Swelling-induced volumetric strains internal to a stressed 

coal associated with CO2 sorption 

C. Karacan 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA. United Slates 

Abstract 

It is generally accepted that typical coalbed gases (methane and carbon dioxide) are sorbed (both adsorbed and absorbed) in the 

coal matrix causing it to swell and resulting in local stress and strain variations in a coalbed confined under overburden pressure. 

The swelling, interactions of gases within the coal matrix and the resultant changes in the permeability, sorption, gas flow 

mechanics in the reservoir, and stress state of the coal can impact a number of reservoir-related factors. These include effective 

production of coalbed methane, degasification of future mining areas by drilling horizontal and vertical degasification wells, 

injection of CO2 as an enhanced coalbed methane recovery technique, and concurrent CO2 sequestration. Such information can 

also provide an understanding of the mechanisms behind gas outbursts in underground coal mines. 

The spatio-temporal volumetric strains in a consolidated Pittsburgh seam coal sample were evaluated while both confining 

pressure and carbon dioxide (C02) pore pressure were increased to keep a constant positive effective stress on the sample. The 

changes intemal to the sample were evaluated by maps of density and atomic number determined by dual-energy X-ray computed 

tomography (X-ray CT). Early-time images, as soon as CO2 was introduced, were also used to calculate the macroporosity in the 

coal sample. Scanning electron microscopy (SEM) and photographic images ofthe polished section ofthe coal sample at X-ray CT 

image location were used to identify the microlithotypes and microstructures. 

The CO2 sorption-associated swelling and volumetric strains in consolidated coal under constant effective stress are 

heterogeneous processes depending on the lithotypes present. In the time scale of the experiment, vitrite showed the highest degree 

of swelling due to dissolution of CO2, while the clay (kaolinite) and inertite region was compressed in response. The volumetric 

strains associated with swelling and compression were between ± 15% depending on the location. Although the effective stress on 

the sample was constant, it varied within the sample as a result of the intemal stresses created by gas sorption-related structural 

changes. SEM images and porosity calculations revealed that the kaolinite and inertite bearing layer was highly porous, which 

enabled the fastest CO2 uptake and the highest degree of compression. 

Keywords: Coal; Carbon dioxide; Sorption; Swelling; Sequestration; X-ray computed tomography; Volumetric strain; Gas outbursts 

1. Introduction 

Sorption of typical coalbed gases (methane and carbon 

dioxide) in the coal matrix forces it to swell, resulting in 

localized stress and strain variations in a coalbed confined under overburden pressure. Such changes impact 
commercial production of coalbed methane, potential

degasification of future mining areas by drilling horizontal and vertical degasification wells, enhanced methane

recovery using CO2 injection, and concurrent sequestration of CO2. Such knowledge can also provide an understanding of the mechanismsleading to gas outbursts in

underground coal mines.

The presence of large amounts of carbon dioxide in a

coalbed due to coalification, hydrodynamics, igneous

activity, or as a result of injection is particularly important. Carbon dioxide has a greater affinity towards

coal than methane and is not only adsorbed on the coal

surface, but is also dissolved in the coal matrix

(Karacan, 2003; Larsen, 2004) like an organic liquid.

This process modifies the physical structure of the coal

matrix and may even extract some of the polycyclic

aromatic hydrocarbons and mobilize them in the

coalbed (Kolak and Burruss, 2006). The modification

in physical structure is associated with a relaxation and

rearrangement of the macromolecular structure of the

coal. The presence of CO2 produces a greater change in

the initial pore structure (Larsen, 2004) than does the

presence of methane. One impact of CO2 interaction

with coal may be more violent and difficult-to-control

gas outbursts, when compared to those caused by methane alone (Lama and Saghafi, 2002). CO2 injection,

with concurrent production of methane, can cause

swelling of the coalbed particularly in the near wellbore

area and a change in its permeability, a factor which

limits the development and potential field application of

this technology (Reeves, 2003). Coal swelling affects

the accuracy of gas adsorption measurements in the

laboratory, too. Since coal swelling is dependent on the

CO2 pressure, both coal surface area and the error due to

coal swelling will change with CO2 pressure (Özdemir

et al., 2003; Romanov et al., 2006a,b).

The molecular rearrangement of coal caused by

molecules dissolving in the coal can be explained by

classical polymer chemistry. Coals are glassy, strained,

cross-linked macromolecular systems that are not at

their lowest energy state (Larsen et al., 1997). The

brittleness of coals is due to their glassy structure, which

has intramolecular interactions greater than the available

thermal energy, and the molecules have limited freedom

to move except for some small-scale vibrations and

rotations. When the coal interacts with a solvent, the free

volume of the polymeric system increases and lowers Tg

(glass transition temperature), the temperature at which

the glass becomes a rubber. For coals having less than

about 85% C, the rearrangement of coal molecules is

towards a more highly associated structure in which the

solubility of liquids is reduced, sometimes by one-half.

