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ARTICLE IN PRESSG ModelJGGC-718; No. of Pages 11

International Journal of Greenhouse Gas Control xxx (2012) xxx–xxx

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International Journal of Greenhouse Gas Control

j ourna l ho mepage: www.elsev ier .com/ locate / i jggc

ffect of coal petrology and pressure on wetting properties of wet coal for CO2

nd flue gas storage

. Shojai Kaveha,b, K-H. Wolfa, S.N. Ashrafizadehb,∗, E.S.J. Rudolpha

Department of Geoscience and Engineering, Delft University of Technology, Delft 2628CN, The NetherlandsDepartment of Chemical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran

r t i c l e i n f o

rticle history:eceived 5 March 2012eceived in revised form 7 September 2012ccepted 12 September 2012vailable online xxx

eywords:O2

lue gas

a b s t r a c t

The injection of carbon dioxide (CO2) or flue gas into coal layers enhances the coal bed methane pro-duction (ECBM) and offers an option for CO2-storage. The success of this process depends on differentfactors, among them wetting behavior of the coal plays an important role, which is a function of pressure,temperature, coal rank and composition of the gas.

The wettability behavior of wet coal samples (semi-anthracite), with respect to injection of syntheticflue gas and pure CO2, was investigated in a modified pendant drop cell at a constant temperature of318 K and pressures varying between 0.2 and 16 MPa. For the semi-anthracite Selar Cornish sample, thewettability alteration from intermediate-wet to gas-wet with CO2 injection was observed at pressures

torageCBMettability

ontact angleigh volatile bituminous coalemi-anthracite coal

above 5.7 MPa. Experimental results with synthetic flue gas revealed that the wettability of Selar Cornishcoal is intermediate-wet at all pressures and the contact angle only slightly increases with increasingpressure.

Comparison between high rank (semi-anthracite) and medium (high volatile bituminous (hvBb)) coalsconfirms that hydrophobicity increases with the coal rank for samples with a similar bulk mineral content.

angl
The results of the contact
. Introduction

Carbon dioxide (CO2) emission from burning fossil fuels haseen identified as a major contributor to the increased atmosphericO2 levels. Carbon capture and storage (CCS) has the potential foremoving CO2 from the atmosphere when biomass is burned (IPCC,005). This option includes storage strategies such as CO2 injection

nto deep saline aquifers (Ofori and Engler, 2011; André et al., 2010;rts et al., 2008; Chadwick et al., 2007; Zweigel et al., 2004; Baklidt al., 1996), depleted oil and gas reservoirs (Arts et al., in press;eeberger and Hugonet, 2011; Meer et al., 2010; Stein et al., 2010;elasquez et al., 2006), and unmineable coal seams (Shojai Kaveht al., 2011; Bergen et al., 2009; Mazzotti et al., 2009; Bergen et al.,006; Siemons et al., 2006).

With injection of CO2 into the coal seams, methane can beeplaced by CO2, since coal has a preferential sorption of CO2 overethane, and consequently coal bed methane (ECBM) recovery

s enhanced. CO2 injected into the reservoir flows through the

Please cite this article in press as: Shojai Kaveh, N., et al., Effect of coal

and flue gas storage. Int. J. Greenhouse Gas Control (2012), http://dx.d

leat system, diffuses into the coal matrix and is sorbed by theoal micropore surfaces. As a consequence, gases with a lowerffinity to coal, i.e., methane are released (Amorino et al., 2005).

∗ Corresponding author.E-mail address: [email protected] (S.N. Ashrafizadeh).

750-5836/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.ijggc.2012.09.009

e experiments are input parameters for field scale reservoir modeling.© 2012 Elsevier Ltd. All rights reserved.

Furthermore, when CO2 is sorbed by the coal, the coal swells. Thelatter reduces the permeability and the injectivity by orders ofa magnitude or more (Shi and Durucan, 2005). This effect mightbe counteracted by increasing the injection pressures (Larsen,2003).

At depths where the pressure is greater than the critical pressureof CO2, it has been injected successfully. Examples are the AllisonProject and in the Alberta Basin in Canada (Gunter et al., 2005).Injection of CO2 into coal reservoirs has enhanced the methanerecovery up to 90% of the gas-initial-in place while conventionalmethods recover only about 50% by reservoir-pressure depletion(Stevens et al., 1996).

Power plants are one of the major sources of CO2 emission. Theemitted flue gases mainly consist of nitrogen and CO2 with tracesof CH4, O2, CO, H2, NOx and SOx (Mazumder and Wolf, 2008). Fordepleted gas reservoirs and aquifers, CO2 is separated from the fluegas prior to its injection for storage; this makes the process lessenergy efficient. Because of the chromatographic behavior of coal,which means here that the sorption of CO2 is much stronger than fornitrogen, direct injection of flue gas could eliminate the necessity offlue gas purification prior to its injection into the coal bed; i.e. CO2

petrology and pressure on wetting properties of wet coal for CO2oi.org/10.1016/j.ijggc.2012.09.009

and the other contaminants get adsorbed at the coal surface whilenitrogen and methane are produced (Shojai Kaveh et al., 2011).However, the injection of flue gas causes the displacement of CO2 byN2 (Florenti et al., 2010), early breakthrough of the nitrogen (Wo

dx.doi.org/10.1016/j.ijggc.2012.09.009

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mailto:[email protected]

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ARTICLEJGGC-718; No. of Pages 11

N. Shojai Kaveh et al. / International Journa

nd Liang, 2004), and contamination of the methane-productiontream.

