Variations in pore characteristics in high volatile bituminous coals: Implications for coal bed gas content

International Journal of Coal Geology 76 (2008) 205–216

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Variations in pore characteristics in high volatile bituminous coals: Implications for coal bed gas content Maria Mastalerz a,⁎, Agnieszka Drobniak a, Dariusz Strąpoć b,1, Wilfrido Solano Acosta a,2, John Rupp a a b

Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, IN 47405-2208, United States Department of Geological Sciences, Indiana University, 1001 East 10th Street Bloomington, IN 47405-1405, United States

a r t i c l e

i n f o

Article history: Received 27 November 2007 Received in revised form 17 July 2008 Accepted 17 July 2008 Available online 25 July 2008 Keywords: Micropores Mesopores Coal Coal bed gas Illinois Basin

a b s t r a c t The Seelyville Coal Member of the Linton Formation (Pennsylvanian) in Indiana was studied to: 1) understand variations in pore characteristics within a coal seam at a single location and compare these variations with changes occurring between the same coal at different locations, 2) elaborate on the influence of mineralmatter and maceral composition on mesopore and micropore characteristics, and 3) discuss implications of these variations for coal bed gas content. The coal is high volatile bituminous rank with R0 ranging from 0.57% to 0.60%. BET specific surface areas (determined by nitrogen adsorption) of the coals samples studied range from 1.8 to 22.9 m2/g, BJH adsorption mesopore volumes from 0.0041 to 0.0339 cm3/g, and micropore volumes (determined by carbon dioxide adsorption) from 0.0315 to 0.0540 cm3/g. The coals that had the largest specific surface areas and largest mesopore volumes occur at the shallowest depths, whereas the smallest values for these two parameters occur in the deepest coals. Micropore volumes, in contrast, are not depth-dependent. In the coal samples examined for this study, mineral-matter content influenced both specific surface area as well as mesopore and micropore volumes. It is especially clear in the case of micropores, where an increase in mineral-matter content parallels the decrease of micropore volume of the coal. No obvious relationships were observed between the total vitrinite content and pore characteristics but, after splitting vitrinite into individual macerals, we see that collotelinite influences both meso- and micropore volume positively, whereas collodetrinite contributes to the reduction of mesopore and micropore volumes. There are large variations in gas content within a single coal at a single location. Because of this variability, the entire thickness of the coal must be desorbed in order to determine gas content reliably and to accurately calculate the level of gas saturation. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Coal gases are retained in coal primarily because of adsorption forces on the surface of the pores (Rightmire, 1984). The pores in coal vary in size; they are commonly divided into micropores (less than 2 nm), mesopores (2–50 nm), and macropores (greater than 50 nm), following the classification system of the International Union of Pure and Applied Chemistry (Orr, 1977). The division into these three groups is arbitrary but important because they reflect different mechanisms of pore filling. Micropores are characterized by volume filling (Dubinin and Radushkevich, 1947; Mahajan and Walker, 1978; Jaroniec and Choma, 1989), whereas larger pores fill with adsorbed gas layer by layer on the internal surface of the pores. Therefore, the volume of micropores (and not the surface area) is thought to be the main control upon gas adsorption (Dubinin, 1975).

⁎ Corresponding author. E-mail address: [email protected] (M. Mastalerz). 1 Present address: ConocoPhillips Company, Houston, TX 77252, United States. 2 Present address: Chevron Energy Technology, Houston, TX 77002, United States. 0166-5162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.07.006

The sizes and volumes of micro-, meso-, and macropores in coal depend on many factors, and coal rank is one of the most important. As coal rank increases, the volume of macropores decreases, whereas the volume of micropores increases. According to Gan et al. (1972), for vitrinite-rich coals having carbon contents above 80% (medium volatile bituminous rank and higher), micropores are the main contributors to the total porosity; for coal having carbon contents between 76 and 84% (high volatile bituminous rank), the bulk of the porosity consists of micropores and mesopores; whereas macroporosity is dominant in coals having a carbon content below 75% (subbituminous rank and below). These size relations were also confirmed by later studies (Clarkson and Bustin, 1996; Levy et al., 1997). Maceral composition is another factor that influences surface area, pore volume, and pore size distribution. Thomas and Damberger (1976) showed that for Illinois coals, specific surface area increased with increased vitrinite content. For high volatile bituminous coal, Harris and Yust (1976, 1979) found vitrinite to be micro- and mesoporous, inertinite mostly mesoporous, and liptinite the least porous, using electron microscopy techniques. These results were confirmed by later studies of coals from various regions (Unsworth et al.,1989). Lamberson and Bustin (1993), studying Cretaceous medium volatile bituminous coal from

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Canada, found that the amount of surface area generally decreased with increasing vitrinite content. Clarkson and Bustin (1996, 1999) document generally higher CO2 micropore adsorption for vitrinite-rich high

volatile bituminous coals from Canada. In the same coals, specific surface area and mesopore volumes seem to increase with increasing inertinite and decrease with increasing vitrinite contents. Gürdal and

Fig. 1. Stratigraphic positions of coal members in the Pennsylvanian System in Indiana and the studied site locations. The Seelyville Coal Member studied is indicated in bold.

