Energy Fuels 2010, 24, 2964–2973 Published on Web 04/08/2010
: DOI:10.1021/ef9015075
Experimental Study on the Porous Structure and Compressibility of Tectonized Coals Zhenghui Qu,†,‡ Geoff G. X. Wang,*,‡ Bo Jiang,† Victor Rudolph,‡ Xinzhao Dou,† and Ming Li† †
School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221116, China, and ‡ School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia Received December 10, 2009. Revised Manuscript Received March 21, 2010
This paper presents experimental investigations on the porous structure and compressibility of six representative tectonized coals, which were geologically formed because of post-formation tectonic deformation of the coal seam, resulting in various brittle and plastic structure development in the seam, such as fissure and fold. The investigations were carried out using microscopy and mercury intrusion porosimetry, providing experimental information about better characterization of the coal structure for coalbed methane recovery from tectonized coal seams. In combination with measurements of vitrinite reflectance and pore size distribution, the mercury intrusion porosimetry data were further analyzed with the fractal theory and used to determine the pore compressibility of the coal samples. The results show that tectonic deformation mainly reformed the bigger pores (pore size above 100 nm). In general, the increased tectonic deformation led to more open pores and, hence, the enhanced connectivity of the pore network. In fractal analysis, a linear relation was used to fit the mercury intrusion data at high pressure instead of the power law that was typically used in previous studies on ordinary coals. The volume-pressure curves obtained by mercury intrusion measurements for all coal samples exhibit a strong dependence relation with the deformation extent. The pore compressibility of these coals obviously decreases as the deformation extent increased, with only one exception for all coal samples studied. This implies that the weak deformation may be corresponding to a high compressibility. Moreover, the results also show that the rank may be responsible for a significant part of the differences in porosity and pore compressibility as well.
changes of permeability of coal seams as the stress field changes. Therefore, a better understanding of the porous structure (pores and/or cleats) and pore compressibility of coals is of significance in improving the application of CBM and CO2-ECBM technologies. Compressibility of material is a parameter that quantifies the relationship between the pressure exerted on a body and the resulting change in its volume. For porous rocks, such as coals, there are two independent volumes, the pore volume and the bulk volume, and also two independent pressures, the pore pressure and the confining pressure. Thus, two different types of compressibility can be defined for coal, giving the pore and matrix compressibilities. Here, the pore mainly consists of mesoand macropores and microfractures or cleats, and the matrix means the coal body represented by coal grains or particles. The current study only focuses on the effective pore compressibility, defined as the effect of the pore pressure variations on the volume of void space contained in coal.3-5 Because it represents the volume of excess fluid that can be stored in or flow through the pore space as a result of an increase of the pore pressure, the pore compressibility has been thought as an important parameter for reservoir analysis. There are considerable experiments designed to measure compressibility. However, most of these experiments focus on the oil and gas reservoirs, and the research on compressibility of coalbed reservoirs is very limited.6,7
1. Introduction Coalbed methane (CBM) has come to prominence worldwide as a low-carbon energy source, evidenced by increasing investments in CBM exploration and exploitation during the past decade. This has led to extensive commercial development and high growth in the coal and energy sectors. Enhanced coalbed methane recovery with CO2 geo-sequestration (CO2-ECBM) in coal seams also becomes attractive because of its dual capabilities, i.e., increasing the CBM recovery from coal seams while reducing the CO2 emission through geo-sequestration of the greenhouse gas.1 However, existing knowledge about the coal structure and its mechanical properties is insufficient to support further technology development in this area and to ensure commercial success. Coal is typically a naturally fractured system containing multi-scale pores (micro-, meso-, and macropores) and microfractures or cleats. These pores or cleats provide access to the hydrocarbon fluid adsorbed in the matrix for CBM recovery and flow paths and storage of CO2 for the geo-sequestration of greenhouse gas. The textural structures of coals are highly heterogeneous, and hence, the flow of fluids (gas and water) in coals exhibits a strong heterogeneity.2 Furthermore, coals can be quite compressible, which mainly results from the compression of cleats under deformation of the coal fabric and induces *To whom correspondence should be addressed. Telephone: þ61-733653928. Fax: þ61-7-33654199. E-mail:
[email protected]. (1) White, C. M.; Smith, D. H.; Jones, K. L.; Goodman, A. L.; Jikich, S. A.; LaCount, R. B.; DuBose, S. B.; Ozdemir, E.; Morsi, B. I.; Schroeder, K. T. Energy Fuels 2005, 19, 659–724. (2) Laubach, S. E.; Marrett, R. A.; Olson, J. E.; Scott, A. R. Int. J. Coal Geol. 1998, 35, 175–207. r 2010 American Chemical Society
(3) Hall, H. H. Pet. Trans. AIME 1953, 198, 309–311. (4) Zimmerman, R. W. J. Appl. Mech. 1985, 52, 606–608. (5) Zimmerman, R. W. Compressibility of Sandstones; Elsevier: Amsterdam, The Netherlands, 1991. (6) Newman, G. H. J. Pet. Technol. 1973, 25, 129–134. (7) McLatchie, A. S.; Hemstock, R. A.; Young, J. W. J. Pet. Technol. 1958, 10, 49–51.