Release of coal bed methane (CBM) into the matrix is largelyontrolled by the interactions among CO2, the reservoir fluid, theoal matrix and the ash. The success of this combined processepends on different parameters. One of these factors is CO2-etting behavior of the coal, which is a function of coal rank, ash

ontent, heterogeneity of the coal surface, pressure, temperature,nd composition of the gas (Shojai Kaveh et al., 2011).

Arnold and Aplan (1989) found for coal, mostly consisting ofnorganic minerals and organic macerals, the biggest difference inhe wetting behavior between hydrophilic inorganic mineral inclu-ions and hydrophobic organic macerals. Furthermore, coal has aetwork of cleats and a matrix pore system ranging from macro-ores (>50 nm) to mesopores (from 2 to 50 nm) and microporesfrom 0.4 to 2 nm) to sub-micro pores (<0.4 nm) (Dubinin, 1960;iemons, 2007). The matrix blocks between the smallest cleat sys-em have diameters of a few tens of microns (Gamson et al., 1993).n the larger cleats, fluid flow occurs according to Darcy’s law.iffusion-driven transport becomes increasingly more important

n the denser network of micro-cleats. For a hydrophobic coal, its expected that the micro-cleats are filled with gas, leading to a

uch faster transport of the injected gas to the coal matrix thanor a hydrophilic coal (diffusion coefficient of CO2 in gas is about.7 × 10−7 m2/s and independent of pressure (Bird et al., 1960)).or a hydrophilic (water-wet) coal, the micro-cleats are filled withater. In this case, the transport of the injected gas to the rock sur-

ace is limited by diffusion of the gas through the aqueous phase.his leads to a much slower transport of the injected gas withinhe coal matrix and micro-cleats network (diffusion coefficient ofO2 in water is 2 × 10−9 m2/s at 10 MPa and 298 K) (Bird et al.,960; Shojai Kaveh et al., 2011; Siemons, 2007). It is worthwhile toote that there are three categories for wettability defined in termsf contact angle, where the contact angle is determined throughhe most dense fluid phase. According to this classification, theample is characterized to be water-wet, intermediate-wet, andas-wet when the contact angles of the (gas) bubble on the surface,n the presence of an aqueous phase, are in the range between 0nd 75◦, 75 and 105◦, and 105 and 180◦, respectively (Anderson,986).

The wetting properties of coal plays a critical role not only inO2 storage processes, but also in coal preparation and utilizationrocesses, viz. froth flotation, oil agglomeration, dust abatement,nd preparation of coal–water slurries (Arnold and Aplan, 1989;uerstenau and Diao, 1992; Laskowski, 1995).

Because of the heterogeneous structure of coal, at both macro-copic and microscopic levels, the results of wettability studies onoal surfaces change from sample to sample, even where the sam-les originate from the same block. Therefore, often a distributionf contact angles is provided instead of reporting one averagedontact angle (Wei et al., 1992). It is also believed that the wett-bility of coal changes as function of several parameters includinghe coal rank (Gutierrez-Rodriguez et al., 1984), chemical com-osition (Brady and Gauger, 1940; Laskowski, 1994; Sun, 1954)ineral matter content (Aplan, 1993; Crawford et al., 1994; He

nd Laskowski, 1992), moisture level (Elyashevitch, 1941), porosityDrelich et al., 2000; Keller, 1987), degree of oxidation (Gayle et al.,965; Gutierrez-Rodriguez and Aplan, 1984; Laskowski, 1994; Sun,954), and the pressure and temperature of the reservoir as well ashe composition of the reservoir fluids (Shojai Kaveh et al., 2011).n addition, chemical heterogeneity due to the presence of differentharacteristic groups of components show different wetting behav-

Please cite this article in press as: Shojai Kaveh, N., et al., Effect of coaland flue gas storage. Int. J. Greenhouse Gas Control (2012), http://dx.d

or. The presence of paraffinic hydrocarbons results in a stronglyydrophobic behavior, aromatic hydrocarbons an intermediate-et, and mineral components in a strongly water-wet behavior

Keller, 1987).

PRESSreenhouse Gas Control xxx (2012) xxx–xxx

Previous studies, which concentrate on the wetting behaviorof coal–water–air systems at atmospheric pressure using con-tact angle measurements, revealed that dry coal (like dry sand)is naturally hydrophobic (Gutierrez-Rodriguez and Aplan, 1984;Gutierrez-Rodriguez et al., 1984). Its hydrophobicity varies fromone sample to another because of variations in the composition ofthe coal, i.e., particularly in the coal rank. Coals become increasinglyhydrophobic with increasing rank (Gutierrez-Rodriguez and Aplan,1984; Gutierrez-Rodriguez et al., 1984; Sakurovs and Lavrencic,2011). A comparative study of contact angle experiments of airbubbles and droplets of either oil, flocculant or coagulant on flatpolished coal surfaces immersed in water was carried out byOrumwense (2001). The experiments show a positive correlationbetween hydrophobicity and coal rank of vitrinite-rich coals. Itcan be concluded that the hydrophobicity of coal decreases withdecreasing of fixed and total carbon content.