This suggests that under long-term storage or injection

conditions with CO2 absorbed in a coal, the coal undergoes a slow rearrangement that will decrease the

solubility of this gas, expelling it from the structure. The

situation with coals having C levels in excess of 85% is

more complicated. Thus, it would not be surprising to

expect different rearrangements, sorption, and rate

behaviors when the coal rank changes.

Karacan (2003) showed the heterogeneous character

of CO2–coal interactions and the behavior of different

microlithotypes within the coal. The study showed that

vitrite, liptite, and clarite densities decreased with

increasing gas pressure. The results indicated dissolution of CO2 in coal, which created some free volume so

that the macromolecular structure of the coal could relax

or swell on the time scale of the experiment. The density

change kinetics of vitrites, liptite, and clarite showed a

“breathing” (expansion–contraction) behavior, during

which the density bounced back by expulsion of excess

CO2 revealing an “overshoot” swelling behavior, a similar phenomenon that was observed before by Gao et al.

(1999) on a bituminous coal. This phenomenon was

caused by the diffusion and solution of gas molecules in

the macromolecular structure of mainly vitrites, followed by subsequent swelling and then rearrangement

to a more highly associated structure.

The changes in Pittsburgh No. 8 coal structure caused

by CO2 sorption and the mechanism of diffusion were

studied by Goodman et al. (2005). They measured the

sorption of CO2 using a high pressure ATR-FTIR

(attenuated total reflectance-Fourier transform infrared)

spectrometer. They demonstrated that after the first

sorption at 55 °C and 0.35 MPa, subsequent sorption

was much faster, indicating that structural changes in the

coal occurred after the first sorption. They also noted

that the rearrangement was not reversed as CO2 was

removed from the coal. They concluded that CO2

plasticized the coals and changed subsequent diffusion

rates, adsorption isotherms, and solubilities.

It has been shown that coals possess anisotropic

behavior in swelling and the degree of swelling rate is

larger perpendicular to the bedding plane, compared to

parallel to the bedding plane, where bond density is

lowest (Cody et al., 1988; Ceglarska-Stefanska and

Czaplinski, 1993; Larsen et al., 1997). This behavior

and the anisotropic structure of coal have been attributed

to the stress conditions under which the coal was generated (Cody et al., 1988). Majewska and Zietek (in

press) observed the anisotropic swelling of cylindrical

coal samples subjected to sorption–desorption cycle

using CO2 by employing strain gauges and acoustic

transducers.
Evaluation of the degree of swelling at particular

thermodynamic conditions is important since significant

coal matrix swelling can affect various processes. In the

case of CO2, significant swelling or volume increases

ranging from 0.75% to 4.18% were observed in a range

of coal samples when they were exposed to CO2 at

pressures up to 1.5 MPa (Reucroft and Sethuraman,

1987). Increases in pressure produced increases in

swelling and a decrease in the time required to reach

maximum swelling. Robertson and Christiansen (2005)

found a linear strain of less than 1.0% in CO2 for

pressures up to 5.3 MPa using optical techniques. Their

linear strain for a sub bituminous coal was 2.1%, which

was more than for a bituminous coal. Levine (1996)

measured a swelling ratio of 0.41% at 3.1 MPa for

bituminous coal samples from Illinois. He suggested

that the swelling function was similar to the Langmuir

isotherm. Harpalani and Chen (1995) found a linear

relation between the coal matrix volumetric strain and

the quantity of gas desorbed. Majewska and Zietek (in

press) reported volumetric strains up to 16% on a

bituminous coal sample from Poland during sorption of

CO2 at 4 MPa. Different volumetric strain measurements in different studies may be related to factors such

as sample composition, experimental method, sample

size, and the temperature of the experiment.

The swelling process may play an important role in

determining the strength of a coal, the speed at which a

coal can absorb additional gas, and the impact of

permeability changes on the gas flow mechanics in the

coalbed reservoir. These are important factors for

evaluating the long-term effects of CO2 injection into

a reservoir for sequestration, for assessing the integrity

of well completions, and for understanding the mechanics of violent CO2-associated outbursts in coal mines.

