Heterogeneous Sorption and Swelling in a Confined and Stressed

Energy & Fuels 2003, 17, 1595-1608

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Heterogeneous Sorption and Swelling in a Confined and Stressed Coal during CO2 Injection C. O ¨ zgen Karacan† The Pennsylvania State University, The Energy Institute and the Department of Energy and Geo-Environmental Engineering, University Park, Pennsylvania 16802 Received June 23, 2003. Revised Manuscript Received September 8, 2003

A confined Pittsburgh seam (DECS-12) coal sample was pressurized with carbon dioxide (CO2) and the changes in the sorbed gas concentration and matrix properties were studied using dualenergy X-ray computed tomography (X-ray CT). The use of dual-energy technique enabled the quantification of spatial and temporal variations in bulk density and effective atomic number. These two quantities were used to calculate separately the amount of sorbed gas and the changes in the coal matrix and to reveal the kinetics of the complex heterogeneous processes occurring with CO2 injection in a consolidated bituminous coal sample kept under a constant effective stress. The swelling and CO2 sorption in coal are heterogeneous processes and different parts of the coal behaved differently. Vitrite, liptite, and clarite microlithotypes swelled due to dissolution of CO2, while the clay + inertite region was compressed, even though it was the region that had the highest CO2 concentration. The vitrite swelled and de-swelled on the time scale of the experiment (5000-7000 min at each pressure) demonstrating that CO2 dissolution enabled rapid coal structure changes. Sorption kinetics were also heterogeneous: CO2 uptake was fastest for the clay + inertite region. Vitrite swelling reached a maximum then decreased with the expulsion of CO2, a behavior that has been observed in polymers.

1. Introduction The injection of CO2 into coalbeds is an attractive option for its geological sequestration. This option envisions the injection and storage of CO2 with the concomitant production of methane. The displacement of CH4 by CO2 is achieved due to the preferential sorption of CO2 under the operational pressure. As the CO2 injection pressure in the coal seam is increased, CO2 derives the CH4 to the fracture system that leads to the production wells. Normally, this process is continued until the CO2 breakthrough occurs from the production wells. A full understanding of the reservoir dynamics, complex mass transfer processes, the interaction of injected CO2 with coal, and the effects of dissolved CO2 on coal are important for process design and to test and evaluate the ability of CBM reservoir simulators to model the process. Coal-seam sequestration of CO2 involves challenging scientific questions due to the chemically and structurally complex nature of coal. This complexity creates challenges in understanding the subsurface behavior of the injected gas and the coal. Different coal macerals which are derived from different organic materials may behave differently. The different macerals usually have a wide distribution of pore sizes and have different adsorption affinities for CO2. Macropores allow fast transport of gas into and out of the porous structures. † Present address: NIOSH, Pittsburgh Research Laboratory, Bldg. 156, P.O. Box 18070, Pittsburgh, PA 15236. Tel: 412-386 4008. Fax: 412-386 6891. E-mail: [email protected].

Micropores have the function of trapping the gas within the micropore network. The micropores add additional molecular interaction between gas molecules and the coal. Coal type and maceral effects on gas emission are poorly understood and usually are overshadowed by coal rank effects. Recent investigations of small, hand-picked samples using high-pressure microbalances provided clear distinctions between coal type and rank with respect to gas emissions.1-3 Gas desorption rate investigations showed that bright, vitrinite-rich coals usually have the slowest desorption rates.4 Some dull, inertiniterich coals may rapidly desorb due to predominance of large, open-cell structures. The effect of coal composition, particularly the organic fraction, upon gas sorption capacity has been investigated for Bowen Basin and Sydney Basin coals.5 Maceral composition influences on gas retention and release (1) Beamish, B. B.; O’Donnell, G. Microbalance applications to sorption testing of coal. Symp. Coalbed Methane Research and Development; Department of Earth Science, James Cook University, Townsville, 1992; Vol. 4, pp 31-41. (2) Crosdale, P. J.; Beamish, B. B. Maceral effects on methane sorption by coal. In New Developments in Coal Geology: A Symposium. Coal Geol. Group, Geol. Soc. of Australia: Brisbane, 1993; pp 95-98. (3) Levine, J. R.; Johnson, P. W.; Beamish, B. B. High-pressure microgravimetry provides a viable alternative to volumetric method in gas sorption studies on coal. Proc. 1993 Int. Coalbed Methane Symp.; University of Alabama, Tuscaloosa, 1993; pp 187-195. (4) Crosdale, P. J.; Beamish, B. B.; Valix M. Coalbed methane sorption related to coal composition. Int. J. Coal Geol. 1998, 35, 147158. (5) Laxminarayana, C.; Crosdale, P. J. Role of coal type and rank on methane sorption characteristics of Bowen Basin; Australia coals. Int. J. Coal Geol. 1999, 40, 309-325.

10.1021/ef0301349 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/16/2003

1596 Energy & Fuels, Vol. 17, No. 6, 2003

were investigated using isorank pairs of hand-picked bright and dull coal in the rank range of high volatile bituminous to anthracite. It was suggested that the importance of maceral composition on maximum adsorption capacity varies with rank. At ranks higher than medium to low volatile bituminous, changes in maceral composition may exert relatively little influence on adsorption capacity. Dull coals desorbed more rapidly than bright coals, and the desorption rate was a function of the coal rank. However, the complications arising during the injection of CO2 into coal are not limited to the difficulties in understanding the transport and adsorption kinetics of the coal organic matrix. CO2 is a fluid that can, like organic solvent, dissolve in the organic coal matrix, thus modifying the physical and possibly the chemical structure of the coal matrix. This physical modification is associated with the relaxation/rearrangement of the macromolecular structure of coal and changes the initial pore structure.6 The swelling process itself may play an important role in determining how fast the coal can take up additional gas and in predicting the long-term effects of injection. The molecular rearrangement of coal caused by molecules dissolving in the coal can be explained by the classical polymer chemistry. Coals are glassy, strained, cross-linked macromolecular systems and they are not at their lowest energy state.7 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. However, when the coal interacts with a solvent, the free volume of the polymeric system increases and lowers Tg, the temperature at which the glass becomes a rubber. The rubbery cross-linked coal has sufficient freedom of motion for the molecules to rearrange themselves to adopt to a new lower-energy, more highly associated physical structure.6 Nuclear magnetic resonance (NMR) has been used to study coals containing large amount of solvents.8 This study also showed that the Tg drops with increasing amount of dissolved molecule and is independent of the identity of the molecule as long as the interactions remain constant. 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 15 atm.9 Increases in pressure produced increases in swelling and a decrease in the time required to reach maximum swelling. This confirms that there is a kinetic process involved in the swelling. Also, it was seen that a lower carbon content correlates with a higher degree of swelling. For the case of organic liquid diffusion in coals, the process is similar in many respects to the diffusion of solvents in glassy polymers (Fickian diffusion or “Case II” - relaxation of macromolecular structure). Therefore, (6) Larsen, J. W. The expected effects of dissolved CO2 on coal structure and properties. Int. J. Coal Geol., submitted. (7) Larsen, J. W.; Flowers, R. A., II; Hall, P.; Carlson, G. Structural rearrangement of strained coals. Energy Fuels 1997, 11, 998-1002. (8) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Phase Behavior and Macromolecular Structure of Swollen Coals. A Low Temperature 1H and 2H NMR Study. Energy Fuels 1994, 8, 266-275. (9) Reucroft, P. J.; Sethuraman. A. R. Effect of pressure on carbon dioxide induced coal swelling. Energy Fuels 1987, 1, 72-75.

