high
pressure rig (Fig. 3) on 38 mm by 76 mm naturally-fractured black coal specimens (Fig.4).
(a) Top surface (Downstream) (b) Bottom surface (Upstream)
Figure 4. Naturally-fractured coal specimen (Perera et al., 2011b).
These tests were carried out for CO2 and N2 injections at 2-20 MPa injection pressures
under 10 to 24 MPa confining pressures at 33 oC. Triaxial undrained tests were conducted for
Cleats
Thesis Summary for Rocha Medal 2014
10
M.S.A.Perera
all the permeability tests as it was necessary to achieve super-critical CO2 conditions inside
the coal mass pore space. Here, it should be noted that the downstream pressure was not
equal to the upstream pressure, and it was therefore necessary to have very high injection or
upstream pressures to create super-critical CO2 conditions in the coal sample. Each coal
specimen was then allowed to swell under sub-critical and super-critical CO2 adsorption and
the corresponding effects on CO2 and N2 permeabilities were examined. Results indicate that
the permeability of naturally-fractured black coal is significantly reduced due to matrix
swelling, which is significantly higher for super-critical compared to sub-critical CO2
adsorption (Fig.5).
0
5
10
15
0 1 2 3 4 5
Per
mea
bil
ity R
edu
ctio
n (%
)
CO2 Injection Pressure (MPa)
0
10
20
30
40
4 5 6 7 8 9 10 11 12
Per
mea
bil
ity R
edu
ctio
n (%
)
CO2 Injection Pressure (MPa)
(a) Sub-critical CO2 adsorption effect (b) Super-critical CO2 adsorption effect
Figure 5. Reductions of CO2 permeability due to swelling after sub- and super-critical CO2
adsorption-induced swelling (Perera et al., 2011b).
This is due to the fact that the amount of coal matrix swelling due to CO2 adsorption
clearly depends on the phase condition of the CO2, and super-critical CO2 adsorption-induced
swelling is about two times higher than that induced by sub-critical CO2 adsorption (see
Fig.6).
Thesis Summary for Rocha Medal 2014
11
M.S.A.Perera
0.000
0.001
0.002
0.003
0.004
0.005
0 4 9 14 19 0
Rad
ial S
train
Incre
men
t
Time (Hours)
Super-critical CO
Adsorption
Sub-critical CO
Adsorption
2
2
Figure 6. Radial strain increments during three 15 hour swelling periods for sub- and super-
critical CO2 adsorptions (Perera et al., 2011b).
An experimental study was then conducted to identify the effect of CO2 phase
condition on CO2 flow behaviour in naturally-fractured black coal, which showed how the
permeability of CO2 varies when it converts to the super-critical state from its sub-critical
state. Permeability tests were carried out for 15, 20 and 25 MPa confinements at 33.5 oC
temperature. Two test scenarios were conducted to investigate: 1) variation of the
permeability behavior of coal with CO2 phase condition; and 2) the potential of nitrogen (N2)
to recover CO2-induced swelling. According to the test results, the permeability of super-
critical CO2 is significantly lower than sub-critical CO2 due to the higher viscosity and
swelling associated with super-critical CO2 (Fig.7). Moreover, at super-critical state there is a
higher decline of CO2 permeability with increasing injecting pressure due to the higher
increments in the associated viscosity and swelling. Although CO2 adsorption-induced
swelling causes permeability of both CO2 and N2 to be reduced, it may also cause CO2
permeability to increase for higher injecting pressures, because CO2 flow behavior may
transfer from super-critical to sub-critical after the swelling due to the decline of downstream
pressure development. Moreover, N2 has the potential to recover some swelling effects due to
CO2 adsorption, and this recovery rate is higher at lower injecting pressures and higher
confining pressures.