Beamish and Crosdale (1998) listed major outburst

incidents in the world. The incidents are usually coincident with the gassy seams in Australia, in Europe and

in China and most of them are originated either purely

from CO2 or from CH4/CO2 gases combined. Outbursts,

besides the degree of shearing of the coal, are associated

with coals of high gas contents, low permeabilities that

lead to high gas pressures, low mechanical strength, and

high stress levels in the seam (Lama and Saghafi, 2002;

Li and Shimada, 2003) which are all conditions arising

from swelling of the coal matrix. In another instance,

CO2 injectivity in the Allison Unit (Reeves, 2003)

decreased because of reduced near-wellbore permeabilities, possibly due to matrix swelling and reductions in

cleat porosity and permeability. Thus, estimations of

coal swelling are important for evaluating coalbed

behavior. Currently, the most widely used predictive

technique to describe porosity changes as a result of gas

sorption and relating those changes to permeability is

the Palmer and Mansoori model (1998). Recently, Pan

and Connell (2007) proposed a predictive surface

energy–elastic energy balance approach to describe

swelling and shrinkage of coal matrix as a result of gas

adsorption by using theoretical and experimental

parameters. Although the model predicts swelling data

reasonably well using literature data, it does not include

the thermodynamics of polymeric behavior. Thus, it

assumes that swelling occurs as a result of only surface

forces, which may be an invalid assumption for coal–

CO2 systems.

This study presents an evaluation of swelling-induced volumetric strains occurring internal to a stressed

coal subjected to CO2 pressure. The evaluation is based

on calculations using the quantitative, time-resolved

spatial maps of coal density and mass percentage of

sorbed CO2 in the coal derived from effective atomic

number computations. The data are presented as swelling/compaction maps and values along profiles. SEM

(scanning electron microscopy) images of different

locations of interest were taken to interpret the observed

behavior. The aim of this study is to document the

swelling behavior of a confined and stressed coal under

CO2 exposure and to identify the behaviors of different

microlithotypes.

2. Experimental

Karacan (2003) has provided a more in-depth

overview of the experimental technique used in this

study. However, the approach and some relevant details

to the present study will be summarized here.

2.1. CO2 sorption experiment

A high volatile bituminous coal from the Pittsburgh

seam (DECS-12) was cored parallel to the bedding

planes to obtain a cylindrical sample (2.5 cm in diameter

and 2.0 cm long). The average elemental composition

of the sample was: 74.78% C, 5.11% H, 1.23% N,

1.12% S, and 7.51% O. The sample had 2.5% moisture

as received and 10.25% ash in proximate analysis.

The CO2 sorption experiment was conducted in a

biaxial pressure cell made up of aluminum, which is

capable of applying CO2 gas pressure and confining

pressure to the sample separately. The pressure cell

containing the sample was placed on the computercontrolled table of a fourth-generation medical X-ray

CT scanner, which was capable of producing a singleenergy-level scan in 4 s. The pixel resolution and the
image thickness were 0.25 mm and 2.0 mm, respectively. During this study, the sample was scanned at two

energy levels: 130 kVand 80 kV with an X-ray intensity

of 65 mA. The 512 × 512 images generated by the

scanner were later cropped to construct 100 × 100 data

matrices focusing only on the coal.

The pressure history of the experiment was as follows. Initially, confining pressure on the sample was

set to 1.36 MPa, which was the effective pressure on

the sample that would be kept constant throughout

the experiment. The sample was scanned to establish the

condition of the sample under vacuum but with the

1.36 MPa confining pressure. The sample was pressurized with 1.70 MPa CO2, while increasing the confining

pressure to 3.06 MPa. The sample was scanned until

equilibrium, which took approximately 7240 min. The

next sorption pressure and confining pressure were

3.06 MPa and 4.42 MPa, respectively. As before, the

sample was scanned for another 7240 min. The procedure was repeated for 4.42 MPa gas pressure and

5.78 MPa confining pressure for the last 5000-minute

period. This experimental methodology enabled the

experiment to be conducted at three different gas

pressures while keeping the sample under constant

effective stress.

After the experiment, the coal sample was recovered

from the core holder, vacuum impregnated with a coldsetting epoxy resin, and cut at the approximate spot of

CT-scan location. The sample was ground and polished to analyze the surface by using the 20-point

ocular procedure defined by the International Committee for Coal Petrology (1963). A more detailed

explanation of this procedure is given in Karacan and

Mitchell (2003).

The polished surface was later coated with carbon to

analyze the locations of interest by SEM using a 20 kV

electron beam and different magnifications. EDX

(Energy Dispersive X-ray) analysis coupled with the

SEM system was used to identify the elements present at

points of interest.