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swelling of coals occurs by processes that have been well documented for other macromolecular (polymer) systems.10 However, unlike other substances, it has been shown that coals possess anisotropic behavior in swelling and the degree of swelling is larger perpendicular to the bedding plane compared to parallel to the bedding plane, where bond density is larger.7,11-12 This behavior and the anisotropic structure of coal have been attributed to the stress conditions under which the coal was generated. Mined coals are in strained state, but since the molecular motion is very slow, they cannot relax from this state rapidly to reach and maintain an equilibrium state with minimum energy conformations until they are swollen with a solvent (e.g., CO2), which plasticizes them into a rubbery state. Although the rearrangement is largely controlled by the coal, the effect of the CO2 as a plasticizer increases the free volume, thus lowering Tg and enabling the rearrangement. For coals having less than about 85% C (dmmf), the rearrangement is toward a more highly associated structure in which the solubility of liquids is reduced, sometimes cut in half. This suggests that under long time storage or injection conditions with CO2 absorbed in a coal, that coal is expected to undergo a slow rearrangement that will decrease the solubility of CO2 in the coal and it will be expelled from the structure.6 The situation with coals having more than 85% C is more complicated. Thus, it would not be surprising to expect different rearrangements, sorption, and rate behavior when the C % changes locally. The changes in the initial reservoir properties of the seam and the resultant gas transport and storage are time- and pressure-dependent. Thus, it is necessary to characterize the subsurface kinetics of sorption-induced swelling and gas transport under changing gas pressures and in the presence of a confining stress. There are not enough data to understand the effects of confinement, and the increased pore and confining pressure on the behavior of coals when they are exposed to CO2 pressure. The availability of such knowledge may allow the adjustment of existing coalbed methane models for the effects of swelling and gas transport changes. Quantitative time-resolved spatial maps of coal density and effective atomic number that enable the analysis of the sorbed CO2 concentration and coal swelling are used to reveal the kinetics of the heterogeneous processes occurring in a consolidated coal kept under constant effective stress as the CO2 injection pressure and confining pressure are increased to different levels. The time-dependent swelling and gas adsorption kinetics have been quantified using dualenergy X-ray CT imaging of the coal under effective stress conditions. Analysis of the two independent X-ray energy data sets yields maps of density and effective atomic number distributions in the coal and these were used to create maps of the amount of sorbed CO2, where it went in the coal, and the changes in the coal matrix. (10) Otake, Y.; Suuberg, E. Temperature dependence of solvent swelling and diffusion processes in coal. Energy Fuels 1997, 11, 11551164. (11) Cody, G. D., Jr.; Larsen, J. W.; Siskin M. Anisotropic solvent swelling of coal. Energy Fuels 1988, 2, 340-344. (12) Ceglarska-Stefanska, G.; Czaplinski, A. Correlation between sorption and dilatometric processes in hard coals. Fuel 1993, 72, 413417.

Heterogeneous Sorption and Swelling in a Coal

Energy & Fuels, Vol. 17, No. 6, 2003 1597

Petrographic analysis of the recovered sample was used to correlate swelling and sorption behavior with local microlithotypes.

(

Z3.8 µ ) F a + b 3.2 E

2. Technical Approach 2.1. Basic Physics behind the Quantitative DualEnergy X-ray Imaging Concept. X-ray CT is a nondestructive testing method based on the attenuation of X-ray energies passing through the objects. The technique was initially developed for medical applications and has been widely used for both medical and industrial applications since then. The technique consists of passing a beam of X-ray through the object from various angles and measuring the attenuation of the X-ray energy from the incident energy. When a material is exposed to a beam of X-rays in CT operation, the radiation is partly reflected and scattered, absorbed, and re-emitted as lower energy electromagnetic radiation or transmitted through the material. The transmitted radiation is received by an array of detectors located around the sample. The basic quantity measured in each element (pixel) of a CT image is the linear attenuation coefficient, µ, which is defined from Beer’s Law:

I ) exp(-µh) Io

The relation between X-ray attenuation and density and atomic number is

(1)

where Io is the incident X-ray intensity and I is the intensity remaining after the X-ray passes through a thickness, h, of the sample. Beer’s Law assumes a wellcollimated beam and a monochromatic source of X-rays. Conventional X-ray computed tomography systems provide cross-sectional images of the investigated object by mapping the local X-ray attenuation values that depend on density and chemical composition, in individual image elements called pixels. The attenuations measured from different angles are collected and processed by a computer to reconstruct the internal structure of the scanned object in terms of attenuation numbers. Unless density and chemical composition effects can be separated quantitatively, the images give qualitative information about the distribution of the materials within the object. 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. The variation of the attenuation properties with different beam energies is significantly different for most materials, making possible the use of two different energies at the same location in order to determine the energy-independent physical parameters, such as electron density and average atomic number. Dual-energy scanning involves use of two different beam energy spectra to distinguish the effects of different components in mixtures. At energy levels below 200 kV, attenuation of X-rays is known to be dependent mostly on photoelectric absorption (a function of atomic number and density of material) and Compton scattering (a function of electron density). Photoelectric absorption dominates at lower energies and becomes more important at higher atomic number. Compton scattering dominates higher energies and becomes more dominant at lower atomic number.