Findings by Sakurovs and Lavrencic (2011) also confirmed thatcoals become increasingly CO2-wet with increasing pressure (gasdensity), and consequently, the penetration rate of CO2 into thecoal increases rapidly. Furthermore, they found that coals with highrank, low ash yield, or both were preferentially CO2-wet at highpressures. Only coals with a high ash and high oxygen contents donot become CO2-wet, even at high pressures.

Comparison of a vitrinite-rich with a vitrinite-poor coal revealedthat the growing of the contact angle with the pressure is morepronounced for a vitrinite-rich coal (Sakurovs and Lavrencic, 2011).

Murata (1981) performed contact angle measurements onpressed pellets of pulverized coal at atmoshperic pressure. He con-cluded that the contact angles depend on the hydrogen and oxygencontent of the coal.

Keller (1987) summarized literature data (Fuerstenau and Diao,1992; Gutierrez-Rodriguez and Aplan, 1984; Gutierrez-Rodriguezet al., 1984; Murata, 1981) on surface properties of coal–water–airsystems. Chi et al. (1988) found that the contact angle in aCO2–water–coal system increases with pressure in the range fromatmospheric up to 6.2 MPa. Additionally, it was observed that whenthe ash content increases, the coal becomes more water wet.

According to literature (Aplan, 1993; He and Laskowski, 1992),the contact angle decreases with increasing mineral-matter con-tent in the coal. Gosiewska et al. (2002) found that increasing theamount of mineral matters in the coal samples reduces the contactangle. The fact that the results are scattered implies that other fac-tors may also be responsible for the variations, e.g. the size of thehydrophilic mineral inclusions (Gosiewska et al., 2002).

So far, only a few experimental data have been reported onthe wettability of different ranks of coals in the presence of fluegas at high pressures and elevated temperatures (Chi et al., 1988;Sakurovs and Lavrencic, 2011; Shojai Kaveh et al., 2011; Siemonset al., 2006). Siemons performed contact angle experiments bymeans of a modified pendant drop cell on two different ranks of wetcoal using supercritical CO2 at a temperature of 318 K and pressuresranging from 0.1 up to 12 MPa (Siemons, 2007). From these data, itcan be concluded that for high-rank coals wetting alteration fromwater-wet to CO2-wet occurs at a pressure as low as 0.27 MPa. For amedium-rank coal, this wetting alteration is observed for pressuresabove 8.7 MPa. It needs to be mentioned that during these exper-iments the water was not fully saturated with CO2 and thus thecomposition of the aqueous phase varied with each experiment.

In this study, two coal samples have been used representingdifferent ranks. The wettability behavior of the wet coal samplesupon injection of a synthetic flue gas (20 mol% CO2/80 mol% N2)and of pure CO2 was investigated by means of contact angle exper-


iments. The experiments were carried out in a modified pendantdrop cell at a constant temperature of 318 K and pressures varyingbetween 0.1 up to 16 MPa. The chosen pressures and temperatureare representative for typical in situ conditions (Huijgens, 1994).

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Table 1Proximate and ultimate analysis and coal petrology of used coal samples (Siemons,2007).

Sample Warndt Luisenthal Selar Cornish

Rank hvBb Semi anthraciteRmax (%) 0.71 2.41Vitrinite (%) 74.40 73.60Liptinite (%) 15.60 0.00Inertinite (%) 9.00 24.60Minerals (%) 1.00 1.80Volatile matter (w.f.) (%) 40.50 10.40Carbon (wt%) 81.30 85.70Hydrogen (%) 5.58 3.36Nitrogen (%) 1.88 1.56Sulfur (%) 0.69 0.68Oxygen (%) 5.47 5.58H/C 0.82 0.47O/C 0.05 0.05

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Ash (w.f.) [%] 2.77 3.94–5.50Fixed carbon (d.a.f.) [%] 58.36 89.27

he experiments were performed with an aqueous phase fully satu-ated with CO2, in order to minimize the effect of dissolution of CO2nd changing the composition and the properties of the aqueoushase during the experiment.

In this work the effect of the coal composition on contact angleas not been studied and only the results of an high-rank coal haveeen directly compared to the results of a medium-rank coal. MicroT images of the coal samples were taken to identify the distribu-ion of the mineral content on the surface.

The mineral matter content on the surface of the used coal sam-les was too low to allow a conclusive interpretation on its infuence

n the wetting behavior. Furthermore, local effects of the variationf the maceral composition of the samples have not been investi-ated.