Thesis Summary for Rocha Medal 2014
12
M.S.A.Perera
0.00001
0.0001
0.001
0.01
5 7 9 11
Per
mea
bilit
y(m
d)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding0.00001
0.0001
0.001
0.01
10 12 14 16 18
Per
mea
bilit
y(m
d)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding0.00001
0.0001
0.001
0.01
12 14 16 18 20
Per
mea
bilit
y(m
d)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding
(a) CO2 at 15 MPa (b) CO2 at 20 MPa (c) CO2 at 25 MPa
0.001
0.01
3 5 7 9
Per
mea
bil
ity
(md
)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding0.001
0.01
5 7 9 11 13
Per
mea
bil
ity
(md
)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding0.001
0.01
10 12 14 16 18
Per
mea
bil
ity
(md
)
Upstream Pressure (MPa)
Initial
After Swelling
After Nitrogen Flooding
(d) N2 at 15 MPa (e) N2 at 20 MPa (f) N2 at 25 MPa
Figure 7. Variation in permeability with varying CO2 and N2 injection pressures for initial
CO2 swollen and N2 flooded samples under 15, 20 and 25 MPa confining pressure conditions
(Perera et al., 2011c).
After the permeability tests a series of strength studies was conducted to investigate
the effect of CO2 sequestration on the strength of coal. First, a strength study was conducted
to investigate the effects of different saturation mediums such as CO2, N2 and moisture on the
strength of low rank coal. However, super-critical CO2 saturation could not be achieved for
the brown coal samples as application of more than 7.38 MPa gas confining pressure may
cause the structure of low strength brown coal to change. A series of uniaxial experiments
was conducted using the newly-developed high pressure rig on 38 mm diameter by 76 mm
high Latrobe Valley brown coal samples with different saturation media (water, N2, CO2) and
pressures (1, 2 and 3 MPa). According to the test results, water and CO2 saturations cause the
uniaxial compressive strength (UCS) of brown coal to be reduced by about 17 % and 10 %
respectively. In contrast, N2 saturation causes it to increase by about 2%. Moreover, the
Young’s modulus of brown coal is reduced by about 8% and 16% due to water and CO2
saturations respectively, and is increased up to 5.5% due to N2 saturation. It can be concluded
that CO2 and water saturations cause the strength of brown coal to be reduced while
improving its toughness, and N2 saturation causes the strength of brown coal to increase,
while reducing its toughness.
Thesis Summary for Rocha Medal 2014
13
M.S.A.Perera
Table 1. Saturation effects on the uniaxial compressive strength (UCS) and Young’s modulus
(E) values of brown coal samples (Perera et al., 2011d)
Specimen UCS △UCS% E (MPa) △E%
Natural 2.40 41.60
Water Saturated 2.00 -16.80 38.42 -7.63
CO2 Saturated at 1 MPa 2.34 -2.53 40.32 -3.07
CO2 Saturated at 2 MPa 2.25 -6.49 33.78 -18.78
CO2 Saturated at 3 MPa 2.17 -9.60 35.08 -15.67
N2 Saturated at 1 MPa 2.41 0.42 41.94 0.83
N2 Saturated at 2 MPa 2.42 0.91 42.52 2.21
N2 Saturated at 3 MPa 2.46 2.23 43.92 5.59
The final experimental study was then conducted to investigate the effects of sub-
critical and super-critical CO2 adsorption on the strength of high rank coals. A series of UCS
tests was conducted on natural intact black coal samples under both sub-critical and super-
critical CO2 saturation conditions. In addition, N2 saturated samples were used to compare the
results with CO2-saturated samples. According to the results, sub-critical CO2 adsorption
causes the UCS and Young’s modulus of the bituminous coal to be reduced by up to 53 %
and 36%, respectively (Fig.8). Super-critical CO2 adsorption causes more significant
modifications to the mechanical properties of the bituminous coal, resulting in 40 % greater
UCS strength reduction and 100 % greater Young’s modulus reduction compared to sub-
critical CO2 adsorption. The greater influence of super-critical CO2 on the UCS of the
bituminous coal is thought to be related to the greater adsorptive potential (and coal swelling
produced) for super-critical CO2. The more significant influence of super-critical CO2 on the
Young’s modulus of the bituminous coal is thought to relate to the greater dissolution (and
thus coal plasticization) potential of the super-critical CO2. N2 saturation was not observed to
have any significant effect on the mechanical properties of the bituminous coal.