)

(2)

where F is bulk density, Z is the bulk atomic number of the material, and E is the X-ray energy. The first term in the parantheses is the Compton scattering term and the second is the photoelectric absorption term. In the equation, a and b are energy-dependent constants. For mixtures of atoms, an effective atomic number (Ze) is used instead of Z. For given experimental conditions, the different interaction probabilities between photons and matter (Rayleigh, Compton, photoelectric, ...) in the case of a single element material is strongly linked to the atomic number Z. In contrast, for multiple element material, the so-called “effective atomic number” Ze can be used. Its aim is to completely characterize the material, in the same way that the Z value is characteristic of a single element.13 There are different definitions of effective atomic number in the literature. The main difference between them is the fact that some methods consider only the atomic numbers of the elements present in the mixture and their concentrations, while others also take into account the scattering angle and the photon energy. One of the most widely used methods for determining the effective atomic number below 150 kV was given by Tsai and Cho:14

∑ fi Zi3.8)1/3.8

Ze ) (

(3)

where fi is the fraction of the total number of electrons contributed by element i, and Zi is the atomic number of element i. In eq 2, fi is treated as electronic percentage and can be defined as14

fi )

(ωi/Ai)Zi

∑i (ωi/Ai)Zi

(4)

where ωi is the mass percentage, and Ai is the atomic mass. Scha¨tzler15 proposed a formula where the atomic number of each element is weighted by the mass percentage, ωi, of each element:

Ze )

∑i ωiZi

(5)

This equation is similar to the one used to calculate the mass attenuation coefficients of a compound:

µ ) F

∑ωi[F]i µ

(6)

However, it does not take into account the energy of photons and the scattering angle. (13) Duvauchelle, P.; Peix, G.; Babot, D. Effective atomic number in the Rayleigh to Compton scattering ratio. Nuclear Inst. Methods In Phys. Res., Sect. B 1999, 155, 221-228. (14) Tsai, C. M.; Cho, Z. H. Phys. Med. Biol. 1976, 21, 544. (15) Scha¨tzler, H. P. Int. J. Appl. Radiat. Isot. 1978, 30, 115.


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Scanning homogeneous calibration materials at two different energies enables determination of X-ray attenuation (µ) in these specimens of known density and atomic number. Plotting the density and atomic number versus the attenuation values of calibration materials allows the determination of the “a” and “b” values by using eqs 2 and 3 in a linear regression. Once these values are defined for two different energies (h: high, l: low), the following equations can be used to extract the bulk density and the atomic number of the unknown material at each pixel position (x,y) independently:

F(x,y) )

Z(x,y) )

(

bhµl(x,y) - blµh(x,y) bhal - blah

(7)

)

(8)

alµh(x,y) - ahµl(x,y) bhµl(x,y) - blµh(x,y)

1/3.8

2.2. Experimental. 2.2.1. Sample. The sample used in the CO2 sequestration test was a high-volatile bituminous coal from the Pittsburgh seam (DECS-12). A bulk sample that was preserved under an argon atmosphere was cored parallel to the bedding planes to obtain a cylindrical sample (2.5 cm in diameter and 2 cm long). The average elemental compositon of the sample was as follows: 74.78% C, 5.11% H, 1.23% N, 1.12% S, 7.51% O. The sample had 2.5% moisture as received and 10.25% ash in proximate analysis. 2.2.2. CO2 Sequestration Test. The CO2 sorption experiment was performed at 298 K constant temperature. The core sample was placed in the rubber sleeve of a biaxial pressure cell, which is capable of applying pore and confining pressure to the sample separately. The schematic drawing of the core holder and how the sample was located inside is shown in Figure 1A. Before putting the sample into the holder, its cylindrical surface was wrapped with Teflon tape. This, together with the confining pressure on the sample, ensures that mass transfer could only occur through the flat surfaces of the specimen. The aluminum core holder containing the sample was attached to the chucks on the computercontrolled table of the fourth-generation medical scanner (Universal Systems HD-250), which was capable of producing a single energy level scan in 4 s. The pixel resolution and the image thickness were 0.25 mm, and 2 mm, respectively, to create a voxel volume of 0.00125 cm3. The two energy levels used in scanning were 130 kV and 80 kV with an X-ray intensity of 65 mA. The 512 × 512 images generated by the scanner were cropped to construct 100 × 100 data matrixes in order to focus only on the coal. Constant pressure CO2 injection was achieved with a 1-L capacity high-pressure gas cylinder. Pressure transducers and a high-precision pump were used to monitor the gas pressure and to keep the confining pressure at a constant level. Since the pore volume of the coal sample was small in comparison to the gascylinder volume, this cylinder was capable of maintaining constant gas pressure on the sample at all times during the experiment. Confining pressure on the sample was controlled with a high-precision fluid injection pump (Quizix, WA). Before starting the sequestration experiment, the confining pressure on the sample was set to 1.36-MPa by

Figure 1. Schematic drawing of experimental set up (A) and confining and gas pressure changes applied during the experiment (B). The data points are the scan times.

injecting water into the pressure vessel, around the rubber sleeve that hosted the coal sample and safely separated it from water. This effective pressure was kept constant throughout the experiment. The sample was scanned to establish the condition of the sample under vacuum. After these scans, the CO2 that was stored in the pressure cell at 1.7-MPa was slowly released into the sample holder, while at the same time the confining pressure on the sample was increased to 3.06-MPa. Sample scanning, more frequently at the beginning, was continued until there was almost no or very little change in the X-ray absorption data due to further gas uptake. The last scanning sequence was taken as the adsorption equilibrium point for that pressure level and the image data represented equilibrium (after 7240 min). After sorption equilibrium was reached, the sample was isolated and the pressure cell was pressurized to 3.06-MPa pressure with additional CO2. After the pressure and temperature reached equilibrium, the gas at 3.06-MPa was applied to the sample and the confining pressure was increased to 4.42-MPa to keep the effective stress constant. Scanning was performed at this pressure level until adsorption equilibrium in the sample was reached (after another 7240 min). The same procedure was repeated when the gas pressure was increased to 4.42-MPa and confining pressure was increased to 5.78-MPa. Equilibrium at this stage was reached after 5000 minutes. The experimental pressure- and scan-time history is shown graphically in Figure 1B. In this graph, the data points are the scan times during the experiment. This