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fracture developed with simple combination fracture developed mostly with simple combination and partly with relatively intensive combination
granulated coal
cataclastic coal
powdered coal
initially cataclastic coal cataclastic coal
gc16
gc21
gc25 gc28
integrated, with intensive joints developed destroyed, composed of coal granule and powder integrated, with rare joints developed integrated, with joints developed
hard, cannot be broken by hand easily hard, can be hardly broken into fragments of more than 1 cm by hand
crazes developed intensively, and the continuity is broken
fracture developed with relatively intensive combination
fracture developed mostly with intensive combination
relatively soft, can be broken into powder and small granules by hand hard, can be hardly broken into particles of less than 1 cm by hand soft, can be broken into powder by hand easily
soft, can be broken into powder by hand easily
destroyed, composed of coal granule and powder destroyed, composed of coal granule gc14
samples
(8) Hou, Q. L.; Li, P. J.; Li, J. L. Foreland Fold-Thrust Best in Southwestern Fujian; Geology Press: Beijing, China, 1995. (9) Jiang, B.; Qin, Y.; Qu, Z. H.; Li, M. Structure of tectonic coals and their reservoir physical properties. Proceedings of the 2008 Asia Pacific CBM Symposium, Brisbane, Queensland, Australia, 2008.
powdered coal
tectonized feature
A total six of the coal samples have been investigated in this study. These coal samples were collected from the Qinglong coal mine in Guizhou, southwest China, named by a number with a prefix “gc” (e.g., gc13, gc14, gc16, etc.). This coal mine is one of the typical tectonized coalfields in China, consisting of several tectonized coal seams featured by the well-developed reverse and layer slip faults. Among these coal specimens, sample gc16 was from coal seam 15, while all others were from coal seam 16. Coal seam 16 is the main minable coal seam in the coal mine, with an average thickness of about 2.88 m, and coal seam 15 is thin and generally unminable. According to the classification schemes proposed by Jiang et al.,9 all six of these samples belong to brittle deformation coals of different types. The brief classification is described in Table 1 and will be interpreted in detail later. Note that the coal samples used were selected on the basis of the key features of tectonized coals, following the two criteria below. The samples must have clear differences in deformation extent and must contain relatively high vitrinite. The numbers of these coal samples are not crucial, but more samples used would enhance the representative of the given tectonized coals. 2.1. Sample Preparation. To prevent coal samples from oxidizing during sample collection, caution was taken by choosing the working and heading faces in the coal mine as the main sampling spots. The coal in these spots was just exposed to air
gc13
2. Experimental Section
strength of coal
Table 1. Texture and Strength Properties of Selected Samples
microscopic texture
Furthermore, the traditional methods that were widely used to measure the compressibility of oil and gas reservoirs could not simply be used to measure the coal compressibility because coal is not so hard as the rocks in oil and gas reservoirs. As such, it is highly desirable to seek alternative approaches for improved measurement of coal compressibility. Tectonized coal is a kind of deformed coal caused by tectonic movements that happened after the diagenesis of coal seam.8 The distribution of different types of tectonized coals depends upon the types and properties of tectonics, and the tectonized coals develop more complexly in areas with repeated tectonic superimposition. In China, tectonized coals are widely distributed with different deformation extents. A coal seam with tectonized coal developed is usually featured by a damaged coal structure and high gas content with low methane pressure and poor permeability. These features make it unsafe in coal mining and difficult for CBM recovery because of difficulties in well completion, control of gas drainage, etc. When coal has been deformed during the tectonic movements, the coal compressibility has also been disturbed, resulting in different mechanical properties, especially strength and hardness, from ordinary coals. This largely limits the further developments of safe mining and effective CBM recovery in tectonized coal seams, and close attention needs to be paid to a fundamental study for better understanding. In this paper, a new method to determine the coal compressibility will be developed and verified on the basis of the analyses of the porous structure of selected coal samples using mercury intrusion porosimetry (MIP) data. All coal samples studied were collected from the coal seams subjected to the tectonic movements. The effective pore compressibility of these coals will be discussed to address the compressible characteristics of the given tectonized coals.