2.2. Processing of dual-energy images

Dual-energy X-ray scanning enables the independent

determination of density and composition from CT data

and thus enables the quantitative investigation of the

material while using two different beam energy spectra

(high: h and low: l) to distinguish the effects of different

components in mixtures:

bhllðx; yÞ−bllhðx; yÞ qðx; yÞ 1⁄4 ð1Þ bhal−blah

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi allhðx; yÞ−ahllðx; yÞ Zeðx; yÞ 1⁄4 3:8 ð2Þ bhllðx; yÞ−bllhðx; yÞ

In these equations a and b are energy-dependent

constants. Calibrating the scanner readings with materials of known density and effective atomic number

enables bulk density (ρ) and effective atomic number

(Ze) maps of the scanned object to be generated at each

pixel position (x,y) independently using linear attenuation of X-rays (μ) by solving for the energy-dependent

constants.

For mixtures of atoms, an effective atomic number

(Ze) is used instead of Z. The relevant literature features

various definitions of effective atomic number. Schätzler (1979) proposed a formula where the atomic number of each element is weighted by the mass percentage,

ωi, of each element:

X Ze 1⁄4 xiZi ð3Þ

i

In order to map the mass percentage of CO2 at any

location, temporal effective atomic number maps were

used. For this purpose, the effective atomic number of

CO2 and the spatial distribution of effective atomic number of the initial coal matrix were considered as simple

elements with Ze electrons per atom (Duvauchelle et al.,

1999). This established an analogy with the classical

atomic model and the use of Eq. (3) to calculate the mass

percentages of CO2 at any given time during CO2

sorption.

By using the relation given in Eq. (4) and the

temporal and spatial distribution of effective atomic

number of coal (Eq. (2)) for each scan time, we can

obtain the mass percentage of CO2 in the coal using

Eq. (5).

X Zeðx; yÞ 1⁄4 xiðx; yÞZiðx; yÞ ð4Þ

i

Ze@tðx; yÞ−Zcoalinitialðx; yÞ xCO2 ðx; yÞ 1⁄4 ð5Þ ZCO2−Zcoalinitialðx; yÞ

In this equation, Ze@t(x,y) is the temporal effective

atomic number map of coal at pixel location (x,y),

ωCO2

(x,y) is the mass percentage CO2, ZCO2 is the effective atomic number of CO2 (7.581), and Zcoalinitial(x,y)

is the effective atomic number of the initial coal material

before sorption of CO2. The images were processed by

using the programs written in PV-WAVE command

language.
3. Results and discussion

3.1. Sample characterization

Visual analysis of the sample showed varying bands

of bright and dull layers of varying thicknesses and

mineral bands. However, it is apparent from Fig. 1 that

one side of the sample can be characterized more by a

dull, high-mineral-matter containing region (light colors), which was highly concentrated with clays and

inertite type microlithotypes, compared to the other side

of the sample, which contained thicker and more

persistent bright layers (darker colors in Fig. 1), mainly

rich in vitrite and clarite. Microlithotype analyses using

a 20-point ocular procedure defined by the International

Committee for Coal Petrology (1963) showed that

the sample had about 32.0% vitrite, 1.2% inertite,

28.5% clarite, 10.2% vitrinertite, 8.3% duroclarite, 1.3%

clorodurite, 7.4% partings, 9.4% carboargilite, and 1.9%

carbopyrite.

Microlithotypes at some specific locations were also

identified using petrographic analysis. These locations

are shown in Fig. 1, and the identifications are listed in

Table 1.

3.2. Mapping of macroporosity

X-ray attenuation is linearly additive for a composite

material such that the fractional contributions from the

Fig. 1. A micro-focus X-ray tomography image showing the internal

structure of coal sample at the scan location. Bright layers are higher

density, clay and rock layers whereas darker layers are organic

lithotypes of varying macerals. The figure also shows various locations

whose microlithotype identification was documented in Table 1.

Straight, diagonal lines are the profiles along which macroporosity

values are shown in Fig. 2.

Table 1

Microlithotypes determined at the positions shown in Fig. 1

Location Microlithotype

a Pyrite particles in a matrix of resinite matrix

b Mineral parting and carbargilite

c Vitrite

d Vitrite and clarite

e Pyrite layer in a matrix of liptite

f Pyrite particles in a matrix of resinite and other

liptite macerals, and vitrite

g Clay and inertite

h Vitrite, inertite and liptite

I Clarite, cutinite and vitrinertite

j N/a

k N/a

attenuation coefficients of each of the pure materials

give the total attenuation. This property helps to

calculate the porosity in each voxel from images taken

at a single energy level. The conventional method of

calculating porosity with single-energy CT scanning is

to saturate the pore space with two different fluids and

determine the change in attenuation of X-rays in the

sample due to the presence of these two fluids in pore

volume.

