International Journal of Geosciences, 2013, 4, 863-870 Published Online July 2013 (
Swelling Measurements of a Low Rank Coal in
Supercritical CO2
Ferian Anggara1,2, Kyuro Sasaki1, Yuichi Sugai1
1Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan
2Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia
Received March 28, 2013; revised April 30, 2013; accepted May 27, 2013
Copyright © 2013 Ferian Anggara et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Coal swelling in the presence of water as well as CO2 is a well-known phenomenon, and these may affect the perme-
ability of coal. Quantifying swelling effects is becoming an important issue to verify the suitability of particular coal
seams for CO2-enhanced coal bed methane recovery projects. In this report, coal swelling experiments using a visuali-
zation method in the CO2 supercritical conditions were conducted on crushed coal samples. The measurement apparatus
was designed specifically for the present swelling experiment using a visualization method. Crushed coal samples were
used instead of block coal samples to shorten equilibrium time and to solve the problem of limited availability of core
coal samples. Dry and wet coal samples were used in the experiments because there is relatively limited information
about how the swelling of coal by CO2 is affected by water saturation. Moreover, some coal seams are saturated with
water in initial reservoir conditions. The maximum volumetric swelling was around 3% at 10 MPa for dry samples and
almost half that at the same pressure for wet samples. The wet samples showed lower volumetric swelling than dry ones
because the wet coal samples were already swollen by water. Experimental results obtained for swelling were compara-
ble with other reports. Our visualization method using crushed samples has advantages in terms of sample preparation
and experimental execution compared with the other methods used to measure coal swelling using block samples.
Keywords: Coal Swelling Experiments; Visualization Method; CO2-Enhanced Coal Bed Methane Recovery; Low
Rank Coal
1. Introduction
CO2 capture and storage (CCS) is a method expected to
reduce emission of CO2 into the atmosphere. In fact,
geological sequestration is most likely to be the only op-
tion that will allow CO2 storage in large enough quanti-
ties over long geological periods of time. One of the
geological sequestration options is injection of CO2 into
coal seams. This option is considered to be a safe and
effective method for permanently storing CO2 with the
added value of enhanced coal bed methane recovery
(CO2-ECBMR). It has been suggested that the CO2 sorp-
tion capacity of coal seams is typically between 2 to 10
times higher than that for CH4 depending on coal rank
CO2 injection into a coal seam increases the rate of
CH4 production considerably [4]. However, CO2 is also
known as a swelling agent for coal. Coal swelling in the
presence of water or CO2 is a well-known phenomenon,
and may affect the permeability of coal. Because the
permeability of a coal is perhaps the most important fac-
tor controlling the economic viability of CO2-ECBMR,
thus quantifying the effect of swelling on permeability
has become an important issue.
A number of studies investigating CO2-induced swell-
ing have been published. The maximum volumetric
swelling has been reported as 1% - 5% depending on
coal rank [5-7]. Reduction of CO2 injectivity has been
observed in some CO2-ECBMR field trials. One of these
projects was carried at Yubari, Hokkaido, Japan. The
Yubari pilot project for CO2-ECBMR identified the
problem of low injectivity of CO2 caused by a reduction
in permeability induced by coal swelling [3]. The decay
ratios of daily CO2 injection rate measured in the Yubari
pilot test are shown in Figure 1.
A field experiment of CO2-ECBMR carried out in the
upper Silesian basin of Poland is another example of a
pilot project that observed decreasing injectivity caused
by swelling. Evaluation of wellhead pressure and tem-
perature data during the pressure buildup and fall-off of
opyright © 2013 SciRes. IJG
the intermittent injection periods, considering phase be-
havior and density changes of the CO2, showed that the
buildup time decreases, and the fall-off time increases
over time. In addition, the steep decrease at the beginning
of the curve during start-up (with water down the hole)
disappeared in November and December 2004. Both ob-
servations indicate that permeability reduced over time
[8]. The reduced injectivity was presumably a result of
swelling of the coal after contact with CO2.
The only successful CO2-ECBMR project so far was
run between 1995 and 2001 at the Allison Unit in the San
Juan Basin in New Mexico, USA, but no monitoring
project was associated with this pilot [10]. It has been
recognized from the pilot test results that reduction of
permeability induced by coal swelling is one of the main
technical issues that needs to be resolved to put economi-
cal, large-scale CO2-ECBMR into practice worldwide.