crazes developed intensively
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integrity of coal
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of coal. However, these measurements are insufficient for quantitative characterizations of pores and/or cleats in coal under conditions of relatively high pore pressure associated with CBM/ECBM recovery and CO2 geo-sequestration. To overcome this deficiency, mercury porosimetry measurements was carried out to characterize the pore volume and the pore size distribution. The mercury porosimetry measurement mainly falls into coverage of the meso- and macropores and microfractures or cleats in coal, which are major concerns in this particular study. Mercury porosimetry measurements provide a series of MIP data, which are usually used to determine the pore volume and the pore size distribution. However, the MIP data could be further interpreted to yield quantitative information about access to the intrinsic structural properties of coal, for example, the pore compressibility of coal. Because the MIP data for coal include the increasing pore volume directly resulting from compression when the intrusion mercury pressure exceeds 10 MPa,10,11 it is possible to retrieve the pore compressibility from the MIP data, which will be discussed as follows. According to the definition of effective pore compressibility of the porous medium discussed above, the pore compressibility of coal can be expressed in the following form: 1 δVp ð1Þ βp ¼ Vp δP T
and was carefully collected using a geologic hammer, enwrapped first by soft paper and then adhesive tape with marks at the end. Then, the collected samples were immediately transported to the laboratory for experimental tests and analyses. The light electronic microscope and vitrinite reflectance measurements use 3 3 3 cm coal bricks made from the bulk coal samples collected above. First, a whole piece (bigger than 3 3 3 cm) was separated from each raw sample, boiled in a glue solution composed of rosin, paraffin, and turpentine in a proportion of 20:4:1, respectively, and cut into cubes with a size that is slightly bigger than 3 3 3 cm. Then, three mutually perpendicular sides of each cubic sample were chosen and ground into smooth surfaces required for microscopic investigation. The specimens used for the MIP measurements are about a 3 g weight containing several small pieces, with a grain size of about 0.5-1 cm, sampled from corresponding remaining coal samples. Before the test, all of the MIP specimens dried at 60 °C in a platform dryer for at least 12 h. 2.2. Experimental Procedures. Before the samples were prepared, a photo of each bulk coal block (hand specimen) was taken by a digital camera, with the coal sample marked and identified corresponding to the coal block for reference. Meanwhile, maximum vitrinite reflectance measurements (500 points) and proximate analysis were performed for all coal samples using a microscope photometer AX10 imager M1m made by the Carl Zeiss Company of Germany and following GB/T 212-2008, a national standard for coal proximate analysis, respectively. All experiments, including microscopic analyses and mercury intrusion tests (see the below), were carried out at the Analysis and Measurement Centre, China University of Mining and Technology, apart from the proximate analysis, which was performed at the Jiangsu Institute of Coal Geology Exploration. Then, the cubic samples prepared above were chosen for microscopic investigation using a light electronic microscope. Snapshots were taken of the representative microstructures, providing a series of microscopic images for further analyses, to characterize the coal textures. These images were taken with a single polarizing filter, giving optical zooms of 40-100. Finally, the mercury intrusion tests were conducted using the MIP specimens prepared above. MIP used in this study is MIP9310 manufactured by the Micromerities Company of America. The working pressure of the MIP9310 is from 0.0035 to 206.834 MPa. The minimum pore diameter that can be measured is 7.2 nm. The analysis in the porosimeter was started with the evaluation, in which the remaining water and air in the coal sample were removed. Following the standard procedure, pressure was controlled under 0.05 mmHg (6.67 10-6 MPa) at this stage, so that most of the remaining water can be degassed from the sample. Then, the sample container (porosimeter) was filled with mercury. The first measurement was taken at about 6.0 10-2 MPa, representing the pore diameter of about 250 μm, according to the Washburn equation at 4.8 10-3 N/ cm of the surface tension of mercury σ and 140° of the mercurycoal contact angle θ. The subsequent measurements up to about 0.15 MPa were then taken, with 10 s intervals for equilibration after each pressure change. After that, the higher pressure was applied, from the maximum pressure in the preceding test until about 206 MPa, corresponding to the pore diameter of about 7.2 nm.