For calculating the voxel-resolved local macroporosity distribution within the coal sample at the X-ray

CT-scan location, the scans taken before, CTt0, and

immediately after, CTt1, introducing 1.70 MPa CO2

were used as the sample was under 1.36 MPa effective stress. The following equation was used for this

calculation:

CTt1ðx; yÞ−CTt0ðx; yÞ /ðx; yÞ 1⁄4 ð6Þ CTCO2−CTair

In this equation CTCO2 and CTair are the CT numbers

(a quantity in Hounsfield units representing the attenuation of X-rays) of pure CO2 and air filling the macropore

space, respectively. CT numbers of CO2 and air were

determined by scanning the free gas phase that was in

the calibration phantom placed in the sample cell for

this purpose. The CT numbers determined as such

were −835 for air and −655 for CO2 (at 1.7 MPa),

respectively.

Macroporosity data have been extracted along the

profiles I–II, III–IV, and V–VI, shown in Fig. 1.

According to the microlithotype identification, profile

I–II is mainly along the clay and inertinite band, profile

III–IV is along the liptite layer with pyrite and mineral

inclusions, and V–VI is within the clarite and vitrite

layers.
Results of the porosity analysis showed that the

microlithotypes with maceral compositions that were rich

in inertinite, mineral, or clay had higher porosities

compared to the microlithotypes rich in vitrite and liptite.

Depending on the location along I–II profile, the porosity

values changed between 2% and 12% (Fig. 2-A). In the

section where the profile is completely within the band

(Fig. 1), the value is approximately 12% and is probably

more representative for clay and inertite, both known to

be porous materials. This was confirmed by SEM images

taken at this section (Fig. 3-A and B). SEM images show

Fig. 2. Macroporosity of the coal determined by computer tomography

method along profiles, A: I–II, B: III–IV and C: V–VI, shown in Fig. 1.

that the porosity is mainly formed by either large

openings or as spaces between the flaky clay sheets.

EDX scans taken on the magnified image (Fig. 3-B)

show that the average elemental composition is 55.5% O,

13.0% Al, and 28.0% Si and minor amounts of K, Fe, and

Mg. These compositional data suggest that the clay is

either kaolinite or kaolinite/illite. In addition to offering

good storage space for free gas, this structure also offers

good permeability and high surface area where the

accessibility, gas transport, and diffusion rates are high

for increased sorption of CO2 either on clay or inertite

(Karacan, 2003).

Porosity of the III–IV profile (Fig. 2-B) is generally

2–3% because of the expected low macroporosity of

liptite, with some regional increases to 6–7% where

mostly irregular mineral-filled fissures (Fig. 3-C) and

intermittent pores in the maceral domain (Fig. 3-D) are

present. Similarly, the porosity along the V–VI profile

averages 2% (Fig. 2-C) because of low porosity of the

vitrite–clarite domain. The SEM image taken at this

location (Fig. 3-E) shows mineral patches with average

compositions of 74.00% O, 8.03% Al, 0.40% Mg,

11.70% Si, 5.22% S, 0.60% K, and 0.30% Fe.

3.3. Evaluation of volumetric strains within the coal as

a response CO2 sorption

One goal of this paper is to demonstrate and to

quantify volumetric strains occurring in confined and

stressed coals as a response to sorption of CO2. This

paper will also show how different microlithotypes are

involved in this process. Previous work documented the

linear and volumetric strain measurements of coal as a

result of CO2 sorption using different techniques, as

presented in the Introduction section. However, it

should be noted that almost all of these studies determined strains from the outside dimensions of the sample

under zero effective stress. This may not represent the

actual strain behaviors and magnitudes that may be

occurring under elevated positive effective stresses, as

seen in coalbed reservoirs. Moreover, the previous studies do not make any distinction between the microlithotypes and different sections of the sample.

The bulk density maps at the end of each CO2

pressure increment were calculated using Eq. (1). Since

the bulk density of coal masks the density increases (or

decreases) in the actual coal material because of sorption

of CO2, the maps were normalized using the density

map of the initial coal. This enabled observations of

only the changes within the coal. Fig. 4A, B, and C give

the calculated bulk density maps at the end of each

pressure regime (1.70, 3.06, 4.42 MPa).
Fig. 3. SEM images taken at the clay and inertite layer (A and B), at a mostly liptite layer (C and D) and at a vitrite and clarite layer (E), using the

polished section of the coal sample prepared from the scan location.