. Experimental

.1. Materials and sample preparation

The following gases were used for the experiments, CO2 with purity of 99.7 mol% and a premixed synthetic flue gas consist-ng of 20 mol% CO2 and 80 mol% N2 (both purchased from Lindeas Benelux). Two coal samples with similar vitrinite content weresed in this study, representing different coal ranks, a Selar Cor-ish (SC), as a semi-anthracite type high rank coal and a Warndtuisenthal (WL), as a high volatile bituminous (hvBb) medium rankoal. The Warndt Luisenthal samples were mined from the intra-ountain Saar basin in Western Germany and the Selar Cornish

amples originate from the Selar colliery in South Wales Coalfield.oth samples were cored in the same direction and parallel tohe bedding plane. The abbreviations SC and WL are respectivelysed to describe the experimental data for these coal samples. Theesults of the ultimate and proximate analysis of the coal samplesre given in Table 1.

For the contact angle experiments, 10 coal samples were pre-ared with dimensions of 30 mm × 12 mm × 6 mm. The samplesere drilled and cut from a larger coal block (>0.25 m3). The sam-le preparation procedure, according to the procedure of Drelicht al. (1997). After preparation, the coal samples were equili-rated at a relative humidity of 96–97%, at 303 K, for at least 48 hASTM standard D 1412), to establish a representative water sat-ration. 2-D and 3-D microscopic surface images were then taken



rom the samples to determine the surface roughness (LEICA 3Dtereo explorer). Finally, a Phoenix Nanotom scanner was used toetermine the mineral matter distribution in the coal samples aticro-level.

PRESSreenhouse Gas Control xxx (2012) xxx–xxx 3

2.2. Experimental set-up and procedure

A modified pendant drop (PD) cell is used to capture the carbondioxide bubbles just below the coal surfaces, at varying pressuresup to 16 MPa and a constant elevated temperature of 318 K (ShojaiKaveh et al., 2011). Determination of the contact angles is doneby image capturing and bubble shape quantification. A schematicdrawing of the experimental set-up is given in Fig. 1. Both sides ofthe cell have a steel cap with a thermal pre-stressed glass window.These glass windows allow the visual observation of the bubble andthe sample surface inside the pendant drop cell.

The experimental set-up consists of a high pressure brine-resistant steel cell, an online density meter (Anton Paar K.G. DMA512), a gas compressor, a thermostatic CO2 storage vessel, a circu-lation pump, two pressure gauges with an accuracy of 1 kPa, twothermometers with an accuracy of 0.1 K, a high-resolution digitalcamera and an endoscope. The maximum allowable pressure of thecell is 60 MPa while that of the lines is 18 MPa. Prior to each exper-iment, a fresh sample was placed inside the cell within a holder insuch a way that the lower surface is horizontally aligned with thecamera and the light source. The cell was then closed and tested forany leakage with pressurized helium, up to a pressure of 18 MPa.

Before starting an experiment all lines and the cell were rinsedwith ethanol and distilled water in order to remove any possibleimpurities which were introduced during the cell assembling. Afterthe leakage tests, the cell was filled with distilled water in such away that the water fully covers the lower end of the sample whileat the same time the cell was not completely filled. Before injectingcarbon dioxide into the cell, the gas was pressurized in a separategas reservoir of known volume. After that, the gas was injectedinto the main cell in order to reach CO2-saturation in the aqueousphase at the given pressure and temperature. Once enough CO2was injected, circulation was started to ensure complete mixing ofthe carbon dioxide with the aqueous phase and to attain equilib-rium. The total composition of the mixture can be determined witha reference equation of state for CO2 (Span and Wagner, 1996) andthrough a simple material balance. The phase equilibrium data ofthe CO2–water system provided by Shyu et al. (1997) was used toensure that the overall composition is such that the experimentsare definitely conducted in a fully saturated aqueous phase, viz. theoverall composition is within the two-phase region. Equilibriumwas achieved if the pressure in the cell stayed constant and thedensity of the re-circulating liquid did not change anymore. Afterestablishing a saturated aqueous phase at the desired pressure, asmall amount of gas was injected via a small needle at the bottomof the cell to create the gas bubble at the surface. If the liquid phasewas completely saturated with CO2, the gas bubble was stable. Inorder to avoid erroneous results, e.g., due to reflection and otheroptic artifacts, the light beam, the substrate, the endoscope, andthe camera must have been aligned horizontally at the beginningof the experiment. At each particular pressure, a number of pic-tures were taken for the subsequent image analysis. After enoughpictures have been taken at a certain pressure, the pressure insidethe cell was increased by introducing more CO2 into the cell.

A more detailed description of the experimental setup and pro-cedure can be found in previous work (Shojai Kaveh et al., 2011).

3. Image analysis

3.1. Microscopic image analysis


For the contact angle experiments, it is of vital importance tohave a coal sample with a very smooth surface. Prior to the contactangle experiments and after preparation of the sample blocks, 2-D and 3-D microscopic images were taken from the coal surfaces.