In the case of CO2-ECBMR, adsorption of CO2 gas,
which has greater sorption capacity than CH4, would
cause coal matrix swelling and thus, in contrast to CH4
desorption, potentially reduce the cleat permeability of a
coal seam. Therefore, it is important to quantify the
swelling of coal induced by CO2 adsorption.
Most swelling experiments have been conducted using
core/block coal samples [6,7,11-13], so the equilibrium
time was long depending on core block size. Indonesia
has many coal resources as well as coal bed methane, so
shows potential for CO2-ECBMR sites. However, there is
a lack of research of coal behavior after introducing CO2;
in particular, coal swelling caused by CO2 adsorption.
Thus, our objective was to investigate the swelling char-
acteristics of Indonesia low rank coal (LRC) up to 10
MPa. Crushed coal samples that were treated as block
samples have been used in the experiments to allow
faster equilibrium time and to solve the problem of lim-
ited availability of core samples from deep underground.
2. Samples and Methods
2.1. Samples
Coal core samples taken from coal seams deep under-
5th 7th 9th
Apr. 19,2006
May 7,2006
Injection Ratio against First Day
Day from start/restart of CO
Base of Daily Injection Ra
y 25,2006
Aug. 28,2006
Figure 1. Decay ratio of daily CO2 injection rate monitored
at Yubari ECBM-pilot test [9].
ground are very limited. It’s costly and technically diffi-
cult to obtain representative core samples from deep un-
derground. Furthermore, coal that is anisotropic causes
other difficulties with regard to analysis and reproduci-
bility. Therefore, to overcome these difficulties, crushed
coal samples were used instead of core samples.
Three coal samples of Indonesia LRC were used in the
swelling experiments. Their properties are summarized in
Table 1.
2.2. Apparatus
A visualization method was used to observe the coal
swelling evaluated by surface movement of a column of
coal particles. Figure 2 shows a schematic diagram of
the measurement apparatus. The measurement apparatus
was designed specifically for this experiment, and con-
sisted of a high-pressure cell with two glass windows, a
hot water bath with a circulation pump, an electric pres-
sure gauge with a precision of 0.01 MPa (GE Druck Ltd,
DPI 282), thermocouples, an illumination source and line
valves. The cell temperature was kept at 48˚C by circula-
tion of hot water through a circular jacket covering the
cell, and the apparatus was positioned in a thermally in-
sulated box.
A digital camera was used to take photographs con-
tinuously during the experiment. All measurement data
from the camera, pressure transducer and thermocouple
were logged into the data acquisition system continu-
2.3. Procedures
The procedure for coal swelling experiments was as fol-
Step 1. Sample preparation
Measurements were performed in a glass cylinder 7.35
mm in diameter and 50 mm in length with a high preci-
sion scale. Two cylinders were placed in the pressure cell
Table 1. Detail coal properties for CO2 swelling experi-
Adb SK-01 SK-02 SK-03
Moisture, % wt 10.68 11.63 7.53
Ash, % wt 4.19 2.44 10.31
Volatile matter, % wt 40.34 41.72 45.17
C, % wt 55.18 57.09 53.76
H, % wt 4.22 4.49 4.62
N, % wt 0.72 0.79 0.73
Sulfur, % wt 0.12 0.19 0.25
R0, % 0.47 0.47 0.45
Copyright © 2013 SciRes. IJG
CO2 Gas Cylinder
Thermal Insulato r Wall
Image Analysis
Plan view of cell
Gas Booste
Releas e Val v e
Coal Sample
Figure 2. Schematic diagram of coal swelling measurement
system using visualization method.
for each measurement. Each cylinder was filled with dry
or water-saturated crushed coal particles.
The crushed coal samples were sieved into grain sizes
of 200 - 250 μm. For dry samples, crushed coal was put
into an incubator at 60˚C for 48 h under vacuum. After
complete drying, approximately 1 g of sample was
placed into a glass cylinder and centrifuged at 3000 rpm
for 10 min. These conditions were equivalent to a rela-
tive centrifugal force (RCF) of 554.4 as defined by Equa-
tions (1) and (2). The results shown in Figure 3 confirm
that there was no difference in the coal column after
3000 rpm.
10 cm RPM
r N
1.12RCF , (2)
where g is the gravitational acceleration of earth, r is the
rotational radius, ω is the angular velocity in radians per
unit time, rcm is the rotational radius measured in centi-
meters (cm) and NRPM is rotational speed measured in
For saturated samples, approximately 1 g of crushed
sample and 1 mL of pure water were filled in the cylinder
and then it was centrifuged at 3000 rpm for 10 min to
introduce air and ensure close packing of the sample.