Thesis Summary for Rocha Medal 2014
14
M.S.A.Perera
0
8
16
24
32
40
0 5 10 15
UC
S (
MP
a)
CO2 Saturation Pressure (MPa)
Super-Critical Region
0
0.5
1
1.5
2
2.5
3
3.5
4
0 5 10 15
Ela
stic
M
od
ulu
s, E
(G
Pa)
CO2 Saturation Pressure (MPa)
Sub-Critical Region
Super-Critical Region
Sub-Critical Region
(a) UCS strength (b) Elastic modulus
Figure 8. Effect of CO2 adsorption on black coal mechanical properties (Perera et al., Under
Review-a)
3. Numerical investigation of CO2 sequestration effects on coal hydro-mechanical
properties
As the CO2 sequestration process in deep coal seams is a highly time- and money- consuming
process, numerical models play an important role in identifying CO2 flow behaviour in coal
prior to field work. It is particularly important for laboratory-scale studies such as triaxial
experiments to analyse the experimental data and extend the experimental findings for field
conditions such as extreme pressures and temperatures, which are normally difficult to
achieve in laboratories. Therefore, a comprehensive study was conducted to develop an
appropriate laboratory-scale model to simulate gas permeability in coal under triaxial test
conditions.
A numerical model was developed using the COMET 3 field-scale simulator to
examine and predict the CO2 permeability values in coal under triaxial drained and un-
drained conditions. A basic model for gas movement in porous media under triaxial drained
test condition was first developed for the movement of a non-reactive gas in a non-reactive
medium (air flow in sandstone), which is the simplest triaxial test condition and therefore,
needs fewer model parameters, resulting in a highly accurate model.
Thesis Summary for Rocha Medal 2014
15
M.S.A.Perera
CO2 Pressure (kPa)
Figure 9. (a) Vertical pore pressure distributions in sandstone sample and, (b) horizontal pore
pressure distribution at the bottom layer of the sandstone sample at the steady state condition
for 180 kPa gas injecting pressure and 220 kPa confining pressure condition (Perera et al.,
2011e).
The model was then extended to simulate the more advanced condition of CO2
movement in naturally-fractured black coal (movement of a reactive gas in a reactive
medium). The triaxial un-drained test condition was used for this case, as the experiments
required for model verification were conducted under the un-drained condition.
(a) (b) x
y
x
z
Thesis Summary for Rocha Medal 2014
16
M.S.A.Perera
Inj-1
Inj
Inj
Inj
Inj
Inj
Inj
Inj
Inj
Inj Inj
Inj Inj
Pro
Pro
Pro
Pro
Pro
Pro
Pro
Downstream
Volume
Impermeable
Cap
Coal Sample
CO2 Injecting
(Bottom) Layer
X
Y
Z
Gas Pressure (MPa)
0.1 3.3 6.5 9.7 12
Figure 10. (a) Selected mesh pattern and well locations for the coal sample and, (b) pore
pressure distribution in the tested coal sample for 12 MPa CO2 injecting pressure and 20 MPa
confining pressure condition (after 30 minutes of injection) (Perera et al., 2012a)
The model calibration and verification were carried out using measured triaxial test
data (newly developed high pressure rig was used) for CO2 movement in naturally-fractured
black coal at the temperature range of 25 to 70 oC. This model was then used to predict
permeability under higher temperature conditions (80 to 200 oC) to understand the effect of
temperature on the CO2 permeability of deep coal seams (Fig.11). According to the developed
laboratory-scale model, there is a clear increase in CO2 permeability with increasing
temperature for any confining pressure at high injecting pressures (more than 10 MPa).
However, for low injecting pressures (less than 9 MPa) the temperature effect is less. With
increasing injecting pressure, CO2 permeability decreases at low temperatures (less than
around 40 oC), and increases at high temperatures (more than 50
oC). Interestingly, the
temperature effect on permeability is significant only up to around 90 oC within the 25 to 200
oC temperature limit. These observations are related to the sorption behaviour of the
adsorbing CO2 during the injection.