The bulk density maps given in Fig. 4 show that the

clay +inertite regions (light colors in figures) can uptake

a large amount of CO2, and their densities may increase

as much as 20% starting from the lower pressures up to

4.42 MPa. The clay type existing in these regions was

identified as kaolinite and illite, as discussed in the

previous section. These structures can uptake CO2 very

rapidly because their porous and flaky nature creates

high amounts of interlayer space, as evidenced by the

SEM pictures and macroporosities. The fast pore diffusion enables gas molecules to penetrate into the macromolecular structure more easily. They are adsorbed on

the large surface area of the clays, retained in the free

volume provided by the interlayer spaces of the clays,

and absorbed by the inertite because of easy accessibility to these sites. Clays are known to provide high

amounts of free space for fluid storage; however, there is

not enough information about CO2 sorption on natural

clays. In one study, Cimlerova and Arlt (2005) report a

surprisingly high loading of the clays (montmorillonite

and kaolinite) that they used in their CO2 adsorption

studies.

On the other hand, Fig. 4A, B, and C show that most

vitrite regions experienced a 6–10% density decrease

compared to the initial density of the coal as a result of

CO2 sorption. Since sorption is expected to increase the
Fig. 4. Normalized bulk densities calculated within the coal using dualenergy computer tomography at the end of A: 1.70 MPa, B: 3.06 MPa

and C: 4.42 MPa, CO2 pressure, while the sample was under 1.36 MPa

effective stress.

density of materials with fixed volumes, this density

decrease can be explained by the swelling of vitrites as

CO2 dissolves in their cross-linked macromolecular

structure. Although the rearrangement is largely controlled by the coal, CO2 acts as a plasticizer that adds

additional free volume to the macromolecular network

of glassy coal and lowers the Tg (glass transition

temperature). This relaxation enables molecular rearrangements in the coal to a lower energy state. Carbon

dioxide that is adsorbed and dissolved in the polymeric

structure of vitrites allows the structure to relax or swell

to conform to a new rubbery structure. Interestingly, this

shows that confined coals also swell. However, swelling

is more pronounced in vitrite and clarite-type microlithotypes compared to other microlithotypes that were

present in the studied coal.

Density maps give a semi-quantitative rating of the

amount of swelling, since density increase due to gas

dissolution and density decrease due to volume increase

counteract each other. The density data presented in

Fig. 4A to C and the mass fractions of CO2 (ωCO2

)

within the sample calculated using Eqs. (2), (4), and (5)

were used to quantify volumetric strains. A mass balance over each image element (pixel) led to calculation

of volumetric strains (defined as ΔV/V) as a function of

time using the following equation:

DV@t qinitialðx; yÞ−q@tðx; yÞ ðx; yÞ 1⁄4 Vinitial q@tðx; yÞð1−xCO2@tðx; yÞÞ

xCO2@tðx; yÞ þ ð7Þ 1−xCO2@tðx; yÞ

In this equation, ΔV/V is the volumetric strain at any

pixel location (x, y) in the coal at a particular time, t,

during gas sorption. ωCO2

(x,y) is the mass percentage of

CO2 in the pixel (x,y) at time t, ρ is the bulk density of

coal.

The volumetric strain maps (as fractions) calculated

using Eq. (7) are shown in Fig. 5A, B, and C at the

equilibrium levels of each pressure step. Comparison of

these maps with the internal structure image of the coal

(Fig. 1) suggests that the coal matrix is mobile and

rapidly exhibits more extensive swelling with increasing

amounts of dissolved gas (increasing pressure), especially in the vitrite bands. These maps show that vitrite

experienced 12.5–18.0% volumetric strain due to swelling at the end of 4.42 MPa pressure. The swellinginduced volumetric strains obtained for vitrites are close

to the volumetric strains (∼16%) measured by Majewska

and Zietek (in press), who used strain gauges for a

bituminous coal sample from Poland during sorption of

CO2 at 4.0 MPa.