where βp denotes the pore compressibility (MPa-1), Vp is the pore volume (L3), and P is the intrusion mercury pressure (MPa). Note that βp here is somewhat different from the effective pore compressibility, because P in eq 1 is not exactly the same as the pore pressure and the confining pressure could not be kept as a constant because of the effect of the pore pressure variations on the volume of void space contained in coal.3-5 However, eq 1 can still be used to investigate the compressible property of coal with some corrections to the relationship between Vp and P resulting from the MIP measurements.12 As a commonly used method for determining the pore size distributions in porous materials, the MIP measurement yields the total volume of pores with sizes greater than the lower limited diameter that are accessible from the surface, manifesting the idea underlying fractal geometry. This concept, originally introduced by Mandelbrot,13 provides an interpretation of measurements of surface roughness. The area of any rough surface depends upon the measuring scale. According to the fractal theory, a surface is defined to be fractal if its area can be described by the power law below SðRÞ∼R2 - D
ð2Þ
3. Analysis
where S(R) represents the rough surface area (L2), R indicates the measuring scale in terms of radius (L), and D > 2 is defined as the fractal dimension. For the special case of a smooth surface, D = 2. Generally, rough surfaces have a value of D > 2.13 The surface fractal dimension can also be determined from the pore volume measurement. Assuming that v(r) µ rs(r), where v(r) and s(r) denote the average volume and surface area
In this study, the coal texture, including pores (mainly meso- and macropores in this case), microfractures or cleats, and their distributions, was characterized by the relatively standard method using a light electronic microscope. It provides the preliminary visualization for the porous structure
(10) Debelak, K. A.; Schrodt, J. T. Fuel 1979, 58, 732–736. (11) Toda, Y.; Toyoda, S. Fuel 1972, 51, 199–201. (12) Friesen, W. I.; Mikula, R. J. Fuel 1988, 67, 1516–1520. (13) Mandelbrot, B. B. The Fractal Geometry of Nature; W. H. Freeman: San Francisco, CA, 1982.
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with respect to the pore size r, the following relationship can be established for a fractal surface:14,15 - dVp ðrÞ µ r2 - D ð3Þ dr
hand easily. Under a microscope, the sample is mainly composed of vitrinite and contains some banding clay. The fractures represent intensively developed crazes (Figure 1a). Sample gc14 is classified as a granulated coal, featured by a massive texture and composed of powder and granules. It exhibits coarse sections and well-developed grinding mirror surfaces. It is relatively soft and can be broken into powder and small granules by hand. Microscope analysis shows that the coal is mainly composed of vitrinite and contains some clay filled into the fractures. The fractures are relatively intensively developed, mostly in small and middle scales, with properties of brittle, tensional, and sheared characteristics. The combination is mostly branch-like, as illustrated in Figure 1b. Sample gc16 is so-called cataclastic coal, and its structure is massive, integrated with intensively developed joints, having irregular sections. For strength property, it is rigid and cannot be easily broken into particles of less than 1 cm by hand. Microscopically, the coal is mainly composed of vitrinite and contains some spot-like pyrites and banding clays (Figure 1c). The fractures are largely developed in small and middle scales, with mechanical properties featured by brittle, tensional, and sheared characteristics. The combination is mostly meshy (Figure 1c). Sample gc21 is also a type of powdered coal, composed of powder and granules, having coarse sections and well-developed grinding mirror surfaces. Its texture is massive. The coal is soft and can be broken into powder by hand easily, and its microscopic composition mainly consists of vitrinite and a few pyrites and clay filled into the fractures. There are massive fractures with intensively developed crazes, resulting in the discontinuous coal structure (Figure 1d). Sample gc25 is an initially cataclastic coal mixed and integrated with two groups of undeveloped joints. The coal has a hard strength and cannot be broken by hand easily, and the mechanical properties exhibited are brittle, tensional, and sheared. The major microscopic composition in this coal is vitrinite. The fractures are developed mostly in small and middle scales and combined in simple forms, such as tensional fractures cut by sheared fractures (Figure 1e). Sample gc28 is also a type of cataclastic coal, mixed and integrated with