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Fig. 1. Schematic representation of th

rom these images, profiles can be extracted allowing determina-ion of the surface roughness. To identify the surface smoothnessuality, a Leica 3D Stereo Explorer was used to determine the Carte-ian surface values. The characterization of the surface is based onhe so-called Pa factor, which is calculated according to the inter-ational standard of EN ISO 4287. A large value of Pa means thathe surface is rough and a small Pa factor indicates a smooth sur-ace (i.e., the Pa value of a glass surface is about 0.1 �m). A moreetailed description on determination of Pa value is given by Shojaiaveh et al. (2011).

.2. Micro CT-scan

The mineral matter distribution in the coal samples at microevel was determined by means of a Phoenix Nanotom micro CTcanner (180 kV/15 W nanofocus computed tomography (nano CT)ystem). With the micro CT scanner the spatial distribution of theineral matter content in the coal sample was obtained. Therefore,

oal samples were cut to a size of 30 mm × 6 mm × 12 mm.The nano CT system consists of a high power nanofocus X-

ay source, a precision object manipulator, a high resolution CCDetector, and a computer for control and re-construction. Digitaleometry processing was used to generate a three-dimensionalmage of the inside of an object from a large series of two-imensional X-ray images taken around a single axis of rotation.hese two-dimensional X-ray images contain information on theosition and density of absorbing features within the small samplesStock, 2009).

.3. Bubble image analysis

Visual observation of the gas bubbles was conducted throughhe endoscope in front of one window while the light source waslaced before the other window. A high-resolution digital cameraas used to take pictures from the gas bubbles. Several picturesere taken from each bubble under the same conditions, in order

o allow determining the reproducibility of the determined contactngles. Contact angles were determined using an in-house MATLABoutine (Shojai Kaveh et al., 2011).

For the image analysis, the profile of the bubble was split intowo parts assuming an axis-symmetric shape for the bubble. Theoung–Laplace equation was then used to describe both halves of


he bubble profiles separately:

(d2y/dx2)

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erimental set-up (pendant drop cell).

where Ra is the radius of the bubble at the apex, � is the interfacialtension between the gas and liquid phases, and �� is the densitygradient between the gas and liquid phases. The bubble profile wasdefined in terms of the Cartesian coordinate system. For each side,a set of parameters (2/Ra) and (��gx/�), were obtained giving therespective best description. Since the values for both sides shouldbe the same, it was assumed that the bubble is axis-symmetric, hav-ing identical half bubble profiles. As a consequence, the differenceof these parameters can be used as a criterion to judge the quality ofthe bubble image and the bubble profile description. Once the dif-ference between the parameters describing the two halves of thebubble showed an area coverage larger than 0.90, the descriptionof the bubble shape was accepted for further determination of thetwo contact angles per bubble. For a detailed description see ShojaiKaveh et al. (2011) and Delavarmoghaddam (2009).

4. Results and discussion

4.1. Microscopic image analysis

The characterization of the surface is based on the so-calledPa factor, which is calculated by using a Leica 3D Stereo Explorer(Shojai Kaveh et al., 2011). The determined Pa factors for the mostsmooth samples of SC and WL coal surfaces before the experi-ments were 0.011 and 0.018 mm, respectively. After the contactangle experiments these values increased to 0.014 and 0.027 mm,respectively. In other words, the surface became rougher during theexperiments. This increase in roughness is most likely attributed tothe coal swelling and differential volume changes between coal andmineral matter (Battistutta et al., 2010) and/or dissolution of min-erals. In addition, fast pressure reduction may cause matrix damageand consequently, surface damage due to the fast escape of CO2.

Fig. 2 shows the side views of the SC and WL substrate surfacesbefore and after the experiment. As it can be seen, the surface ofthe WL substrate (Fig. 2a) is more rough than that of the SC sample(Fig. 2b) and even became more rough after the experiments as well(Fig. 2c and d).

Note: The surface reconstruction of the contact planes wereconducted with the Leica Stereo Explorer directly after the experi-ments. Sometimes this gives spikes that are attributed to unwantedreflection of the water coming from the matrix.


4.2. Micro CT-scan

A Phoenix NanotomTM scanner was used to determine the min-eral matters and void distribution based on density differences in

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N. Shojai Kaveh et al. / International Journal of Greenhouse Gas Control xxx (2012) xxx–xxx 5

F rimenw , y, z d

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ig. 2. 3-D side view of the surface of (a) WL sample, (b) SC sample before the expeith a LEICA 3D stereo explorer. The blue, red and green lines give the orthogonal x

he coal samples. 2-D sections through the WL and SC reconstructedT images after the experiments show the heavier minerals as redreas and the fracture network and cracks as blue areas; the greenhades represent the coal matrix (Fig. 3). In this work, for clarifica-ion purposes, the original coal part represented by gray scales waseplaced by green shades. This was only for visualization purposesnd not for quantification. Meanwhile direct comparison betweenhe green shades of two different images was not possible, because

different gray scale calibrations were used for different images.he side of the samples coated with a stabilizing epoxy layer showsllipse-shaped voids of various sizes.