Heterogeneous Sorption and Swelling in a Coal Table 1. Microlithotypes Determined at the Particular Positions Shown in Figure 4 location

microlithotype

a b c d e f

pyrite particles in a matrix of resinite matrix mineral parting and carbargilite vitrite vitrite and clarite pyrite layer in a matrix of liptite pyrite particles in a matrix of resinite and other liptite macerals, and vitrite clay and inertite vitrite, inertite, and liptite clarite, cutinite, and vitrinertite N/a N/a

g h i j k

experimental methodology enables the experiment to be conducted at three different gas pressures to follow the kinetics of gas storage and matrix property changes at three different gas pressures while keeping the sample under constant effective stress. After the experiment, the coal sample was recovered from the pressure vessel and processed for microlithotype analysis. 2.2.3. Microlithotype Identification. The oriented core segment that had been subjected to high-pressure CO2 sorption was first glued to the bottom of a 32-mm diameter mold and vacuum impregnated with a coldsetting epoxy resin. As the core segment was rather friable, this procedure ensured stability so that different sections of the core could be cut to reveal the approximate location of different CT-scan regions. Because the bedding plane of the coal was oriented parallel with the long-direction of the core, cuts were made perpendicular to the bedding plane approximately 1.4 mm above the regions of each CT-scan interval and the samples were glued to the bottom of separate 32-mm diameter molds and re-imbedded in epoxy resin. After hardening, the sample was ground and polished using a series of grit papers (400 and 600 grit) and alumina polishing slurries (0.3 µm and 0.05 µm using a highnap cloth and silk, respectively). The grinding and polishing procedure removes about 1.3-1.4 mm of material and thus results in an analysis surface in very close proximity to the original CT-scan region. In this investigation, the 20-point ocular procedure defined by the International Committee for Coal Petrology16 was employed and the identification was made on the basis of the so-called 5%-rule, which stipulates that macerals accounting for less than 5% in the assemblage are disregarded.17 Table 1 provides the microlithotype analysis on some particular locations. 2.2.4. Use of Phantom Materials for Dual-Energy Calculation. The use of dual-energy X-ray scanning to determine physical parameters such as density and atomic number requires determining the X-ray attenuation properties of materials of known density and atomic composition as previously explained. For this purpose, water, benzyl alcohol, and fused quartz phantoms were scanned in the core vessel sample to determine their attenuation properties at 130 and 80 kV and 65 mA after removing the coal. The phantom materials (16) International Committee for Coal Petrology. International Handbook of Coal Petrology; Centre National de la Recherche Scientifique: Paris, 1963. (17) Stach, E.; Mackowsky, M.-Th.; Teichmuller, M.; Taylor, G. F.; Chandra, G.; Teichmuller, R. Stach’s Textbook of Coal Petrology, 3rd ed.; Gebruder Borntraeger: Berlin, 1982; 535 pp.

Energy & Fuels, Vol. 17, No. 6, 2003 1599 Table 2. X-ray Attenuations (as CT numbers), Densities, and Effective Atomic Numbers of These Substances That Were Used To Determine Constants a and b of Eq 2

benzyl alcohol water fused quartz

F (g/cm3)

Ze

CT number 130 kV

CT number 80 kV

1.04 1.00 2.65

5.92 7.54 11.81

65.9 82.4 930.9

260.1 322.0 1455.9

were placed in the pressure vessel in exactly the same way that the coal sample was placed in order to obtain artifact-free calibration coefficients. The phantom substances were selected in order to cover the possible range of density and atomic number of CO2 and coal. Table 2 gives the X-ray attenuations (in Hounsfield units), densities, and effective atomic numbers (eq 3) of phantom substances that were used to determine constants a and b of eq 2. The CT numbers given in Table 2 were used to correlate density and effective atomic numbers of the known substances to generate a general equation, for the particular scanning conditions, with constants a and b to extract the local bulk density and effective atomic number of the coal sample with and without CO2. 3. Results and Discussion 3.1. Assessment of Bulk Density and Atomic Composition Changes in the Confined Coal During CO2 Injection. Sorption and gas transport kinetics are essential for predictive calculations and reservoir modeling of CO2 sequestration in coal seams. Evaluation and quantification of sorption and transport kinetics using consolidated cores kept under a constant effective stress have been performed before to investigate the sorption kinetics and behavior of different microlithotypes under CO2 pressure.18-20 In Karacan and Mitchell19 and Karacan,20 single-energy X-ray CT was used. With proper calibration and image quantification, the application of a high energy X-ray beam resulted in maps that were mostly functions of density variations in the sample due to CO2 injection. In these studies, the regions where the injected gas preferentially goes and is stored have been observed. It was possible to calculate the sorption kinetics in different microlithotypes. It was found that high storage capacity and fast diffusion rates were associated with clay + inertite layers. In some of the microlithotypes (e.g., liptite), surface diffusion increased with increasing pressure or surface loading due to their large surface area and relatively low adsorption affinity. A net density decrease was observed in vitrite layers due to coal swelling. The change in density in those layers could not be directly attributed to the net amount of swelling because it was not possible to decompose the amount of gas stored from the matrix effects with a single X-ray energy level. In this study, dual-energy scanning was used for the direct characterization of both their density and chemi(18) Karacan, C. O ¨ .; Okandan, E. Adsorption and gas transport in coal microstructure: investigation and evaluation by quantitative X-ray CT imaging. Fuel 2001, 80, 509-520. (19) Karacan, C. O ¨ .; Mitchell, G. D. Behavior and effect of different coal microlithotypes during gas transport for carbon dioxide sequestration into coal seams. Int. J. Coal Geol. 2003, 4, 201-217. (20) Karacan, C. O ¨ . An effective method for resolving spatial distribution of adsorption kinetics in heterogeneous porous media: application for carbon dioxide sequestration in coal. Chem. Eng. Sci. 2003, 58 (20), 4681-4693.


Figure 2. Normalized bulk density maps after each pressure applied to the coal (A: 1.7-MPa, B: 3.06-MPa, C: 4.42-MPa).

cal composition of the studied materials. Figures 2A,B,C and 3A,B,C give the calculated bulk density and atomic number maps of the coal using the calculation and calibration techniques discussed in section 2.1 and 2.3. The maps (A, B, and C) given in these figures are the normalized bulk density and effective atomic number distributions in the coal at the end points of each pressure regime (1.7-, 3.06-, 4.42-MPa). The normalization was performed by dividing the temporal and spatial data by the corresponding maps of the coal at the initial stage before injecting CO2. Figure 2 shows the temporal and spatial variation of the bulk density in the coal after injecting CO2 at different pressures. Here, Figure 2, parts A, B, and C,

