Investigation of the Effect of Carbon Dioxide ...€¦ · Investigation of the Effect of Carbon...

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


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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.


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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


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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.


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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


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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.


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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.


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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


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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


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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).


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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.


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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)


Initial

After Swelling


0.0001

0.001

0.01

12 14 16 18 20

Per

mea

bilit

y(m

d)


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

)


Initial

After Swelling


0.01

5 7 9 11 13

Per

mea

bil

ity

(md

)


Initial

After Swelling


0.01

10 12 14 16 18

Per

mea

bil

ity

(md

)


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.


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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



N2 Saturated at 1 MPa 2.41 0.42 41.94 0.83



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.


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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.


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


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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)


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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)


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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)


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


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


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)


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)


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


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

)


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)


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


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)


Model Predicted

Experimentaly determined

0

0.001

0.002

0.003

0.004

0 2 4 6

Perm

eab

ilit

y (m

d)


Model Predicted

Experimentaly determined0

0.0004

0.0008

0.0012

0.0016

0 2 4 6

Perm

eab

ilit

y (m

d)


Model Predicted


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


0

0.001

0.002

0.003

0.004

0.005

0 5 10 15 20

Perm

eab

ilit

y (m

d)


Model Predicted

Experimentaly determined0

0.002

0.004

0.006

0.008

0 5 10 15 20P

erm

eab

ilit

y (m

d)


Model Predicted


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)


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.


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


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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*Papers resulting from this study

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