The distribution of mineral matters on the samples and contacturfaces is shown in Fig. 4. Although the mineral content of thesed WL substrate is higher than the one of the used SC substrateFig. 4a and b), we would like to emphasize that it cannot be useds explanation for differences in the wetting behavior of these twoifferent kinds of coal. In order to see the influence of the mineral



ontent on the wetting behavior, the mineral content of the surfacen which the bubble sits, needs to be compared (Fig. 4c and d).owever, the analyses of the mineral matter content of both contact

urfaces shows that there are no considerable differences.

Fig. 3. Side view of the (a) WL and (b) SC sample after the experiment, using a P

ts, (c) WL sample, and (d) SC sample after the experiments. The pictures are takenirections used for determination of the Pa factor.

The spatial distributions of voids before and after experiments(Fig. 5) clearly show that a fracture pattern, comparable to aface/butt-cleat type of system, developed in the SC-sample (Fig. 5aand b). The medium rank WL coal shows that the void volumeincreases sub-parallel to the coal layering (Fig. 5c and d).

4.3. CO2 wetting behavior on a wet semi-anthracite coal sample(Selar Cornish)

The contact angles of the CO2–water–semi-anthracite coal sys-tem at a constant temperature of 318 K are depicted in Fig. 6.Injection of a gas bubble into an equilibrated system which consistsof water and CO2 induces an initial fluctuation of the determinedcontact angle values. These fluctuations are mainly due to the ini-tial counter-current mass transfer between the gas bubble and thesurrounding aqueous phase which is accompanied by a continu-ous change in the composition of the gas and liquid phases before


reaching equilibrium. However, due to the fact that the overall com-position is within the two-phase region, the releasing of a new gasbubble only gives a small disturbance of the equilibrium which doesnot last long.

hoenix Nanotom micro CT-scanner. The images have a 10 �m resolution.

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Fig. 4. Distribution of mineral matter content determined with the micro-CT scanner: (a) three dimensional view of the SC sample, (b) 3-D view of the WL samples, (c) 2-Dv d areas

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iew of the SC contact surface, and (d) 2-D view of the WL contact surface. The rehow cracks.

After equilibrium has been achieved, a constant value of the con-act angle is identified which is called the “stable contact angle”.he contact angle (�) can be described by a positive linear cor-elation (regression: � = 92.84◦(±4.3◦) + 0.42 × P [MPa], r2 = 0.94 at5% confidence). The wetting properties of SC coal change from

ntermediate-wet towards CO2-wet with increasing pressure, i.e.,t a temperature of 318 K the coal surface becomes hydrophobic atressures higher than 5.7 MPa. The implication on field scale coulde that for CO2 storage in semi-anthracite coal, the injection pres-ure has to overcome a pressure threshold of 5.7 MPa to wet theurface and thereby to enhance the storage capacity.

This behavior is partly in contrast with the contact anglexperiments of Siemons et al. (2006) and the capillary pressurexperiments of Plug (2007). According to Plug’s experiments theettability of the same high rank coal, SC, is CO2-wet during pri-ary imbibition experiments from low pressures up to 9 MPa. This


s in agreement with the contact angle data of Siemons et al. (2006).egarding the fact that both Plug and Siemons conducted theirxperiments using water which was not fully saturated with CO2,ne may attribute this observation to the dissolution of CO2 in the

s show the heavier minerals (i.e. silicates, carbonates, oxides, etc.) and blue areas

aqueous phase. The comparison of the SC contact angles result-ing from the work of Siemons et al. (2006) and from this work(Fig. 7) shows that the values of Siemons are generally higher thanthose of this work. In Siemons’ experiments the SC samples behaveCO2-wet already at pressures above 0.26 MPa, with contact anglesvarying between 100 and 140◦. The contact angles in this workshow a relatively steady increase from 90 to 120◦ versus pressure.A possible explanation for the difference in the contact angle behav-ior might be attributed to the different experimental conditions ofthe two studies. In the work of Siemons the solution of CO2 andwater was not pre-equilibrated and thus the mass transfer of CO2and water into the aqueous and the gas phase might have possi-bly interfered. This explanation is also supported by the fact thatin the experiments conducted by Siemons the CO2 bubble disap-pears within 60 min at low pressures and in about 20–30 min athigh pressures and that the contact angle data of Siemons strongly


scatter upon increasing pressure (Siemons et al., 2006). Siemonset al. (2006) showed that the disappearance of the CO2 bubblewas due to dissolution and not due to CO2 sorption on the coalsurface.

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F le befoe ubble

lbtsvca

wscsatfocbbt

ig. 5. Three dimensional view of the void distribution (dark blue) in (a) SC sampxperiment and (d) WL sample after the experiment. Note that all obvious elliptic b

Throughout this research, the system was always carefully equi-brated before a CO2 bubble was released for the measurement. Thisubble was stable throughout the contact angle determination athat specific pressure. It only disappeared when the pressure of theystem was changed. However, even with this cautious method aery slight initial change in the contact angle was always observedaused by slight initial pressure fluctuations and/or mass transfers mentioned above (Shojai Kaveh et al., 2011).