Karacan

correspond to the end points of 1.7-, 3.06-, and 4.42MPa CO2 pressures, respectively. The legends in these figures show that in some regions of the coal, the bulk density increased as much as 20% on the basis of the initial density, especially after the 4.42-MPa pressure. These regions can be characterized as the clay + inertite structures that uptake a large amount of CO2. In these units, gas is transported rapidly due to the relatively open pores of the inertite microlithotype and the interlayer spaces of clay.19 The fast pore diffusion enables gas molecules to go into the macromolecular structure more easily. Additionally, possibly more gas is stored due to the large surface area of the clays and the free volume provided by the interlayer spaces of the clays. However, most vitrite regions appear as dark colors in the images. The injection of CO2 resulted in 6-10% density decrease compared to the initial density of the coal. Since addition of an extra mass (injection of CO2) to the coal would normally not cause a density decrease, this density decrease can be explained by the swelling of cross-linked macromolecular structure as the CO2 dissolves in the vitrite as discussed in the Introduction section. Although the rearrangement is largely controlled by the coal, the effect of the CO2 as a plasticizer is to add additional free volume to the macromolecular network of glassy coal and to lower Tg (glass transition temperature) thus enabling the molecular rearrangement to a lower energy state. For coals having less than about 85% C (dmmf), the rearrangement is toward a more highly associated structure.6 Thus, in our case, the injected CO2 was likely both adsorbed and dissolved in the polymeric structure of the coal material in those regions. The dissolution of CO2 in coal created some free volume so that the polymeric structure of the coal could relax or swell to conform itself into a new rubbery structure. This shows that confined coals also swell. However, swelling is heterogeneous within the coal depending on the maceral composition of the coal material. This should not be surprising. Since coal is chemically very heterogeneous, the interaction of CO2 with different macerals or microlithotypes should be different, resulting in differing gas solubilities and swelling. It must be confirmed whether the observed density decrease is actually caused by the dissolution of CO2 in the polymeric network of the coal material in a confined coal. If that is the case, then there should be a mass increase due to the presence of CO2 in the matrix. However, since the density increase due to CO2 was shadowed by the density decrease due to swelling, it is impossible to determine this with density maps. To answer this question, effective atomic number maps calculated and normalized as in the case of bulk density are presented in Figure 3. The maps given in the figure show very clearly that the effective atomic number in the coal increased, especially in the clay + inertite that showed density increases and in mostly vitrite that showed density decreases due to swelling. The increase of atomic number especially within the swollen regions confirms the presence of dissolved CO2 in those regions. The data given in Figures 2 and 3 confirm that the sorption and swelling behavior of coal is heterogeneous and dependent on the microlithotypes present in the coal.


Figure 3. Normalized effective atomic number maps of coal after each pressure applied (A: 1.7-MPa, B: 3.06-MPa, C: 4.42-MPa).

3.2. Kinetics of Bulk Density Change within Confined and Stressed Coal. The temporal and pressure-dependent changes in the bulk density of coal were quantified for some selected locations during CO2 injection. The relative positions of these locations are shown on the bulk density map calculated after 4.42MPa (Figure 4A) and on the CT number image of the coal (Figure 4B), which gives structural information. The bulk density change kinetic data at different positions listed in Table 1 are shown in graphs given in Figure 5A,B. The end-point bulk density data and the net change at the end of each pressure level are given in Table 3.


In Figure 5A,B, the kinetics of bulk density change during CO2 uptake at positions b, e, j, and g were different than other locations. These points corresponded mainly to mineral partings and rock (b), clay + intertite (g), and pyrite + liptite (e) materials where sorption of CO2 caused a net increase in the bulk density. This suggests that swelling did not occur or was not pronounced at these locations. Table 3 shows that particularly in the clay + inertite region (g), the bulk densities are 7.7%, 20.1%, and 24.7% more than the initial density after injecting CO2 at 1.7-, 3.06-, and 4.42MPa, respectively. In location b, where there was mostly mineral parting and rock materials, there was almost no change in the density. This suggests that there was no CO2 sorption and swelling of that material. Figures 5A and 5B show that the behavior of a, c, d, f, h, i, k were quite different than the other locations mentioned previously. In these microlithotypes, the bulk density decreased with time as gas pressure increased. This indicates the net effect of matrix swelling on density. However, the amounts and kinetics of swelling at each of these points (microlithotypes) were different. With 1.7-MPa CO2, their densities decreased 1%-3% depending on the microlithotype. Increased pressure resulted in a further decrease in bulk density by 4%11% in 3.06-MPa, and by 6%-13% in 4.42-MPa. For example, in vitrite (particularly locations c and h) the bulk densities were 5.3% and 7.8% less than the initial densities at the end of the sequestration test. It is also interesting to follow the kinetics of microlithotype density changes at points c, f, h, and k. As plotted in Figure 5A,B, the bulk density showed a decreasing trend especially when the gas pressure first increased. This decrease was followed by a density increase until the next pressure disturbance (increase) was created. The initial density decrease is probably due to diffusion and dissolution of gas molecules in the macromolecular structure of the vitrites, accompanied by swelling of their structures. This process resulted in a net decrease in the bulk density although there was an additional amount of gas in the coal. After this initial density decrease due to swelling, there was a period of slight density increase in each pressure level. This suggests that after the initial swelling for each pressure step, the coal structure rearranged to a more stable and more highly associated structure in which solubility was reduced and the extra CO2 was expelled from the structure. This behavior may be favored on a plasticized and swollen polymer structure by the confining pressure that acts in the direction opposite to expansion. From the reservoir-engineering point of view, the immediate consequences of this behavior may be changes of coal permeability during gas injection. Rapid permeability decreases following CO2 injection have been reported before.21 Since swelling is probably responsible for the decreased permeability of the coal seam, one can expect an immediate permeability decrease at the beginning stages of injection process, followed by a slight increase in permeability. A similar behavior in the injectivity of CO2 wells has been observed in the Allison Unit of San Juan Basin after CO2 injection has started; initially, (21) Skawinski, R. Considerations referring to coal swelling accompanying the sorption of gases and water. Arch. Mining Sci. 1999, 44, 425-434.