Centrifugation was used to allow accurate measurement
of initial column height and ensure that the sample was
fully saturated with water. Both samples were placed in
the pressure cell for 24 h before CO2 injection.
Step 2. Swelling Measurement
The cell temperature was set at 48˚C and the CO2 in-
jection pressure was increased in a series of stepwise up
to 10 MPa. The system was maintained at each pressure
step at least for 48 h and even more. The equilibrium
time required for swelling of coal induced by CO2 ad-
sorption depends on the size of the sample. A bigger coal
sample will take longer to reach equilibrium and vice
versa. In our experiments, most of the swelling occurred
Figure 3. Column length of change as a function of relative
centrifugal force in 10 minutes.
within 24 h for each pressure step. After measurement at
the maximum step, the pressure was decreased to at-
mospheric conditions to allow comparison of final and
initial heights of the coal column.
The upward movement of the column surface was
measured against pressure and termed change of column
length (α). The proportion length of change, α (%), and
volumetric swelling, β (%), are defined by Equations (3)
and (4).
where h is increase length caused by CO2-induced
swelling, K is compressibility factor (MPa1), V0 is initial
volume (cm3), r is radius of cylinder (cm), p is pressure
(MPa) and p0 is initial pressure (MPa).
3. Results and Discussion
3.1. Swelling Experiments
Crushed samples with particle sizes of 200 - 250 μm
were used to avoid particle-packing problems. The sam-
ples were centrifuged to ensure that the particles packed
into interstitial spaces and were distributed evenly. In
addition, Busch et al. [14] reported that sorption rate did
not change significantly with increasing grain size >100
μm, thereby increasing ash content during grinding can
be eliminated.
Swelling was represented by upward surface move-
ment of the coal packing column, as shown in Figure 4.
As shown in Equation (5), the total initial volume
is equal to the volume of each particle i and CO2 gas
filling the interstitial space. Because swelling of coal
Copyright © 2013 SciRes. IJG
Copyright © 2013 SciRes. IJG
Crushed sam
les Centrifugalize
Inside the pressure cel
condition Afte r CO
Coal particles
Modele d li
coal block
Figure 4. (a) Model showed the experimental procedure for crushed samples which was modeled like coal block; (b) Picture
taken during experiment showed different column height after put in the centrifuge (left) and inside the pressure cell before
and after CO2 injection (right).
samples is proportional to adsorbed CO2 gas molecules
and CO2 mass was kept constant at each pressure step, it
was considered that the total initial volume was equiva-
lent to the total volume of each particle. Moreover, the
swelling ratio was the ratio of the total volume of the
particles at a certain pressure (Pn) to the initial volume of
the particles, as given in Equation (6).
0, (5)
vP V
. (6)
Because upward surface movement of the coal column
was assumed to be the total expanded volume for each
particle, consequently, crushed samples were modeled
like coal block. A diagram illustrating this model is
shown in Figure 4.
Most of the swelling in the samples occurred within 24
h of exposure to CO2; after that, swelling increased
slightly over a considerable length of time, as depicted in
Figure 5. The length of both dry and wet sample col-
umns increased with increasing CO2 pressure. A rapid
increase in length was observed in the low pressure range
(6 MPa), and the maximum length was reached between
8 and 10 MPa.
The results of volumetric swelling for three coal sam-
ples as a function of pressure and gas density are plotted
in Figure 6. Previous studies observed that CO2-induced
coal swelling fitted the Langmuir model [11,13]. The
Langmuir model has been widely used and is used as a
standard by many industries. However, it gave a poor fit
in the pressure range above 6 MPa [6]. As an alternative,
Day et al. [6] suggested that the Dubinin-Radushkevich
(D-R) adsorption model can give a better fit. The D-R
model uses gas density instead of pressure according to
Equation (7) [7]:
max eag
, (7)
where εmax is the maximum volumetric swelling of the
coal, ρg is the density of the gas at the temperature and
pressure, ρa is the density of the condensed phase on the
coal surface, and D is a constant that is related to the en-
thalpy of adsorption. ρa for CO2 is 1028 kg m-3 and both
D and εmax are considered empirical curve-fitting pa-
rameters. The gas density (ρg) at each pressure step was
calculated using the PROPHAT program. Figure 6
shows a plot of the D-R model. A summary of the results
for both the D-R and Langmuir models is presented in
Table 2.
Based on these results, the maximum volumetric swell-
ing ratio measured is very similar to ones reported by
[5,6,15-17], who used blocks and crushed coal samples.