relatively developed joints, having irregular sections in the hand specimen. It is hard to break into more than 1 cm fragments by hand, and the mechanical properties also exhibit brittle, tensional, and sheared features. It is mainly composed of vitrinite and contains some pyrites, partly developed spot-like and partly filled into the fractures. The fractures are developed mostly in small and middle scales, with just a few big-scale fractures developed as well. The combination is mostly simple with en echelon and intersecting arrays (Figure 1f). In other words, structural characterization of tectonized coals provides first-hand information for evaluation of the correlation between tectonic deformation and coal structure. The structural characterization above shows that the tectonic deformation extent for the given coal samples generally varies from weak to strong, following the order gc25 f gc28 f gc16 f gc14 f gc13 f gc21. 4.2. Pore Size Distribution. Figure 2 shows the pore size distributions of all coal samples, which were retrieved from the original MIP data using Matlab. According to Hotot’s classification on coal pore size,16 the four pore size ranges can
where Vp(r) is the total volume of pores with sizes greater than r that are accessible from the surface or the cumulative pore volume. In combination with the Washburn equation12 P ¼ - 2σ cos θ=r ð4Þ Equation 3 can be rewritten as dVp ðPÞ µ PD - 4 dP
ð5Þ
In eq 4, σ is the surface tension of mercury (N/L) and θ is the mercury-solid contact angle (degree). Thus, the cumulative pore volume Vp(r) can be estimated by varying the intrusion mercury pressure using eq 5. Given the uncertainty of the relationship between Vp(P) and MIP measured quantity Vm(P), in most instances, Vp(P) needs to be replaced by Vm(P). In that case, eq 5 can be rewritten as Vm ðPÞ ¼ A þ BPD - 5
ð6Þ
with integral constants A and B after integration, which can be determined together with the fractal dimension D by fitting the MIP data. First, according to eq 5, plotting the MIP data in the form of log(dVm(P)/dP) versus log(P) allows for any power law behavior to be revealed. The value of the fractal dimension D can be determined by fitting straight lines to the calculated points. Then, applying the least-squares fitting of the data to eq 6 obtains the constants A and B, giving the relationship between Vm(P) and P as indicated in eq 6. Finally, substituting the eq 6 determined by the preceding procedure into eq 1 allows for the determination of the pore compressibility of coal. This method has been used in similar studies by previous researchers on ordinary coals, and the results showed an excellent agreement between the fitted curves and the MIP data.12 4. Results and Discussion Figure 1 shows the typical images of coal texture captured by microscopy. The results of the proximate analysis, mercury intrusion tests, and vitrinite reflectance measurements are summarized in Table 2. The ash-free basis porosity for all samples was corrected on the basis of the ash and porosity data and was also presented in Table 2 as well. The details about the mercury intrusion tests, including the evaluation of the pore size distribution and the characteristics of the pore shapes and connectivity using MIP data, will be discussed later. Moreover, the MIP data were further interpreted to determine the pore compressibility of the coals using fractal theory. 4.1. Structural Characterization. Sample gc13 is a powdered coal with massive texture. This coal is composed of powder and granules, having coarse sections and well-developed grinding mirror surfaces. It is very soft and can be broken into powder by (14) Friesen, W. I.; Mikula, R. J. J. Colloid Interface Sci. 1987, 120, 263–271. (15) Pfeifer, P.; Avnir, D. J. Chem. Phys. 1983, 79, 3558–3565.
(16) Hotot, B. B. Coal and Gas Outburst; Song, S. Z., Wang, Y. A., Translators; China Industry Press: Beijing, China, 1961.
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Figure 1. Micro photos of all selected samples. Table 2. Results of Proximate Analysis and Vitrinite Reflectance and MIP Measurements proximate analysis samples gc13 gc14 gc16 gc21 gc25 gc28
MIP
Mada (wt %)
Adb (wt %)
Vdafc (wt %)
vitrinite reflectance Ro,max(%)
total pore volume (cm3/g)
total pore area (m2/g)
εdd (%)
εdafd (%)
1.98 0.68 0.82 2.46 2.24 1.34
7.10 7.94 10.55 6.27 6.36 10.69
7.72 7.4 7.39 7.30 6.69 7.75
3.05 3.24 2.52 2.42 2.58 2.80
0.0571 0.0583 0.0379 0.0961 0.0268 0.0266
9.4965 7.8337 7.1068 8.2055 6.7377 6.6487
7.58 7.83 5.52 12.19 3.71 3.76
8.16 8.50 6.17 13.01 3.96 4.21
a Mad = moisture on an air dry basis. b Ad = ash content on a dry basis. c Vdaf = volatile on a dry ash-free basis. d εd and εdaf = porosities on a dry basis and a dry ash-free basis, respectively.