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Figure 4. Map of normalized bulk density at the end of 4.42-MPa (A) and the CT-number image (B) that gives the structural details and the locations of the selected points of interest within the coal. Table 3. Bulk Density of the Analysis Locations at the Final Stages of Each Pressure Regime and the Changes Relative to the Initial Density of the CO2-free Coal bulk density (g/cm3) a b c d e f g h i j k

fraction of initial density

initial

1.7-MPa

3.06-MPa

4.42-MPa

1.7-MPa

3.06-MPa

4.42-MPa

2.134 2.929 1.791 1.463 1.568 1.729 1.888 1.362 1.335 1.326 2.027

2.114 2.926 1.724 1.402 1.732 1.719 2.034 1.339 1.319 1.393 1.893

2.108 2.922 1.686 1.374 1.857 1.689 2.266 1.282 1.290 1.443 1.792

2.059 2.914 1.697 1.300 1.850 1.697 2.354 1.255 1.293 1.504 1.763

0.990 0.999 0.963 0.958 1.105 0.994 1.077 0.983 0.988 1.050 0.934

0.988 0.998 0.941 0.939 1.185 0.977 1.201 0.942 0.967 1.088 0.884

0.965 0.995 0.947 0.888 1.180 0.982 1.247 0.922 0.969 1.134 0.870

injectivity declined significantly, then it began a long period of improvement.22 This process may be repeated at each increase in pressure in the seam. Because the chemical and physical structures of each of the locations discussed were different, their behaviors were also different during CO2 uptake. The behavior of the coal matrix and the different density change kinetics confirmed that the rate of density change would be different at each different pressure step. Figure 6A-C shows the average rate of bulk density change maps in the coal. These maps were calculated by dividing the density change at each pressure by the total duration of that pressure step. This figure clearly shows that the inertite + clay layer sorbed gas to increase its density with a high rate at 1.7-MPa pressure. At 1.7MPa, the density of vitrite layers decreased with an average rate of 5.54 × 10-6 to 1.2 × 10-5 g/cm3/min due to the gas diffusion, dissolution, and matrix relaxation processes previously discussed. At 3.06-MPa pressure, the density of the clay + inertite layer continued to increase. However, at some regions along that layer, the rate of density increase has declined. In the vitrite (22) Reeves, S. The COAL-SEQ Project: Results of the Allison and Tiffany ECBM field studies. Proc. 2nd Annual DOE/NETL Conf. Carbon Sequestration, 5-7 May, Alexandria, VA, 2003.

layer, which was in the middle of the sample, the swelling rate decreased. When 4.42-MPa CO2 gas pressure was applied, the rates of bulk density changes increased again. The kinetic plots given in Figure 5 and the rates of bulk density changes given in Figure 6 show that the gas storage and matrix swelling kinetics are heterogeneous and depend on the microlithotypes present. The rates are not proportional to the gas pressure applied in every part of the coal. The kinetics of these processes are complex functions of diffusion, adsorption, absorption, the swelling of the macromolecular network of coal with time, and the gas pressure applied. The average rate and kinetics even in the same microlithotype can vary, probably due to the transformations in the coal that lead to new structures with every rearrangement. When the CO2 dissolves in a particular microlithotype in the coal, it serves as a plasticizer enabling a structural rearrangement so that the next absorption at a different pressure will be into a new and different structure. 3.3. Quantification of Net Amount of Gas Sorption and Physical Changes in the Coal Matrix. The density and effective atomic number changes and the maps given until now are combinations of the sorbed gas in the coal matrix (gas amount) and the response



Figure 5. Kinetics of bulk density changes at the locations given in Table 1 and marked in Figure 4 (P1 ) 1.7-MPa, P2 ) 3.06-MPa, P3 ) 4.42-MPa).

of the coal to the changes caused by gas interactions (swelling/shrinkage). Dual-energy scans can be used to separate the amount of gas and the changes in the matrix by using the data obtained from bulk density and effective atomic number. To map the actual amount of gas in the coal and the amount of matrix change as a response to gas sorption, the temporal bulk density maps and the effective atomic number maps were used. For this purpose, the effective atomic number of CO2 and the spatial distibution of effective atomic number of the initial coal matrix were considered as simple elements with Ze electrons per atom.13 This allowed establishing an analogy with the classical atomic model and the use of eq 5 to calculate the mass percentages of CO2 and coal matrix at any given time during CO2 injection. By using the relation given in eq 9,

Ze(x,y) )

∑i ωi(x,y)Zi(x,y)

(9)

and the temporal and spatial distribution of effective atomic number of coal (eq 8) for each scan time, we can obtain the mass percentage of CO2 in the coal using eq 10.

|ωCO2(x,y)|@t )

|Ze@t(x,y)| - |Zcoalinitial(x,y)| ZCO2 - |Zcoalinitial(x,y)|

(10)

Figure 6. Average rate of bulk density change within each pressure level (A: 0-1.7 MPa, B: 1.7-3.06 MPa, C: 3.064.42 MPa).

In this equation, Ze@t(x,y) is the temporal effective atomic number map of coal at 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. Absolute value bars denote the 100 × 100 data matrixes. After calculating the mass percent of CO2 and coal (1 - ωCO2) at any time, they were used to calculate the separate mass of each phase in the bulk mass data generated by multiplying bulk density maps with the volume of each voxel (0.00125 cm3). The temporal maps of amount of sorbed gas in the coal and the amount of matrix change that were used


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Figure 7. The maps of net gas amount stored in coal (left row) and the change in the coal matrix (right row) at the end of each pressure step (A, D: 1.7-MPa after 7240 min; B, E: 3.06-MPa after 7240 min; C, F: 4.42-MPa after 5000 min).

to investigate the kinetics of the changes were calculated by using eqs 11-12, respectively.

|S.G.(x,y)(g/g)|@t )

|ωCO2(x,y)|@t‚|Fcoal bulk@t(x,y)| |Fcoalinitial(x,y)|

(11)

|C.M.change(x,y)(∆g/g)|@t ) [|1 - ωCO2(x,y)|@t‚|Fcoal bulk@t(x,y)|] - |Fcoalinitial(x,y)| |Fcoalinitial(x,y)|

(12)

In these equations, S.G.(x,y) is the sorbed gas amount per initial mass of the coal, C.M.change is the differential

change of the mass of coal matrix per mass of initial coal at any time during injection, and Fcoal bulk is the bulk density of coal at the initial stage or during CO2 injection, depending on the subscript. Using these equations, the final conditions for the amount of gas sorption and matrix changes were evaluated, mapped, and correlated for the equilibrium levels of each pressure step. Figure 7 gives the map of net gas present (Figure 7A-C) in the coal, and the amount of net physical change in the coal matrix during CO2 injection (Figures 7D-F). The maps presented in this figure belong to the last stages of each pressure level and have been calculated by using eqs 11 and 12. The same data for