(a) (b)
Thesis Summary for Rocha Medal 2014
17
M.S.A.Perera
0.00016
0.0002
0.00024
0.00028
0.00032
0 50 100 150 200
Per
mea
bilit
y (
md
)
Temperature (oC)
8 MPa9 MPa10 MPa
11 MPa12 MPa
0.00005
0.00006
0.00007
0.00008
0.00009
0.00010
0 50 100 150 200
Per
mea
bilit
y (
md
)
Temperature (oC)
9 MPa
10 MPa
11 MPa
12 MPa
13 MPa
Figure 11. Model-predicted temperature effects on CO2 permeability at two different depths
(Perera et al., 2012a).
After conducting a comprehensive study on the development of a laboratory-scale
model for the CO2 sequestration process in coal under triaxial test conditions, an appropriate
field-scale model was developed using the COMET 3 software to identify the field CO2
sequestration process.
Figure 12. Selected geometry of the model grid blocks and the CO2 distribution with time for
15 MPa CO2 injection pressure at 30 oC (Perera et al., 2011f).
(a) 0.77 km depth (b) 0.92 km depth
(a) Grid system
(b) Distribution of CO2 after
the 1st year of injection
(c) Distribution of CO2
after 10 years of injection
0 2 4 6 8 10 12 14 15
Pore Pressure (MPa)
Thesis Summary for Rocha Medal 2014
18
M.S.A.Perera
The model was then used to investigate the effects of coal mass physical properties
such as moisture content, temperature, injecting gas pressure and production and injection
well operations on the CO2 storage capacity in a selected deep coal seam. This kind of
numerical modelling provides important information about how these parameters should be
controlled in the field to enable the injection of the optimum amount of CO2 into a coal seam.
According to the model, the temperature and moisture content of the coal seam and CO2
injection pressure have significant effects on CO2 storage capacity. According to the model
results, CO2 storage capacity decreases with increasing coal seam water saturation and
increases with increasing temperature, and this observation relates to the adsorption
behaviour of the injected CO2. If the effect of CO2 injection pressure is considered, during the
initial injection period, the increase of CO2 injection pressure causes the CO2 injection rate to
be significantly increased. However, this injection rate decreases with time due to coal matrix
swelling and may again increase suddenly due to the formation of fractures.
0
500
1000
1500
2000
2500
3000
0 5 10 15 20 25
CO
2 S
tora
ge
Cap
acit
y in
Ten
Yea
rs (m
3)×
10
7
CO2 Injecting Pressure (MPa)
0
5
10
15
20
25
30
35
0 20 40 60 80 100
CO
2 S
tora
ge
Cap
acit
y i
n
Ten
Yea
rs (m
3)×
10
7
Temperature (oC)
1
10
100
1000
0 0.2 0.4 0.6
CO
2 S
tora
ge C
apac
ity
in
Ten
Yea
rs (m
3)×
10
7
Water Saturation (fraction))
Figure 13. Variation of coal seam CO2 storage capacity with (a) injection pressure, (b)
temperature and, (c) moisture content (Perera et al., 2011f).
The developed model was then extended to investigate the effect of injecting and
production well operations on CO2 storage capacity in deep coal seams. According to the
(a) (b)
(c)
Thesis Summary for Rocha Medal 2014
19
M.S.A.Perera
model results, the maximum storage capacity can be obtained by having two injection wells
for the selected coal seam (Fig 14). However, a further increase in the number of wells (up to
four) causes the storage capacity to be reduced due to the coincidence of injecting wells
creating pressure contours (Fig 15). This is confirmed by the fact that, for the two injecting
wells condition, when the distance between the injecting wells increases from 150m to
around 570m, the CO2 storage capacity increases by about 270%. Considering all of these
observations, it can be concluded that, not only the number of injecting wells but also the
distance between the injecting wells should be calculated using an appropriate numerical
model, before starting any field CO2 injection to enable the injection of the optimum amount
of CO2 into a selected coal seam.