In our previous work (Shojai Kaveh et al., 2011), we related theettability behavior of the wet WL system to the CO2 density, the

olubility of CO2 in water, and the sorption of CO2 on wet WLoal. In Fig. 8, the dependency of these parameters on the pres-ure is compared based on relative values. These relative valuesre defined as the actual value of a certain property divided byhe respective maximum value in the investigated pressure range,rom atmospheric up to 20 MPa. We assumed that all of the previ-usly mentioned properties could be responsible for the wettability



hange at a pressure of about 8.5 MPa. In this pressure range, it cane seen that the density and the solubility of CO2 strongly increaseefore they become almost constant at higher pressures. Based onhese results, it was concluded that the maximum CO2 sorption on

re the experiment, (b) SC sample after the experiment, (c) WL sample before thes are in the epoxy. The resolution is 10 �m.

the coal sample, and the wettability alteration from water-wet togas-wet occurred at aprroximately the same pressure (Fig. 8, lightblue area).

The same relative properties have been plotted for the wet SCcoal system (Fig. 9) and show that the wettability alteration and themaximum of the CO2 sorption are also observed in the same pres-sure range as for the WL experiment. In contrast with the wet WLsystem, the change in the density and the CO2 solubility in waterdo not coincide in the same pressure range. This leads to the con-clusion that the sorption of CO2 on the wet coal determines thewettability behavior rather than the CO2 properties, such as den-sity and solubility. The lower pressure range (light blue area) forwettability alteration in a wet SC-system is due to the higher coalrank.

4.4. Synthetic flue gas wetting behavior on a wet semi-anthracitecoal sample (Selar Cornish)


A series of experiments were performed to recognize the wet-ting properties of flue gas on a wet semi-anthracite coal. The stablecontact angles of the synthetic flue gas–water–semi-anthracite coal

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Fig. 6. Stable contact angle as a function of pressure for the wet SC coal system for experdashed line gives the best linear fit through the data and is used to guide the eye.

Fig. 7. Comparison of determined contact angle values for the CO2–wet SC coalsystem. Triangles display the work of Siemons et al. (2006) for which the aqueousphase was not saturated with CO2; diamonds give the data of this work in a pre-saturated aqueous phase.

Fig. 8. Dimensionless density (Span and Wagner, 1996), CO2 solubility in water (Baranenon wet WL coal (Siemons, 2007) at a constant temprature of 318 K and a pressure ranging

iments with injection of CO2 and synthetic flue gas at a temperature of 318 K. The

system as a function of pressure at constant temperature (318 K)are depicted in Fig. 6. The contact angles of the flue gas on wetSC coal are usually smaller than those of CO2. The experimentsrevealed that the wettability of Selar Cornish coal is intermediate-wet at all pressures. The increase in contact angle at low pressuresis stronger (red dashed line) than that at pressures above 2.5 MPa(green dashed line). For pressures below 2.5 MPa, the contactangle increases with an inclination of about 2.88◦/MPa, while forpressures above 2.5 MPa, it is 0.41◦/MPa.

For the relevant in situ pressures, it can be concluded thatthe contact angle only slightly increases with increasing pressure.Thus, the wettability of SC coal with flue gas injection remainsintermediate-wet at all pressures between 0.2 and 16 MPa. For realapplications on field scale this implies that there is no pressurethreshold when injecting the flue gas.

The contact angles of the flue gas bubbles are all smaller than


those of the CO2 bubbles. This is clearly visible from the imagesshown in Fig. 10. This phenomenon could be explained by consid-ering the ratio of 20 mol% CO2 to 80 mol% N2 in the flue gas bubble.The solubility of nitrogen in water and the sorption on coal are

ko et al., 1990), stable contact angle (Shojai Kaveh et al., 2011) and sorption of CO2

from atmospheric up to 20 MPa.

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Fig. 9. Dimensionless stable contact angle, sorption of CO2 on wet SC coal (Siemons, 2007), density (Span and Wagner, 1996) and solubility of CO2 (Baranenko et al., 1990)at a constant temprature of 318 K and pressures ranging from atmospheric up to 20 MPa.

F t 318 K

sov

4

agto

ig. 10. Digital images of gas bubble on the SC coal surface in the presence of water a

ignificantly lower than those of CO2. Therefore, the contributionf nitrogen on the variation of the contact angle is small and theariations of the contact angle is dominated by the behavior of CO2.

.5. Comparison of experimental results with respect to coal rank

The values of the determined contact angles for a high-rank and medium-rank coal samples with similar vitrinite contents areiven in Fig. 11. The data of the medium-rank WL coal sample areaken from previous work (Shojai Kaveh et al., 2011). The followingbservations were made:

(1) For the hvBb rank WL sample and injection of pure CO2, thewettability of the coal surface changes from intermediate-wetto CO2-wet at 8.5 MPa.

(2) For the same WL sample when injecting the synthetic fluegas, the alteration from water-wet to intermediate-wet wasobserved at pressures above 10.5 MPa.

(3) For the semi-anthracite rank Selar Cornish sample, it is foundthat upon injecting pure CO2 the contact angle increaseswith pressure and the wettability alteration occurs at around5.7 MPa.

(4) Experimental results with synthetic flue gas revealed that thewettability of Selar Cornish coal is intermediate-wet at allpressures and the contact angle only slightly increases withpressure.