Table 4. Final Amount of Sorbed Gas Calculated Using Eq 11 (g CO2/g of Initial coal mass) at the End of Each Pressure Level and the Differential CO2 Sorption between Each Pressure Step sorbed gas (g/g coal)

differential sorbed gas (g/g coal)

location

1.7-MPa

3.06-MPa

4.42-MPa

0-1.7 MPa

1.7-3.06 MPa

3.06-4.42 MPa

a b c d e f g h i j k

0.0116 0.0019 0.0290 0.0066 0.0061 0.0066 0.0614 0.0119 0.0047 0.0111 0.0544

0.0167 0.0030 0.0400 0.0039 0.0154 0.0226 0.0675 0.0112 0.0424 0.0095 0.0814

0.0394 0.0098 0.0540 0.0418 0.0148 0.0251 0.0814 0.0355 0.0496 0.0534 0.0716

0.0116 0.0019 0.0290 0.0066 0.0061 0.0066 0.0614 0.0119 0.0047 0.0111 0.0544

0.005 0.001 0.011 -0.003 0.009 0.016 0.006 -0.001 0.038 -0.002 0.027

0.023 0.007 0.014 0.038 -0.001 0.003 0.014 0.024 0.007 0.044 -0.010

Table 5. The Amounts of Matrix Change Calculated from Eq 12 [(mcoal - mcoal-i]/mcoal-i) at the End of Each Pressure Level and the Differential Change between Successive Pressure Levels matrix mass change/initial mass (∆g/g)

differential change (∆g/g)

location

1.7-MPa

3.06-MPa

4.42-MPa

0-1.7-MPa

1.7-3.06-MPa

3.06-4.42-MPa

a b c d e f g h i j k

-0.0207 -0.0041 -0.0826 -0.0490 0.0951 -0.0062 0.0323 -0.0212 -0.0230 0.0373 -0.1162

-0.0283 -0.0124 -0.1580 -0.0654 0.1610 -0.0522 0.1403 -0.0528 -0.0718 0.0690 -0.1854

-0.0742 -0.0148 -0.2271 -0.1539 0.1610 -0.0457 0.1849 -0.1086 -0.0880 0.0790 -0.1981

-0.0207 -0.0041 -0.0826 -0.0490 0.0951 -0.0062 0.0323 -0.0212 -0.0230 0.0373 -0.1162

-0.008 -0.008 -0.075 -0.016 0.066 -0.046 0.108 -0.032 -0.049 0.032 -0.069

-0.046 -0.002 -0.069 -0.089 0.000 0.007 0.045 -0.056 -0.016 0.010 -0.013

the selected analysis locations discussed before are given in Tables 4 and 5. Figure 7A-C shows that gas was sorbed mainly in the inertite + clay layer at the end of 1.7-MPa pressure regime. The amount of gas present in this region was between 0.05 and 0.07 g/g depending on the location in that layer. There were other scattered regions in vitrite, liptite, and clarite layers that take up CO2 at this pressure, and their capacity ranged from 0.02 to 0.04 g/g. However, the clay + inertite region was the most distinguishable one on the basis of its capacity for CO2 at this pressure. The point “g” analyzed in this layer sorbed 0.0614 g/g CO2 as shown in Table 4. The points characterized by a (in resinite), c (in vitrite), d (in vitrite and clarite), f (in liptite and vitrite), and h (in vitrite, clarite, liptite) sorbed 0.0116, 0.029, 0.0066, 0.0066, and 0.0119 g/g net CO2, respectively. This shows that at this pressure level, clay and inertite are more accessible to CO2 molecules than the other microlithotypes. This may be due to the highly porous and permeable nature of their structures. Comparison of Figure 7A to Figure 7D shows the changes in the matrix associated with gas sorption at 1.7-MPa pressure. Figure 7D shows that there were not severe changes in the matrix properties at this pressure level. However, in mostly clay and other inorganic material containing regions, a mass increase of 0.070.08 ∆g/g. has occurred. Conversely, in predominantly vitrite regions, a mass decrease of 0.06-0.10 ∆g/g took place. This suggests that vitrites started to swell with gas dissolution and their structures relaxed. The incremental densification of porous and inorganic regions is suggested to be a response to the expansion of vitrites since the coal was in a confined system. In the case of clay layers, the compression was probably possible due to the interlayer spaces of the clay plates. The amounts

of coal matrix mass change (decrease) per initial mass at locations a (in resinite), c (in vitrite), d (in vitrite and clarite), f (in liptite and vitrite), and h (in vitrite, clarite, liptite) were 0.020, 0.0826, 0.049, 0.0062, and 0.0212 ∆g/g, respectively (Table 5), due to swelling. The matrix at points e and g has been compressed as a response to the swelling to create a mass change of 0.0951 ∆g/g and 0.0323 ∆g/g, respectively. As the pressure was increased to 3.06-MPa (Figure 7B), one notices immediately that the vitrite and other organic regions in the coal have sorbed large quantities of gas. This suggests that gas sorption immediately in vitrite-dominated layers is an activated process that is driven by the pressure. It could be either enhanced diffusion or enhanced dissolution. The amounts of gas present and the incremental amount of gas sorption for specific analysis locations are given in Table 4. The sorbed gas in vitrite-dominated regions was between 0.02 and 0.04 g/g, which corresponds to as high as 0.011 g/g. gas increase at point c. There was some additional gas uptake also at point g (in the clay + inertite layer); however, the additional gas amount at that location was small compared to the increase at point c. The quantitative changes in the matrix properties at 3.06-MPa pressure are given in Figure 7E. Comparison of this map with the map of sorbed gas (Figure 7B) shows the intimate relation of gas storage in coal with the changes occurring within the matrix. At this pressure level, the changes in the mass of the coal matrix are clearer due to swelling in the vitrites during gas dissolution. Compression in the clay and clay-mixed layers as a response to that swelling also became more pronounced. At this pressure level, the matrix of vitrites was relaxed to decrease the matrix mass by about 0.050.25 ∆g/g. depending on the location. For example, at point c, there was a decrease of matrix mass by 0.158


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Figure 8. Positions of the locations described in Table 1 on the gas sorption map (A) and on the matrix properties change map (B), computed at the final scan stage of 4.42-MPa pressure. These maps are also shown in Figure 7C,F.