0
5
10
15
20
25
30
35
1 2 3 4
2
Number of Injecting Wells
Figure 14. Variation CO2 storage capacity with number of injection wells (Perera et al.,
2012b).
Figure 15. CO2 pressure developed after 10 years of injection for two to four injection wells
(Perera et al., 2012b).
Injecting
Well 1
Injecting
Well 2
Injecting
Well 1Injecting
Well 2
Injecting
Well 3
Injecting
Well 4
Injecting
Well 1Injecting
Well 2
Injecting
Well 3
300m
300m
200m
220
m
200m
175m
175m
175m
175m
0 5 10 15 20CO2 Partial Pressure (MPa)
(a) 02 Injecting wells (b) 03 Injecting wells (c) 04 Injecting wells
Thesis Summary for Rocha Medal 2014
20
M.S.A.Perera
Moreover, CO2 storage capacity can be significantly increased by inserting a water
production well, because it causes the pore pressure inside the coal seam to be reduced.
However, this storage capacity increment will reduce with time due to the reduction of
mobile water inside the coal seam. Therefore, the effectiveness of the production well will
reduce and disappear with time. Furthermore, when the distance between the production and
injecting wells was reduced from 700m to 400m, the CO2 storage capacity was increased by
around 30% for the selected coal seam (Fig.16), because the pore pressure reduction effect
from the production well will more quickly affect the CO2 injecting area. However, further
reduction of distance among the wells causes the storage capacity to be reduced due to the
mixing of injecting CO2 with desorpting CH4 from the coal seam. Therefore, it is also
important to calculate the distance between the production and injecting wells.
8
12
16
20
0 200 400 600 800
Distance Between the Injecting and Production Wells (m)
Figure 16. Variation of CO2 storage capacity with distance between the injection and
production wells (Perera et al., 2012b).
Conducting a safe CO2 sequestration process is as important as studying the CO2
sequestration process in coal and the ways to optimize it. Therefore, a field- scale model was
then developed to investigate the effect of CO2 injection pressure on cap rock deformation. In
this case, the COMSOL Multiphysics simulator was used instead of the previously used
COMET 3 simulator as it was necessary to model coupled flow and strength behaviour to
identify the cap rock and coal mass deformations upon CO2 injection.
CO
2 S
tora
ge
Cap
acit
y
(m3)×
10
7
Thesis Summary for Rocha Medal 2014
21
M.S.A.Perera
Coal Layer
1000m
X
3m
5m
1.5m
1.5m
Cap Rock (Mud Stone)
Hard Rock
Y
Bottom Mud Stone Layer
(0 m, 0 m) (200 m, 0 m)
(200 m, -1011 m) (0 m, -1011 m)
CO2 injecting point (100m, -1004.5 m)
Figure 17. Location of the coal layer (Perera et al., Under review-b).
In addition to being caused by the injecting gas pressure, cap rock deformation also
occurs with CO2 adsorption-induced coal mass swelling. This factor was also considered in
the model development to find the total deformation of the cap rock on CO2 injection. The
developed model shows that during the CO2 injecting period, the cap rock is vertically
deformed by CO2 pressure (Fig.18b). Moreover, the maximum vertical deformation of the
cap rock and the CO2 spread rate along the interface increase with the CO2 injecting pressure
(Fig 18a). This vertical deformation of the cap rock increases from around 0.2 mm to15 mm
when the gas injecting pressure increases from 10.2 MPa to 30 MPa.
10
11
12
13
14
15
16
0 50 100 150 200
Pre
ssu
re (
MP
a)
Horizontal Distance Along the Boundary (m)
01 Month 02 Months
03 Months 04 Months
05 Months 06 Months
01 Year 02 Years
03 Years 04 Years
05 Years 10 Years
(a)
Thesis Summary for Rocha Medal 2014
22
M.S.A.Perera
0
1
2
3
4
5
0 50 100 150 200
Cap
Ro
ck B
ou
ndar
y (m
m)
Horizontal Distance Along the Boundary (m)
01 Month 02 Months
03 Months 04 Months
05 Months 06 Months
01 Year 02 Years
03 Years 04 Years
05 Years 10 Years
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
Horizontal Distance Along the Boundary (m)
01 Month 02 Months
03 Months 04 Months
05 Months 06 Months
01 Year 02 Years
03 Years 04 Years
05 Years 10 Years
Figure 18. (a) Pressure, (b) cap rock deformation, and (c) CO2 spread along the cap rock
boundary throughout the 10 years (Perera et al., Under review-b).