The maps presented in Fig. 5 also show that as

swelling occurred, the clay and inertite layer was compressed to create space for the internal volume change in
Fig. 5. Volumetric strain maps (in fractions) internal to the coal as

a result of CO2 sorption at the end of various CO2 gas pressures

(A: 1.70 MPa, B: 3.06 MPa, C: 4.42 MPa), while the sample was under

1.36 MPa effective stress. Vertical lines represent the profiles (I–II and

III–IV) along different positions that have been used to extract the

volumetric strain data presented in Fig. 6.

a closed system. The SEM images presented in Fig. 3A

suggest that, in the case of clay layers, the pores and the

interlayer spaces of the clay plates made this compression possible. Volumetric strain maps indicate that the

amount of compression in clay and inertite layers was

around 10.0–17.5%, which is very close to the volumetric strains created by swelling of vitrites, suggesting

that the amount of compression was a reaction to swelling and was approximate in magnitude to the strains

created by the vitrites. Volumetric strains (as compression) observed in the clay layer are also close to the

calculated ∼10–12% macroporosity, suggesting that the

compression is made possible largely by the availability

of macroporosity within kaolinite and inertite layers.

Fig. 6A and B present the volumetric strains along

the profiles between I–II and III–IV shown in Fig. 5A to

C for different gas pressures. The change of volumetric

strains presented in these figures shows that the changes

are quite variable within the sample based on the location and the pressure. In these plots, negative volumetric

strains refer to the compressed regions, whereas positive

volumetric strains refer to the swelling. The data in

swollen locations show that the differential change in

Fig. 6. Volumetric strain data (as fraction) extracted along the profiles

shown in Fig. 5 as a function of location in the coal and CO2 pressure.
volumetric strains associated with increased pressure

may not be the same at different locations even if the

identified microlithotypes are the same. The observed

changes also show that the strains may not necessarily

increase in a linear or Langmuir manner as pressure

increases. This may be a consequence of spatial variations in local stresses experienced within the coal.

These variations may change the local sorption characteristics and glass transition temperature depressions

within the microlithotypes, resulting in changes in

volumetric strain differentials of the same or similar

microlithotypes between pressure increments. The effect

of local stress variations on the volumetric strains occurring within the sample may be similar to the sorption

and swelling behavior of polymers analyzed by Wissinger and Paulaitis (1987). They showed that at temperatures below Tg, the swelling and sorption behavior

depend on the glass transition pressure (Pg) in the presence of compressed CO2. If the pressure corresponding

to the maximum gas solubility is reached before Pg, then

swelling and sorption level off and reach limiting values

at elevated pressures. However, if Pg is less than the

pressure corresponding to maximum gas solubility in

the polymer, then sorption and swelling continue to

increase with pressure and the effect of pressure on

swelling is more pronounced than its effect on sorption.

It has been observed for polymers that hydrostatic

pressure usually increases the glass transition temperature Tg of a polymer (Bianchi, 1965; Wang et al., 1982).

However, if the pressurizing medium is soluble in the

polymer, a decrease in Tg with pressure is expected as

the polymer is plasticized by the environment, as

evidenced by both ultrasonic measurements of Young's

modulus and static measurements of the creep compliance of the polymer (Wang et al., 1982). Thus, in the

case of hydrostatically pressurized polymers, the

interplay between the effect of solubility and the effect

of pressure determines the onset of glass transition. The

effect of constant confining stress or effective stress, on

the coal during CO2 sorption has not been reported

before. This study proves that confined coals also can

swell due to CO2 dissolution in the coal matrix.

However, it does not document the effect of different

effective stress levels on swelling behavior. It has been

suggested (Karacan, 2003) that confining pressure may

favor the expulsion of extra CO2 by acting in the

opposite direction to expansion in a confined volume,

after the initial rapid swelling of coal due to structural

rearrangements to a more stable structure in which

solubility is reduced. A constant, prevailing, effective

stress on the coal may also change the glass transition

onset (Tg or Pg) for rubberization and thus may also

change swelling by affecting the thermodynamic parameters. This is an important consideration for underground coal reservoirs in relation to gas transport and

coal strength during CO2 injection.

The volumetric strain maps and graphical data in

Fig. 6 indicate that the volumetric strains within a coal

sample during gas sorption are very heterogeneous and

are functions of the local microlithotypes. The observed

heterogeneity in swelling behavior within a consolidated

and confined coal also suggests that a single sample may

not be adequate for determining the impact of CO2

sorption on volumetric strain, especially if the sample

lithology does not represent the average composition of

the coalbed.

The implications of the calculated volumetric strain

behavior can be significant for coalbed reservoir dynamics during a CO2 injection process or when CO2 intrusion occurs into the coalbed by natural processes, such

as igneous activity or hydrodynamics. Since the clay

layer was highly porous and probably very permeable,

the compression imposed by swelling vitrites on the clay

would likely decrease the permeability of the sample. If

a similar process occurs in a reservoir where primary

permeability is also controlled by clay veins, injectivity

losses or permeability reductions may be inevitable.