be figured out based on the critical pore sizes of 10, 100, and 1000 nm, corresponding to pore 1 (1000 nm), respectively. The pore volume contrast histogram of these pores for the studied coal samples was retrieved as well, as illustrated in Figure 3. As can be seen, the total pore volumes and the porosity (including εd and εdaf) of the selected coal samples basically ascends as the tectonic deformation increases, following the same order that resulted from structural characterization, i.e., gc25 f gc28 f gc16 f gc14 f gc13 f gc21. This can also be demonstrated by micro photos in Figure 1, showing that the amount of fractures increases following the order above. This confirms that tectonic deformation caused increased
fractures and pores. It was further noticed that, except for sample gc13, the cumulative pore volumes for pores 3 and 4 in the coal samples become bigger and bigger as tectonic deformation increases (Figure 2). As a result, the shape of the cumulative pore curves changes accordingly and the slopes of the cumulative curves for pores above 100 nm increase as well. For the staggered pore volumes, the curves vary in a similar way when the pore size is under 100 nm. However, when the pore size is above 100 nm, the changes of the staggered pore volumes become more complicated as the tectonic deformation extent increases. This phenomenon implies that tectonics might mainly reform bigger pores (pore size is larger than 100 nm) in coal, which can be further confirmed by the pore volume contrast histogram (Figure 3). As shown in Figure 3, the changes of the pore 2968
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Figure 2. Pore size distribution curves of various coal samples.
volumes in the coal samples, excluding sample gc13, are slight for pores 1 and 2 and the changes for pores 3 and 4 ascend as tectonic deformation increases. This also suggests that tectonics might generate more fractures and pores. Sample gc13 is exceptional in this study. It might not have been reformed by tectonics only and needs further study. Although a clear relationship between the tectonic deformation extent and the porosity of the samples has been
found, it does not mean tectonic deformation is the only reason leading to a higher porosity. The vitrinite reflectance is for sure playing a role in the differences of the porosity. For anthracite, the higher the rank, the lower the porosity and the smaller the pore as well. These small pores usually exhibit a slit shape between the relatively well-ordered aromatic clusters. Nevertheless, there are clear differences in the rank, between 2.42 and 3.24% in this study. These are necessarily 2969
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from high porosity and poor strength and, hence, has a high potential for the risk of coal and gas outburst in coal mines. 4.4. Pore Compressibility. As discussed before, eqs 1, 5, and 6 provide a method to calculate the fractal dimension and the pore compressibility of coal.12,19 This method has shown to be effective with ordinary coals12,14,20,21 but has not been verified with tectonized coals. As implied in eqs 5 and 6, there exists a linear relation between log(dVm(P)/dP) and log(P) for the MIP data. Plotting the MIP data obtained for all tested tectonized coals in the form of log(dVm(P)/dP) versus log(P) gives the results as illustrated in Figure 5. It can be seen from Figure 5 that the data can be fitted by two linear relations corresponding to the lower and higher pressures for all tested coals. In the ranges of higher pressures with P > 10 MPa, the slopes of fitting lines are nearly horizontal, which is different from the negative values obtained by above researchers. The results indicate that, in this study, the relationship between mercury intrusion volume and mercury pressure in higher pressures for the tectonized coals studied should be linear (which could be thought as a special power law, with the power equal to 1) rather than the typical power law, which is different from the results observed by the above researchers. Meanwhile, a linear relation of log(dVm(P)/dP) versus log(P) implies that the power law behavior changes in the mercury intrusion volume with the mercury pressure, which may be true if the increment of the mercury intrusion volume is caused only by the pores filled with mercury. However, the volumetric increment at higher pressures is actually controlled by both the mercury-filled pores and the pore compressibility in coal, which are hard to distinguish. Therefore, the power law relation between the mercury intrusion volume and mercury pressure would be improper when the pore compressibility becomes the dominant factor that controls the increment of mercury intrusion volume. In this case, a linear relation should be used to fit the MIP data for determination of the pore compressibility. Several researchers have observed this phenomenon and used the fitted straight lines to determine the relationship between mercury intrusion volume and pressure.11,22-24 It was probably the tectonic behaviors that changed the strength property of coals, which made them more sensitive to pressure during the MIP measurement. As a result, the increment of mercury intrusion volume in higher pressures mostly depends upon the pore compressibility of coal. Figure 6 shows that the mercury intrusion volume in all coal samples is sharply increased under lower pressures and the increment is apparently slowed down in the range of higher pressures. It can be further found that a linear relationship fits the MIP data well under higher pressures and the slopes of these fitted lines have little difference varying from 0.1127 to 0.1241. Except for sample gc13, other coal samples exhibit almost the same increment in mercury intrusion volume as the intrusion pressure increases under conditions of higher pressures, implying that the tectonic deformation has less effect on the volumetric increment in
Figure 3. Pore volume contrast of selected samples in different pore size ranges.