This model was then used to determine the optimum CO2 injection pressure into a
selected coal seam to avoid any hydraulic fracture or shear failure in the cap rock, which
cause the injected CO2 to back-migrate into the atmosphere some time after CO2 injection.
According to the model, as a result of gas injection, the cap rock may fail at around 21 MPa
due to hydraulic fracture, and at around 19 MPa due to shear failure (Fig.19). Considering the
lower value and a 1.2 safety factor, the maximum gas injecting pressure should not exceed 16
MPa to avoid any failure in the cap rock of the selected coal seam.
(b)
(c)
CO
2 C
once
ntr
atio
n (
kg/m
3)
V
erti
cal
Def
orm
atio
n o
f th
e
Cap
Rock B
oundar
y (
mm
)
Thesis Summary for Rocha Medal 2014
23
M.S.A.Perera
8
13
18
23
28
10 15 20 25 30
Max
imu
m P
ore
P
ress
ure
/
Th
ird
Pri
nci
ple
Str
ess
(MP
a)
Maximum Pore Pressure on the Cap Rock InterfaceMaximum Third Priciple Stress
on the Cap Rock
Failure point at
21 MPa injecting pressure
0.1
0.2
0.4
0.8
1.6
3.2
6.4
10 15 20 25 30
Shear S
tren
gth
/Str
ess
(M
Pa)
Maximum Shear Stress on the Cap Rock Interface
Shear Strength of the Cap Rock
Failure point at
19 MPa injecting pressure
Figure 19. Optimum CO2 injection pressure to avoid (a) hydraulic fracture and, (b) shear
failure in the cap rock (Perera et al., Under review-b).
4. Development of a theoretical model for coal cleat permeability under CO2
movement
Theoretical and empirical models play an important role in the CO2 sequestration process in
deep coal seams as they obviate laboratory experimentation and speed the identification of
coal mass properties. Cleat permeability is the most important parameter which can be used
to determine the CO2 sequestration potential of any coal seam, as it represents the balance
between effective stress and matrix shrinkage and swelling. To date, many studies have been
conducted on this aspect and various theoretical and empirical relationships have been
proposed for cleat permeability in coal. However, these models have been developed on the
basic assumption that lateral strain is zero, which is not applicable under laboratory
experimental conditions with triaxial testing. Therefore, by applying the theory of elasticity to
the constitutive behaviour of fractured rocks, a theoretical relationship between permeability
and gas injecting pressure, confining pressure, axial load and gas adsorption in triaxial tests
has been developed.
Shea
r S
trength
/ S
tress
(M
Pa)
CO2 Injection Pressure (MPa) CO2 Injection Pressure (MPa)
Thesis Summary for Rocha Medal 2014
24
M.S.A.Perera
h
e q Z
X
Y
σA
σC
σCO2
σC
Figure 20. Stresses and fractures in the coal specimen used for the model development
(Perera et al., Under review-c).
The developed model is shown below:
−
−
∆−×
−
−
∆−×
−
−
−+
∆×
+−
−
−
∆=)21(2
)1(9
exp')21(
)1(
exp')21(2
)1(1
2
)1(3exp
2
)1(3
')21(2
)1(3
exp2 ϑ
αϑ
ϑ
ϑ
σϑ
ϑϑ
σϑ
ϑ
ϑ
σ
sf
f
bulk
f
f
c
bulk
f
f
f
f
A
f
f
bulk
f
f
COo
E
E
Sk
E
E
k
E
E
EEk
E
E
kk
[1]
where, k is the cleat permeability ko is the initial cleat permeability, σC is the confining stress,
σA is the axial stress and σCO2 is the CO2 injecting pressure into the coal sample under triaxial
test condition, ϑ is the matrix Poisson’s ratio, ϑf is the fracture Poisson’s ratio, E is the
sample matrix Young’s modulus, Ef is the fracture Young’s modulus, S is the adsorption
mass, α is the volumetric swelling coefficient, bulkk ' is the bulk modulus.