M. Mastalerz et al. / International Journal of Coal Geology 76 (2008) 205–216 Table 1 Reproducibility of BET surface area, BJH mesopore volume (adsorption branch), D–R micropore surface area, and D–A micropore volume for a single coal sample using ASAP2020 BET surface area, m2/g

BJH ads. mesopore volume, cm3/g

D–R micropore surface area, m2/g

D–A micropore volume, cm3/g

14.38 14.46 14.47 14.41 14.40 Average: 14.42 St. deviation: 0.039115

0.017108 0.017332 0.017615 0.017108 0.017223 Average: 0.0172772 St. deviation: 0.000210539

120.6 121.5 120.8 120.9 121.0 Average: 120.96 St. deviation: 0.3362

0.059254 0.057828 0.058726 0.058321 0.057988 Average: 0.0584234 St. deviation: 0.000577932

Yalçin (2001), on the other hand, did not see relationships between pore volume and specific surface area versus maceral composition in bituminous coals from Turkey. Mineral-matter content is expected to have a negative effect on the porosity of coal, assuming that it is organic matter and not minerals that contribute the dominant amount of pore volumes. Clarkson (1994) noticed this negative relationship between micropore volumes and mineral-matter content, but did not detect, however, any relationship between mineral-matter content and mesopore characteristics. Although parameters such as specific surface area, the amounts of mesopore and micropore volumes, and pore sizes are undoubtedly important for understanding gas adsorption and desorption processes, these relationships, although extensively studied, have not been rigorously tested and are still not very well understood. Previous researchers usually used sets of samples carefully selected to address their specific approaches (Gan et al., 1972; Lamberson and Bustin, 1993; Clarkson and Bustin, 1996; Prinz et al., 2004; Prinz and Littke, 2005). However, it is still not known what range of variations in specific surface area, pore volumes, and pore size distribution can be expected in a coal bed from a single location, in a coal that has identical coalification (same rank) and post-coalification (same depth, pressure, and temperature) history. Understanding these local variations is necessary to correctly interpret differences that we see between various coals from different locations.

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This paper investigates variations in coal properties: maceral composition, specific surface area, mesopore and micropore volumes, and pore size distribution, in closely spaced samples of the Pennsylvanian Seelyville Coal Member (Fig. 1) from three locations in Indiana. All these coal samples were canister-desorbed, and the relationship between gas content and coal properties is also evaluated. 2. Methods 2.1. Sampling strategy Three drillholes in the Seelyville Coal: one in Sullivan County, one in Knox County, and one in Gibson County, were selected for detailed examination. The sampling locations were chosen so that samples from the Seelyville Coal could be taken at different depths: the most shallow, in Sullivan County, was from 107.01 m (351 ft), the deepest was in Gibson County at 237.98 m (781 ft). In each location, the whole thickness of the coal was housed in 30-cm-tall desorption canisters, resulting in multiple samples from each location. This sampling strategy allowed us to compare very local in-seam variations to those occurring within the same coal, but at different depths and in geographically different locations. 2.2. Techniques Prior to the placing the coal in the canisters, the canisters were flushed with argon to remove the air. Canister desorption measurements were carried out for about 3 months until there was no gas to be desorbed. The remaining (residual) gas was determined on crushed coal samples. Crushing was carried out in a hammermill crusher (Holmes Bros. Inc.) and after crushing, the whole sample was put immediately back into the desorption canister and sealed for determination of the remaining gas. Crushing and transferring of the crushed coal back into the canister took about 2 min, and the size of the crushed coal was b250 micrometers (60 mesh). The amount of the gas lost during these 2 min was estimated based on the rate of desorption of the crushed coal during the first hour, when gas measurements were taken every 2 min. The desorption of the crushed

Table 2 Proximate analysis and petrographic composition (vol.%, mineral-matter-free basis) of the coal samples studied Moisture, wt.%

Ash wt.%, dry

S wt.%, dry

Heating value MJ/kg, daf

R0 %

Vitrinite, vol.%

Ct, vol.%

Cd, vol.%

Liptinite, vol.%

Inertinite, vol.%

Location 1 — Sullivan County 005-325-6 107.01–107.32 2005-787-1 107.32–107.62 2005-787-2 107.62–107.92 2005-787-3 107.92–108.23 2005-787-4 108.23–108.54 2005-787-5 108.54–108.84 Average