superimposed to the tectonic fragmentation, which impacts the coal porosity. As can be seen in Table 2, the porosity of sample gc21 is dramatically higher than the other five samples. This may be the result of not only tectonic deformation but also coal rank because this sample has the highest deformation extent and the lowest coal rank among all selected coal samples. 4.3. Pore Shape and Connectivity. The effective pores in coal are what MIP can measure, including open, semi-closed, and flask pores. The MIP curves of coals with different contents of the three kinds of effective pores show different hysteresis loop characteristics.17,18 In general, the open pore has a hysteresis loop, while the semi-closed pore does not have the hysteresis loop because its mercury intrusion pressure equals the mercury extrusion pressure. The flask pore, with two mercury extrusion pressures corresponding to its neck and body, has a special type of hysteresis loop featured by sudden declining, and the mercury extrusion curve exhibits an upper convexity. Figure 4 shows the mercury intrusion and extrusion curves for the various coal samples. It can be seen that the mercury extrusion curves of all of the studied coal samples are apparently down convexities. The curves for sample gc21 also show the partially upper convex feature (Figure 4). These results mean that there is a certain content of semi-closed pores contained in all tested coal samples, and the flask pore would be formed when tectonic deformation was strong. Additionally, except for sample gc13, the volume difference between mercury intrusion and extrusion increases with tectonic deformation. This indicates that tectonics would result in more open pores and the connectivity of the pore network could be enhanced with increased tectonic deformation. On the basis of the pore structure analyses above, they suggest that tectonized coals, such as samples gc25 and gc28, containing low porosity are generally not suited for CBM exploitation and the coals, such as samples gc16 and gc14, meet the basic conditions for CBM recovery because of their relatively high porosity and open-pore structure, while the coals, such as samples gc13 and gc21, have high porosity and poor strength and, hence, are neither suitable for CBM exploitation nor safety for coal mining because of the increased risk for coal and gas outbursts. This is particularly important for mining of the coal, such as sample gc13, because it exhibits a relatively low open-pore content apart
(19) Xu, L. J.; Liu, C. L.; Xian, X. F.; Zhang, D. J. Colloids Surf. 1999, 157, 219–222. (20) Fu, X. H.; Qin, Y.; Xue, X. Q. Coal 2000, 9, 1–3. (21) Wang, W. F.; Xu, L.; Fu, X. H. Coal Geol. China 2002, 14, 26–33. (22) Zwietering, P.; Krevelen, W. V. Fuel 1954, 33. (23) Spitzer, Z. Powder Technol. 1981, 29, 177–186. (24) Li, Y. H.; Lu, G. Q.; Rudolph, V. Part. Part. Syst. Charact. 1999, 16, 25–31.
(17) Qin, Y. Micropetrology and Structural Evolution of High-Rank Coals in People’s Republic of China; China University of Mining and Technology Press: Xuzhou, China, 1994. (18) Yan, J. M. Absorption and Solidificaiton: Surface and Pore of Solid Substance; Science Press: Beijing, China, 1986.
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Figure 4. Semi-log V-P plots for selected samples.
this circumstance. However, both the total volume in lower pressures and the starting pressure at which the linear relationship can be fitted to the MIP data well increase with the tectonic deformation. As discussed before, the tectonic deformation extent in the studied coals varies from weak to strong, following the same order obtained by the structure and pore size distribution analyses: gc25 f gc28 f gc16 f gc14 f gc13 f gc21. Note that sample gc13 is exceptional, which has a less total volume in lower pressures than that the coals with weaker tectonic deformation. This coal sample exhibits the highest starting pressure for well fitting the linear relationship and a maximum slope of the fitted lines in higher pressures as well. This might result from its particular physical properties caused by tectonics, which needs to be studied for further research.