The new model was then verified using experimentally-determined permeability data
(newly developed high pressure rig was used) of two coal samples. Results indicate that the
new model can quite accurately predict the combined effects of effective stress and coal
matrix swelling on cleat permeability for both CO2 and N2 injections at various injection
pressures. The model also provides quite accurate prediction of the effect of confining
Thesis Summary for Rocha Medal 2014
25
M.S.A.Perera
pressure on cleat permeability for both CO2 and N2 injections. The model includes parameters
for fractured rock properties namely Poisson’s ratio and Young’s modulus. The model can be
applied to predict cleat permeability, regardless of cleat size.
0
0.002
0.004
0.006
0.008
0 2 4 6
Perm
eab
ilit
y (m
d)
CO2 Injecting Pressure (MPa)
Model Predicted
Experimentaly determined
0
0.001
0.002
0.003
0.004
0 2 4 6
Perm
eab
ilit
y (m
d)
CO2 Injecting Pressure (MPa)
Model Predicted
Experimentaly determined0
0.0004
0.0008
0.0012
0.0016
0 2 4 6
Perm
eab
ilit
y (m
d)
CO2 Injecting Pressure (MPa)
Model Predicted
Experimentaly determined
0
0.002
0.004
0.006
0.008
0 5 10 15 20
Per
mea
bil
ity (m
d)
Confining Pressure (MPa)
Model Predicted
Experimentaly determined
0
0.001
0.002
0.003
0.004
0.005
0 5 10 15 20
Perm
eab
ilit
y (m
d)
Confining Pressure (MPa)
Model Predicted
Experimentaly determined0
0.002
0.004
0.006
0.008
0 5 10 15 20P
erm
eab
ilit
y (m
d)
Confining Pressure (MPa)
Model Predicted
Experimentaly determined
Figure 21. Variation of model-predicted and experimentally-evaluated CO2 permeability
values (a) with injecting pressure at 5 MPa confining pressure, (b) with injecting pressure at
10 MPa confining pressure, (c) with injecting pressure at 15 MPa confining pressure, (d) with
confining pressure at 2 MPa injecting pressure, (b) with confining pressure at 2 MPa injecting
pressure, (c) with confining pressure at 4 MPa injecting pressure conditions (Perera et al.,
Under review-c).
5. Development of an analytical model for gas adsorption capacity in coal
Adsorption plays an important role in the carbon dioxide sequestration process in deep coal
seams as it is the main gas storage mechanism inside the coal mass. Therefore, correct
estimation of gas adsorption capacity is most important for the related field-work to estimate
the CO2 storage capacity in a selected coal seam. Although the D-R equation is one of the
most widely- used gas adsorption models, experimental evaluation of micropore capacity is
(d) (e) (f)
(a) (b) (c)
Thesis Summary for Rocha Medal 2014
26
M.S.A.Perera
necessary to use this equation. Therefore, a new descriptive model for the gas sorption
capacity of coal as a function of various factors is proposed. The new model is based on the
existing D-R equation, which has been modified by inserting a new expression for the term
for micro-pore capacity.
If T> Tc (T is the temperature and Tc is the critical temperature),
{ }
−+++=
−
2
2
2
min log}%))(%)(%)(1
log{
1
logρ
ρ
β
ρ
ρ
s
a
cvit
s
BTgWcVbVa
T
V [2]
If T < Tc,
( ) { }
−+++=
2
2
2
min log}%))(%)(%)(1
log{logp
pBTgWcVbVa
TV s
a
cvitβ
[3]
where, ρ is the adsorbing agent density (kg/m3) and ρs is the density of adsorbed phase. p is
the pressure (MPa) and ps is the saturated vapour pressure of the adsorbed gas (MPa), βa is
the affinity coefficient of the adsorbate, and dg is the effective molecular diameter of
adsorbate (nm).