∆g/g as compared to the initial mass of that location as shown in Table 5. A further increase in the pressure to 4.42-MPa created similar behavior in gas sorption and matrix changes. At this pressure level, both the gas sorption amount and the area of gas penetration increased (Figure 7C). A comparison of Figure 7F and Figure 7C shows again the relation of gas dissolution to the changes in the matrix properties. At 4.42-MPa, the extent of matrix swelling became more severe at the locations where more gas was dissolved. The matrix is mobile and rapidly exhibits more extensive swelling with increasing amounts of dissolved gas, especially in vitrites. In a confined system, as in this study, the clay or more porous structures compress more as a response to swelling in other regions of the coal in a fixed overall volume. The gray level legends in Figure 7 give the average amount of changes in gas sorption and in coal matrix. The data for specific points are listed in Tables 4 and 5. 3.4. Kinetics of Gas Sorption and Coal Matrix Changes. To help to follow the kinetics at selected locations in different microlithotypes in the coal, they were marked on the gas sorption map (Figure 8A) and matrix change map (Figure 8B). The kinetic data for the change in gas sorption and matrix changes are presented in Figure 9 and Figure 10, respectively. The kinetic behavior of gas sorption at different locations is shown in Figure 9A-C. The data show that in most of the locations, especially in vitrite, clarite, and liptite microlithotypes (c, d, f, h), the amount of CO2 uptake increased when gas pressure was first applied to the coal. However, after a while the CO2 amount started to decrease in the coal although the CO2 pressure was kept constant. The gas remaining in the coal was found to be about 30-50% less than the maximum sorption amount achieved. As higher pressures were applied, the amount of gas at the end points and the maximum amount sorbed during uptake were higher compared to lower pressure results. But the “breathing” (expansion-contraction) behavior was always repeated

with different kinetics. This behavior in gas sorption is consistent with the expected consequences of the gas dissolution process.6 When the gas pressure was first applied, CO2 diffused into the polymeric network of the coal material and caused it to swell and lowered its Tg. After the initial swelling, the coal rearranged to a more stable and associated structure in which gas solubility was reduced and some of the CO2 was expelled from the structure. This process caused a reduction in the CO2 sorption in those microlithotypes. The observed overshoot occurs because the rearrangement of the coal molecular segments is slower than CO2 uptake and the coal expands past its thermodynamic equilibrium state and then slowly relaxes to it. When the pressure was increased to a higher level, more gas dissolved, but this was followed by another rearrangement. Thus, different sorption and rearrangement kinetics were observed. Location “g” in the clay + inertite layer, however, demonstrated a different behavior. In this location, the dissolution-induced sorption behavior discussed in the previous paragraph was not observed. The amount of gas sorption was related to the time allowed during a pressure step and the amount of pressure. The gas amount stored in this location followed a Langmuir-like behavior suggesting that the process was controlled mainly by pore diffusion and adsorption rather than dissolution. The kinetics of the changes in the coal matrix are presented in Figure 10A,B. The data in the graphs show that the locations e, g, j, corresponding to the porous and inorganic-rich sections of the coal, were compressed as a consequence of the pressures created by the swelling of other structures in the coal. In the other locations, the kinetics of swelling and rearrangement processes have been followed. It is apparent from the kinetics of c and h that, especially in vitrites, increased pressures created more drastic changes in the swelling rate and amount as confirmed by the sudden changes after introducing 3.06-MPa and 4.42-MPa pressures. In almost all microlithotypes (other than e, g, j), the decrease of coal matrix mass due to swelling was



Figure 10. Kinetics of the coal matrix mass change at the locations given in Table 1 and marked in Figure 4 and in Figure 8 (P1 ) 1.7-MPa, P2 ) 3.06-MPa, P3 = 4.42-MPa).

Figure 9. Kinetics of net gas sorption at the locations described in Table 1 and marked in Figure 4 and in Figure 8 (P1 ) 1.7-MPa, P2 ) 3.06-MPa, P3 ) 4.42-MPa).

followed by a slight increase due to rearrangement into a new more stable structure. This behavior correlates well with the observed behavior in gas sorption kinetics graphs in Figure 9. 4. Conclusions A density increase of as much as 20% of the initial density was observed in clay + inertite layers with gas sorption. In these units, gas is transported rapidly due to the relatively open pores of the inertite microlithotype and the interlayer spaces of the clay. With 1.7-MPa CO2, vitrite, liptite, and clarite densities decreased 1%-3%, depending on the microlithotype. Increased pressure resulted in a further decrease in bulk density by 4%-11% at 3.06-MPa, and by 6%-13% at 4.42-MPa. The dissolution of CO2 in coal created some free volume so that the macromolecular structure of the coal could relax or swell on the time scale of the experiment.

Confined coals swell. However, swelling is heterogeneous within the coal, depending on the maceral composition of the coal material. The density change kinetics of vitrites shows a “breathing” (expansion-contraction) behavior. This is 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 bulk density change rates are not proportional to the gas pressure applied in every part of the coal. The variability of average rate and kinetics even in the same microlithotype can be due to the transformations in the coal that occurred in the previous dissolution so that the next sorption at a different pressure will be into a new and different structure. The net sorption amount data indicate that the highest amount of gas is stored in inertite + clay regions (∼0.08 g/g of coal), followed by vitrite layers (∼0.06 g/g). In vitrite-dominated layers, the dissolution had an important effect, as evidenced by the swelling of those regions. As the gas dissolved and the matrix swelled and clay-rich sections were compressed to compensate the volume increase in a confined system. In vitrite, liptite, and clarite regions, the amount of gas sorption usually increased drastically, followed with a falloff period. This process shows that some of the initially sorbed gas is expelled from the structure due to rearrangement of the matrix to a structure where gas solubility is less. The results presented in this paper have important implications from the reservoir-engineering point of


view. In enhanced coalbed methane recovery or direct injection of CO2 into the seam, a long-term storage is aimed and usually the injection pressures are varied, depending on the injectivity changes in the well. The immediate consequences of the swelling kinetics may be on the change of permeability of the coal during gas injection. One can expect an immediate permeability decrease at the beginning stages of the injection process, followed by a slight increase in permeability. The injectivity results of CO2 injection wells in the San Juan Basin confirm this behavior. This process may be repeated at each increase in pressure in the seam. Also, a decrease in gas solution may be observed and some of the gas that is measured or

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predicted to be in the seam will be expelled from the coal structure. Acknowledgment. Dr. J. W. Larsen is gratefully acknowledged for reviewing an earlier version of this paper and for making valuable comments and for fruitful discussions. Dr. A. S. Grader, and Dr. P. M. Halleck of the Pennsylvania State University are thanked for making the X-ray CT facility available for this study and for useful discussions. The author also thanks Gareth D. Mitchell of Pennsylvania State University for performing microlithotype analysis on the sample. EF0301349

Heterogeneous Sorption and Swelling in a Confined and Stressed

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