The observed swelling behavior of a confined coal in

response to CO2 sorption is based on the coal microlithotypes present. The restriction of permeable pathways in a swelled coalbed may have important

implications for the occurrences of catastrophic gas

outbursts in coal mines as well as injectivity losses

experienced during enhanced coalbed methane production operations. Gas outbursts during underground coal

mining generally associate with coalbeds of high gas

content but with low permeabilities that lead to

formation of high pressure gas pockets and with low

mechanical strength (Lama and Saghafi, 2002; Li and

Shimada, 2003). Li and Shimada (2003) showed that

coals in compressive environments and exhibiting

tightly compressed and broken fractures with inefficient

connectivity for gas flow to release gas pressure buildup

were the most likely candidates for gas outbursts. These

conditions can occur with swelling of coal matrix.

Ettinger and Serpinsky (1991) showed that gas sorption

changes the chemical potential of the coal and causes its

swelling. Overburden pressure constrains the swelling,

although internal forces lead to elevated internal

pressures and compressions within the system. The

swelling also increases the distance between coal atoms

that decrease the mechanical strength of the coal.

Mining of the coal changes the stress state of the coal

and may lead to a sudden gas outburst.
3.4. Evaluation of CO2 sorption

The strains created internally as a response to CO2

sorption raise the question how much CO2 was spatially

Fig. 7. CO2 sorption amounts (in mmol/g) within the coal at the end of

various CO2 gas pressures (A: 1.70 MPa, B: 3.06 MPa, C: 4.42 MPa),

while the sample was under 1.36 MPa effective stress.

sorbed within the coal. The computed total sorption

amounts using image data were mapped (in mmol/g) in

Fig. 7A to C for different CO2 pressures. The maps show

that the sorption amounts are variable within the sample,

depending on the microlithotypes. The data indicate that

most of the gas is retained by vitrite regions and by the

clay and inertite layers. The amount of CO2 sorbed by the

vitrites is between 2 and 4 mmol/g, while the clay and

inertite layers have comparable sorption capacity.

Although it is not surprising that the vitrites can uptake

high amounts of CO2, it is surprising to observe similar

quantities for clay and inertites. There are insufficient

data about CO2 sorption on natural clays, other than the

recent work of Cimlerova and Arlt (2005), who reported

a surprising high loading of the clays (montmorillonite

and kaolinite) in CO2. Based on the present work and the

work of Cimlerova and Arlt (2005), it can be argued that

the easy access of gas molecules to the high available

surface areas of clays may be one of the factors that

promote high sorption amounts. Particularly if the clays

are mixed with the inertites, as in the case of the sample

discussed in this paper, the flaky clay plates and the

macroporosity they create will provide rapid access to

inertites where CO2 can be sorbed.

4. Conclusions

It has been demonstrated that confined coals swell

during CO2 sorption. The swelling is possibly due to

dissolution and subsequent molecular rearrangements

following the glass transition temperature depression.

Swelling-induced volumetric strains are heterogeneous

within the coal depending on the maceral composition

of the coal material. It has been calculated that vitrites

are the most-swelling lithotype, and the volumetric

strains they create are in the order of 12.5–20.0% level

at 4.42 MPa pressure. As the positive volumetric strains

occur within the sample, they tend to compress any

other region with higher porosity or any open fissures

within the sample to create volume to expand. In this

work, a clay and inertite layer, which had open pores

and a highly porous flaky structure, was compressed.

The amount of negative volumetric strains experienced

by this layer was close to the amount of swelling.

Compression of high permeability clay bands, open

fissures, and cleats within the coal is the reason

for possible permeability reductions. Vitrites are the

responsible lithotype group for this reduction because of

the high volumetric strains they create upon CO2

sorption.

Heterogeneity of calculated volumetric strains within

the confined coal suggests that multiple samples should
be tested, possibly from different locations within the

seam or even from the same block of coal in order to

determine the volumetric strains as a response to gas

sorption. Otherwise, calculated volumetric strains may

not be representative of the coal seam being studied.

The calculated CO2 sorption on different microlithotypes shows that the highest amount of sorption occurred

on vitrites and on a clay–inertite layer (2-4 mmol/g). A

possible explanation for the high amount of sorption on

clay–inertite may be related to the SEM-evidenced

structure of that layer: a porous, flaky, and possibly

highly permeable structure enables fast gas transport and

improved access to the large surface area provided by

clay plates and inertite, where sorption can take place.

Acknowledgements

Dr. J.W. Larsen is gratefully acknowledged for

reviewing an earlier version of this paper and for

making valuable comments. Dr. Isaac Zlochower of

NIOSH is thanked for his help in SEM analysis.

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