Two types (CO2 and N2) of gas adsorption data for coal from five different locations
(British Colombia and Alberta in Canada and Victoria, Sydney and Bowen in Australia) at
three different temperatures (273, 296.5 and 318 K) were considered for the model
development. According to the model, the new gas adsorption equations can quite accurately
predict the adsorption capacity of coal.
Thesis Summary for Rocha Medal 2014
27
M.S.A.Perera
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Vo
lum
e A
dso
rbed (
cm
3/g
)
fro
m N
ew
Mo
del
Volume Adsorbed (cm3/g) from Existing D-R Equation
1:1
Canadian (Sedimentary Basin) Coal at 273 K Australian (Bowen Basin) Coal at 296.5 K
Australian (Yallourn Basin) Coal at 318 K Australian (Sydney Basin) Coal at 296.5 K
Figure 22. Plot of gas sorption capacity derived from the new equation and existing D-R
equations for all the coal types considered (Perera et al., 2012c).
6 Conclusions
A comprehensive study has been conducted using experimental, numerical, theoretical and
analytical approaches and the main conclusions drawn from the study can be listed as
follows:
� The permeability of coal is significantly reduced due to swelling, which starts as
quickly as within 1 hour of CO2 injection.
� The amount of coal matrix swelling due to CO2 adsorption clearly depends on the
phase condition of the adsorbing CO2, where super-critical CO2 adsorption-induced
swelling is up to two times greater and therefore permeability is significantly lower
than sub-critical CO2.
� N2 has the potential to reverse some swelling areas created by CO2 adsorption.
� CO2 adsorption has a more significant influence on the mechanical properties of black
coal than brown coal due to the well-developed cleat system in black coal. Sub-
critical CO2 adsorption causes the UCS and Young’s modulus of brown coal to be
reduced by about 10 % and 16 % respectively, and of black coal to be reduced by
53% and 36 %, respectively.
y=x+3
y=x-3
Thesis Summary for Rocha Medal 2014
28
M.S.A.Perera
� Super-critical CO2 adsorption has a much greater influence on coal’s mechanical
properties compared to sub-critical CO2, and it causes the UCS and Young’s modulus
of black coal to be reduced by up to 78 % and 71 %, respectively.
� CO2 movement in naturally-fractured black coal under triaxial test conditions can be
successfully modelled using the COMET 3 field-scale numerical simulator.
� According to the developed laboratory-scale model, there is a clear increment in CO2
permeability with increasing temperature under any confinement for high injecting
pressures (more than 10 MPa), while for low injecting pressures (less than 9 MPa)
temperature has little effect on permeability.
� According to the developed field-scale models, CO2 storage capacity increases with
decreasing bed moisture content and increasing temperature and CO2 injection
pressure. However, the injection pressure effect will be gradually reduced by the
process of coal matrix swelling. It is also important to have an adequate distance
between the injection wells to inject the optimum amount of CO2 into any coal seam,
as reduced distance may cause the storage capacity to be reduced due to the
coincidence of the pressure contours created by each injecting well.
� Cap rock deforms considerably in an upward direction due to the CO2 injection
pressure and the amount of deformation depends to a great extent on the injecting gas
pressure and duration. Therefore, high injection pressures may cause back-migration
of the injected CO2 into the atmosphere.
� An accurate theoretical relationship for coal cleat permeability under triaxial, non-
zero lateral strain conditions can be obtained by using basic geotechnical engineering
fundamentals. This model is expected to be highly useful in future, laboratory-scale
CO2 sequestration studies to establish the required triaxial test conditions.
� An accurate descriptive model for CO2 adsorption capacity in coal can be developed
by applying a multi-linear regression approach to the micro-pore capacity term in the
existing D-R equation.
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coal properties pertinent to carbon dioxide sequestration in coal seams: With special
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Recommended