(5) Gutierrez-Rodriguez et al. (1984) and Sakurovs and Lavrencic(2011) found that the hydrophobicity of a wide variety of coalsdecreases with decreasing rank, fixed carbon, total carboncontent, and with increasing oxygen and hydroxyl contents.

and 5.25 MPa; (a) CO2 bubble, �avg. ≈ 104.08◦ , and (b) flue gas bubble, �avg. ≈ 89.97◦ .

We observed similar results; i.e. hydrophobicity (the contactangle) of a coal sample increases with the coal rank and pres-sure.

(6) When injecting CO2, for the semi-anthracite sample, the wett-ability alteration occurs at a lower pressure than that for thelower rank hvBb sample.

(7) When injecting flue gas, the contact angles on the semi-anthracite Selar Cornish sample are higher than those onWarndt Luisenthal coal.

(8) For the Selar Cornish coal sample the pressure does onlyslightly affect the wettability no matter CO2 or flue gas hasbeen injected. Warndt Luisenthal coal becomes apparentlymore hydrophobic with increasing pressure. In other words,coal rank is a parameter that controls the degree of pressuredependency of the wettability.

(9) For the samples used in this work the mineral matter contentsin the contact surface are comparable. In this study, the WLcoal sample contains more total mineral matters than the SCcoal sample. However, the difference between mineral mattercontents of the two coal surfaces is negligible. Although ingeneral a coal with a higher content of mineral matter is morewater-wet (Gosiewska et al., 2002), the results of this study donot allow to attribute the variations in the wettability of theused samples to the amount of mineral matters in the coal,due to the mineral matter content on the surface of the usedcoal samples was too low and similar.


(10) The found wettability behavior of the coal ranks used in thiswork can be related to the different surface chemistry of thecoal samples. The fact that the hvBb-rank coal (WL) is morewater-wet is due to hydrophilic functional groups, mainly

dx.doi.org/10.1016/j.ijggc.2012.09.009



F (a) ina

(

ec

5

efF

•

•

•

•

•

ig. 11. Contact angle as a function of pressure for coal samples of different ranks;re extracted from Shojai Kaveh et al., 2011).

carboxylic and hydroxyl groups, which are more abundant inthe hvBb rank coal (Crawford et al., 1994; Gutierrez-Rodriguezet al., 1984).

11) Although the underlying mechanisms causing the observedwettability alterations have not yet been identified, it is foundthat for CO2 injection the wettability alterations “for our coaltypes” are closely related to the CO2 sorption behavior on thissame coal. The maximum sorption of CO2 on the wet-coal sam-ples of different ranks was found in the same pressure rangeas the wettability alteration.

It is thus expected that the behavior found in this study is gen-rally applicable to coals with the same rank and with similarompositions disregarding the ash content of the coals.

. Conclusions

Two coal types were studied with respect to their wetting prop-rties when injecting CO2 or flue gas at various pressures, rangingrom atmospheric up to 16 MPa, at a constant temperature of 318 K.rom this study it was found that:

In general, the stable contact angles of CO2 as well as the syntheticflue gas increase with pressure. The increasing rate is influencedby the coal rank and the gas bubble composition.When injecting CO2, the wettability of the semi-anthracite coalsurface changed from intermediate-wet to CO2-wet at a pressurearound 5.7 MPa. The implication on field scale could be that forCO2 storage in semi-anthracite coal, the injection pressure has toovercome a pressure threshold of 5.7 MPa to wet the surface andthereby to enhance the storage capacity.Results with injection of synthetic flue gas revealed that SelarCornish coal is intermediate-wet at the investigated pressuresand that the contact angle only slightly increases with pressure.For real applications on field scale this implies that there is nopressure threshold to wet the surface when injecting the flue gas.For both coal samples the contact angles of the flue gas bubblesare smaller than those of the CO2 bubbles. Based on this wettingbehavior, injection of pure CO2 into SC and WL coal could be more


efficient than injection of flue gas for field-scale purposes.The pressure shows lower effect on the contact angles on thesemi-anthracite sample than on those on the Warndt Luisenthalsample independent whether CO2 or flue gas has been injected.

jection of CO2 and (b) injection of synthetic flue gas (the results for the WL sample

• In general, the hydrophobicity of the coal samples increases withcoal rank and pressure. This behavior can be related to the differ-ent surface chemistry of the two coal samples used.

• The fact that the wettability of different coal ranks changes dif-ferently reveals that on field scale coal wettability is definitelyimportant for the evaluation of the efficiency of CO2 storage pro-cess.

Acknowledgements

The research reported in this paper was carried out as a partof the CATO2 project (CO2 capture, transport and storage in theNetherlands). The financial support from GRASP, founded by theEuropean Commision, is also acknowledged. The research was con-ducted in the Dietz laboratory of the Department of Geotechnology,Delft University of Technology. Our grateful thanks go to the tech-nical staff of the Dietz laboratory, specially J. Etienne, M. Friebel,K. Heller, J. van Meel and W. Verwaal. We also thank Prof. HansBruining and Dr. P. van Hemmert for insightful discussions.

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