11.9 10.2 8.1 9.6 8.7 11.7 10.0

7.3 10.6 37.6 16.2 8.2 10.2 15.0

4.3 5.5 5.1 6.1 4.6 5.7 5.2

34.2 34.0 31.5 33.4 34.1 33.9 33.5

0.57 0.58 0.56 0.56 0.57 0.59 0.57

86.6 80.3 71.9 86.1 78.7 82.9 81.1

72.4 68.9 59.7 61.4 60.7 60.1 63.9

14.2 11.4 12.1 24.7 18.0 22.8 17.2

6.7 10.6 2.4 6.1 7.8 4.6 6.4

6.7 9.1 25.7 7.8 13.5 12.5 12.6

Location 2 — Knox County 2005-629-6 186.23–186.53 2005-325-5 186.53–186.84 2005-205-6 186.84–187.15 2005-629-5 187.15–187.45 Average

8.4 11.6 9.2 9.5 9.7

25.9 10.6 18.3 12.6 16.8

4.8 6.0 10.6 4.6 6.5

30.2 34.0 33.3 33.4 32.7

0.56 0.57 0.59 0.6 0.6

82.6 83.5 78.8 74.4 79.8

73.7 76.1 63.4 64.2 69.4

8.8 7.4 15.4 10.2 10.5

4.7 9.1 3.3 6.3 5.9

12.6 7.4 17.8 19.3 14.3

Location 3 — Gibson County 2005-629-1 237.98–238.29 2005-629-2 238.29–238.59 2005-205-8 238.59–238.90 2005-325-3 238.90–239.20 2005-325-4 239.20–239.51 2005-325-2 239.51–239.81 Average

8.9 6.3 7.3 7.7 8.7 6.5 7.5

12.7 33.2 19.7 9.7 10.2 29.3 19.1

4.0 2.4 11.9 5.5 3.7 4.8 5.4

33.3 32.0 32.9 34.1 34.0 33.1 33.2

0.6 0.64 0.6 0.59 0.59 0.61 0.6

84.3 86.8 74.8 80.3 82.9 77.9 81.2

73.7 81.9 55.8 62.4 72.5 60.4 67.8

10.6 4.9 19 17.9 10.4 17.5 13.4

6.9 5.4 9.2 11.1 6.5 5.7 7.5

8.8 7.8 16 8.6 10.6 16.4 11.4

Sample

Depth, m

Ct — collotelinite, Cd — collodetrinite.

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coal took usually 2–3 days. After desorption on the crushed coal had been completed, total gas content was calculated. Canister desorption of the cores and data processing was done following general guidelines outlined by the Gas Research Institute (GRI, 1995). After canister desorption and residual gas determinations had been completed, each crushed sample (~ 250 µm in size) was split manually into several representative sub-samples (using the standard coning and quartering technique) for proximate analysis, petrographic composition (maceral composition and vitrinite reflectance), specific surface area, and pore size distribution. Specific surface area, mesopore volume, and mesopore size distribution were determined using an ASAP 2020 porosimeter, with low-pressure (b10.13 kPa) nitrogen as the adsorptive at the boiling point temperature of liquid nitrogen (77.35 K at 101.3 kPa). Micropore volume, micropore specific surface area, and micropore size distribution were determined using low-pressure CO2 at a temperature of 273 K. Sample weight varied between 1 and 2 g. Both in the nitrogen adsorption and CO2 adsorption techniques, sample evacuation time was 960 min and the target temperature was 110 °C. During the analysis, an equilibrium detection algorithm of the software determined when equilibrium had been satisfactorily established between the adsorbed and the unadsorbed phases prior to the collection of the isotherm data point. The equilibrium interval (time over which the pressure must remain stable within a very small range) was set at 30 s and the pressure tolerance was set at 0.6666 kPa (5 mm Hg). Consequently, the equilibrium time varied from sample to sample. Among the parameters calculated from the nitrogen adsorption analysis, BET surface area and BJH mesopore volume (adsorption branch) are discussed in this paper, whereas Dubinin–Radushkevich (D–R) micropore surface area and Dubinin–Astakhov (D–A) micropore volumes were calculated from the CO2 adsorption analysis. Definitions and discussion of these parameters are given elsewhere (e.g., Gregg and Sing, 1982). Based on nitrogen and CO2 adsorption, mesopore and micropore size (width) distributions are also generated. Size distribution is calculated using the Kelvin equation that assumes emptying of condensed adsorptive in the pores in a stepwise manner as the pressure decreases. Reproducibility of the results of a coal sample from the study area is shown in Table 1.