The maximum swelling from [6] with Ro 0.89% was
~2.6% at around 8 MPa and 40˚C. Moreover, the maxi-
mum swelling of lignite samples reported by [5] was
around 4.18% at 1.5 MPa.
Day et al. [6] used block coal with dimensions of 30 ×
10 × 10 mm, while [5] used pencil-shaped samples that
were 10 mm in length and 4 mm in diameter. Such sam-
ples are difficult to prepare because LRC tends to break
when cut into small pieces using a rock cutter with water
as a lubricant. Therefore, swelling experiments using
crushed coal are advantageous in terms of sample prepa-
ration and experimental apparatus compared with ones
using blocks.
CO2 adsorption equilibrium is reached faster for coal
of smaller grain size [18]. For the crushed samples used
in this study, swelling equilibrium time, which correlates
Figure 5. Length of change as a function of time and pres-
Table 2. D-R and Langmuir’s parameters for three coal
SK-01 SK-02 SK-03
Dry Wet Dry Wet Dry Wet
D-Rs Model
εmax (%) 2.81 1.45 3.33 1.57 3.04 1.51
D 0.0859 0.0827 0.0761 0.0910 0.0827 0.0953
Langmuirs Model
εL (%) 3.53 1.96 4.53 2.34 3.95 2.17
PL (MPa) 4.53 5.07 4.82 6.58 4.63 6.46
strongly with adsorption equilibrium time, was shorter
than that for blocks. Reucroft and Sethuraman [5] em-
ployed an exposure time of 200 h in their study. In our
experiments, maximum swelling occurred within 12 - 24
h for each pressure step. This was another benefit of us-
ing crushed samples.
One of the remaining concerns of using crushed sam-
ples was whether the samples return to their original po-
sition or not. Coal swelling experiments using block
samples showed that all of the coal samples return to
their original dimensions after removal from CO2 [6,7].
In the present study using crushed coal, we observed the
same tendency. Unlike some liquid solvents, exposure to
gas does not impart enough molecular mobility to allow
relaxation of the structure of coal. Thus, the coal remains
elastic, and the swelling process is reversible [19].
Otake and Suuberg [17] reported that the packing den-
sity of particles could affect swelling measurements and
introduce significant artifacts. To solve this problem, we
used small particle sizes (200 - 250 μm) and sample col-
umns were centrifuged to ensure that the particles packed
into the interstitial spaces and were distributed evenly.
Our results suggest that the processes of swelling are
similar in crushed and block coal samples.
The experimental results shown in Figure 6 indicate
that volumetric expansion of wet coal is lower than that
of dry samples. Swelling of wet coal decreased by
Figure 6. Volumetric expansion of the three coal samples at
48˚ under exposure to CO2, the line plot represents the mo-
dified D-R model results.
Copyright © 2013 SciRes. IJG
around 50% compared with that of dry coal. In accor-
dance with [7], the effect of moisture on the amount of
swelling is higher in LRC samples.
When considering the wet samples, one should re-
member that they are actually already swollen because of
the presence of moisture. It has been suggested that the
mechanism of swelling is the same in both water and gas
[20]. Therefore, the total volumetric swelling of wet coal
is the sum of swelling induced by both water and gas.
Considering the crushed coal samples used in this
study, instead of swelling both perpendicular and parallel
to the bedding plane, experimental results only showed
“iso-swelling”. The term iso-swelling was used to show
that volumetric swelling was calculated without consid-
ering the difference between linear swelling parallel and
perpendicular to the bedding plane.
In fact, based on some previous studies, linear swell-
ing is significantly different between perpendicular to
bedding plane and parallel to the bedding plane. Swelling
varies by 30% - 400% depending on the coal properties,
and perpendicular swelling is always greater than parallel
To investigate this phenomenon, some equations have
been used to predict linear swelling parallel and perpen-
dicular to the bedding plane. In swelling experiments
using a strain gauge where volumetric swelling was cal-
culated as twice the strain parallel to the bedding plus the
strain perpendicular to the bedding [11,12], the following
equation was used:
123ls lslsvs
2lpar per
 , (8)
where εvs is volumetric swelling, and εls1, εls2, and εls3 are
the principal linear strain. The volumetric swelling can
be determined by adding the linear strain,
, (9)
where εlpar and εlper are strains parallel and perpendicular
to the bedding plane, respectively. To formulate Equation
(9), it was assumed that the strain perpendicular and par-
allel to the bedding plane could be predicted using Equa-
tion (10),
par par
3.2. Dependence of CO2 Swelling on Coal
Because the samples used in these experiments have
similar coal rank, the difference between swelling results
was too small to allow meaningful comparison. Mean-
while, some authors [5,7,11,19] concluded that swelling
decreased with increasing coal rank. Figure 7 shows the
maximum coal swelling (ɛmax) as a function of coal rank
from our results and those reported [6,7,11-13,21,22].