Because the linear relation can fit the data in higher pressures instead of the power law, the linear equations indicated in Figure 6 rather than eq 6 are used to estimate the pore compressibility by combining with eq 1. The results are summarized in Table 3 and Figure 7, giving the pore compressibility of all selected coal samples at several representative mercury intrusion pressures. The results show that the pore compressibility of each selected coal decreases with the pressure increasing. As for the different coal samples, excluding sample gc13, all others show that the pore compressibility generally declines as the tectonic deformation extent increases. This means that a weak tectonic deformation in coal corresponds to the high pore compressibility. However, it might also be possible, as implied by the exception, that the coal with higher tectonic deformation 2971
Energy Fuels 2010, 24, 2964–2973
: DOI:10.1021/ef9015075
Qu et al.
Figure 5. Log(dVm/dP)-log(P) plots for selected samples.
Figure 6. Linearly fitted V-P plots of various coal samples.
exhibits higher pore compressibility. In addition, as shown in Figures 6 and 7, the volume-pressure curves (Figure 6) and the values of pore compressibility (Figure 7) for samples gc25 and gc28 are almost the same. Both of these two samples have a relatively weak tectonic deformation. This implies that the tectonic deformation may have less impact on the
change in the pore compressibility of coal when the deformation extent in coal is weak. Moreover, the difference in coal rank may also significantly contribute to the differences in pore compressibility, and the structure of the lower rank anthracite is more susceptible of being modified during mercury intrusion 2972
Energy Fuels 2010, 24, 2964–2973
: DOI:10.1021/ef9015075
Qu et al.
Table 3. Pore Compressibility of Selected Coals Corresponding to Different Pressures pore compressibility (MPa-1) samples gc25 gc28 gc16 gc14 gc13 gc21 a
10 MPa
20 MPa
30 MPa
40 MPa
50 MPa
75 MPa
100 MPa
125 MPa
150 MPa
200 MPa
26.71 27.44 7.85 a 5.50 a
21.08 21.53 7.28 a 5.21 a
17.41 17.72 6.79 a 4.96 1.69
14.83 15.05 6.36 a 4.72 1.66
12.91 13.08 5.98 3.29 4.51 1.64
9.76 9.86 5.20 3.04 4.05 1.57
7.85 7.91 4.60 2.83 3.68 1.51
6.56 6.60 4.13 2.64 3.37 1.46
5.64 5.67 3.74 2.48 3.11 1.41
4.40 4.42 3.15 2.20 2.69 1.31
No value for the corresponding pressure under the starting pressure indicated in Figure 6.
The experimental investigations show that tectonics mainly reformed the bigger pores (pore size is larger than 100 nm). In general, the increased tectonic deformation forms more open pores and, hence, the enhanced connectivity of the pore network. It has been found that a linear relationship has been used to fit the MIP date in higher pressures in this study, instead of the typical power law. Following this approach, the volume-pressure curves of MIP for the six tectonized coals have been further investigated. Excluding sample gc13, the results show that both the total mercury intrusion volume under conditions of lower pressures and the starting pressure at which the linear relationship can be fitted to the MIP data well increase with the tectonic deformation of the coals. This implies that the values of these two parameters increase as the fractures and pores in coals increased. The results further show that the pore compressibility generally declines as the tectonic deformation extent increases. This suggests that weak deformation in coal would correspond to high pore compressibility. However, it might also be possible, as implied by the exception, that the coal with a higher tectonic deformation exhibits a higher pore compressibility. This exceptional behavior needs to be further studied.
Figure 7. Pore compressibility for various coal samples.
experiments. These may result in the compressibility of sample gc28 being slightly higher than that of sample gc25. 5. Conclusions Tectonized coal is a type of coal geologically formed under different tectonic deformations after coal formation and has unique textural and mechanical properties compared to other ordinary types of coal. The porous structure and compressibility of six representative tectonized coals from the Qinglong coal mine in southwest China have been investigated by means of microscopy and MIP. It provides experimental information about better characterization of such coals, which is one of the major concerns in safe coal mining and CBM recovery from tectonized coal seams. In combination with other ordinary analyses, such as measurements of vitrinite reflectance and pore size distributions, the fractal theory has been applied to MIP data to determine the pore compressibility of these coal samples.
Acknowledgment. This work was supported by the Natural Science Foundation of China (40672101 and 40730422) and the Chinese National Key Scientific Research and Development Project (2008ZX05034). The authors acknowledge the China Scholarship Council (CSC), the Australian Research Council (ARC), and the Queensland International Fellowship program for their support. The CSC provided one of the co-authors (Mr. Zhenghui Qu, from the China University of Mining and Technology) with financial support for his visit as a Ph.D. candidate at the School of Chemical Engineering, The University of Queensland.
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