There is a dependence between coal rank and swelling;
higher swelling corresponds to lower coal rank.
Some considerations have to be made for the LRC
where the data are scattered. Coal swelling by CO2 injec-
tion is strongly controlled by CO2 adsorption volume.
Based on [23], the dependence of CO2 sorption on coal
properties is controlled not only by individual factors but
also the combination by several factors; e.g., coal com-
position, TOC, ash content, dull and bright coal as well
as coal rank. Meanwhile, gas sorption tends to increase
with increasing rank, although some exceptions were
found in [18]. Beulah-Zap lignite, which was the lowest
rank of all samples, showed the highest sorption capac-
Coal of higher rank has been subjected to a more se-
vere stress history than LRC, so it in general exhibits
more structural deformation and is thereby more difficult
to swell. Larsen et al. [19] explained that higher rank
coal rearranges in the opposite direction to LRC to form
a less-associated structure. Thus, coal properties should
be well characterized to evaluate coal swelling because
swelling does not only rely on rank.
Within the sample set studied, a general tendency was
observed for D to increase for wet samples, although
sample SK-01 was an exception. Increasing D is related
to the enthalpy of adsorption based on Wood’s equation
(Equation (11)), whereas E of CO2 is reduced in the pres-
ence of moisture [7,24]. This apparent reduction in en-
thalpy may be caused by higher energy adsorption sites
being occupied by water molecules in the wet samples
(by hydrogen bonding), thus restricting gas sorption to
sites of lower energy [7].
Figure 7. The maximum coal volumetric swelling (ɛmax) as a
function of coal rank (black cube represent swelling data
from present experiment results).
Copyright © 2013 SciRes. IJG
where R is the universal gas content, T is the temperature
and β is a scaling factor to account for the different af-
finities of sorbates for the sorbent material.
In the case of published data [7,24], high rank coal
with vitrinite reflectance (R0) more than 0.76 [24] and
0.62 to 1.40 for [7] was used. However, the lowest rank
sample in [7] showed decreasing D value, which they
attributed to moisture preferentially occupying polar sites
on the coal surface. In dry samples, it appears that CO2
and CH4 can readily adsorb on these polar sites, but when
water is present, they become unavailable to other sor-
bates. Thus, in moist LRCs with a high proportion of
polar sites, much of the internal surface of the coal is
already covered by water molecules. As a result, there is
relatively little surface available for gas adsorption and
therefore little swelling.
Based on our data using LRC, both decreasing and in-
creasing D values were observed. Figure 8 shows D
value as a function of coal rank based on current experi-
ments and reported data [6,7,11-13,21,22], which indi-
cates that D value does not have a strong dependence on
coal rank. Thus, because CO2 swelling induced by CO2
adsorption correlates strongly with the amount of CO2
adsorbed, care should be taken when considering the
factor(s) causing coal swelling.
4. Conclusions
In this study, a visualization method was used to measure
Figure 8. D value as a function of coal rank (black cube
represent swelling data from present experiment results).
swelling of crushed coal samples in a pressure cell with
CO2 pressure of 0.1 to 10 MPa and temperature of 48˚C.
Crushed coal samples provide benefits in terms of sam-
ple preparation and experimental ease compared with
block coal samples. The other benefit of this method is
the relatively short equilibrium time needed to reach
maximum swelling.
Coal swelling was represented by upward surface
movement of columns filled with crushed coal particles.
The total expanded volume of coal particles was modeled
like the swelling volume of coal block. Present swelling
results were comparable with published data.
The maximum swelling volume ratios for the three
coal samples achieved by CO2 adsorption were around
3% at 10 MPa of CO2. Wet coal samples showed lower
volumetric expansion than dry ones because moisture
considerably reduced the degree of gas-induced swelling.
Although wet coal swells less than dry samples, wet
samples are in fact already partially swelled because of
the presence of water. In addition to calculating swelling
perpendicular and parallel to the bedding plane instead of
volumetric swelling, the equation developed in this study
can be used.
5. Acknowledgements
This study was financially supported by the JSPS KA-
KENHI Grant-in-Aid for Scientific Research (B); Num-
ber 24360372.
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