(Gregg and Sing, 1982; Bustin and Clarkson, 1999). The shape of the hysteresis loop in the samples corresponds to a de Boer loop Type B, indicating slitted-shaped pores (Gregg and Sing,1982). All samples show a lack of total closure of the low-pressure hysteresis loop, which has been interpreted as being due to coal swelling or adsorption in micropores (Gregg and Sing, 1982), both reasons being plausible for the samples studied. The quantities of the adsorbed nitrogen in individual samples at a given location vary widely. This is especially evident in samples from the shallowest location (Sullivan County, Fig. 2A), where the sample from the depth interval of 107.01 to 107.32 m (351.1 to 352.1 ft) adsorbed three times as much nitrogen as the sample from the 107.92

3. Results 3.1. Coal characteristics Basic coal-quality characteristics and petrographic composition are presented in Table 2. Average moisture content ranges from 10% in the Sullivan County location to 7.5% in the Gibson County location. In each location, there is a range of ash and sulfur contents. Vitrinite reflectance values of 0.57 to 0.6%, and heating values of 32.7 to 33.5 MJ/kg (14,056 to 14,394 Btu/lb) indicate basically iso-rank highvolatile C/B bituminous coal. The average maceral composition of the coal is also similar in samples from all three locations (vitrinite content 79.8–81.2%), although there are variations in maceral composition among individual samples in each location (Table 2). 3.2. Mesopore characteristics Low-pressure adsorption isotherms of the samples (Fig. 2) correspond to Type IV isotherm of the classification of Brunauer et al. (1938). This type of isotherm is considered to be associated with mesoporous solids. All samples from three locations show a distinct hysteresis loop between 0.45 and 0.95 relative pressure, with the hysteresis becoming less pronounced with increasing depth (Fig. 2A, B, C). The presence of a pronounced hysteresis indicates that evaporation from pores is a distinctly different process than condensation within the pores and suggests that capillary condensation occurred within the mesopores

Fig. 2. Low-pressure nitrogen adsorption isotherms (linear plots) of coal samples from locations studied: A — Sullivan County, B — Knox County, C — Gibson County. All isotherms show distinct hysteresis. Note that the largest variations in the quantity of adsorbed nitrogen occur in the samples from the Sullivan County location (shallowest); the samples having the least variation are from the Gibson County location (deepest).

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to108.23 m (354.1–355.1 ft) interval at the maximum relative pressure of 0.99. Significant differences, although less extreme, exist at deeper locations in Knox and Gibson Counties (Fig. 2B, C). These variations in the nitrogen adsorption translate into differences in BET surface area and BJH mesopore volume, especially large for the coal in the Sullivan location (Table 3). Interestingly these variations still remain when data are presented on ash-free basis. Averaging all the samples for each location, we observe a decrease in surface area and mesopore volume with an increase in depth (Table 3). An example of the distribution of the individual pore width is presented in Fig. 3 for the Sullivan location. Three samples from this location (107.01–107.32, 107.32–107.62, and 108.23–108.54) have the largest pore volumes within the 5.2- to 12-nm size range. These three samples adsorbed the highest N2 quantities (Fig. 2A). For two other samples (107.62–107.92 and 108.54–108.84), the 28- to 75-nm pore width range has the highest pore volume. This significant contribution of large pores could be explained by a relatively high inertinite content (Table 2). The lowest-adsorbing sample, 107.92–108.23 (Fig. 2A), shows the lowest pore volumes among all pore sizes (Fig. 3).

209

Fig. 3. Incremental mesopore size distribution in coal samples from Sullivan County.

Table 3 Nitrogen adsorption derived parameters: BET surface area and BJH adsorption on raw basis and ash-free basis Nitrogen adsorption Depth, m BET surface BET surface BJH adsorption area m2/g, area m2/g, mesopore volume, raw basis ash-free cm3/g, raw basis Location 1 — Sullivan County 2005-325-6 107.01– 22.9 107.32 2005-787-1 107.32– 18.5 107.62 2005-787-2 107.62– 11.4 107.92 2005-787-3 107.92– 7.1 108.23 2005-787-4 108.23– 18.4 108.54 2005-787-5 108.54– 12.0 108.84 Average 15.0 St. deviation 5.85 Location 2 — Knox County 2005-629-6 186.23– 3.3 186.53 2005-325-5 186.53– 4.6 186.84 2005-205-6 186.84– 4.3 187.15 2005-629-5 187.15– 5.7 187.45 Average 4.5 St. deviation 1.01 Location 3 — Gibson County 2005-629-1 237.98– 4.9 238.29 2005-629-2 238.29– 5.3 238.59 2005-205-8 238.59– 4.3 238.90 2005-325-3 238.90– 1.8 239.20 2005-325-4 239.20– 4.1 239.51 2005-325-2 239.51– 2.9 239.81 Average 3.8 St. deviation 1.31

BJH adsorption mesopore volume, cm3/g, ash-free

24.8

0.0339

0.0366

20.6

0.0274

0.0306

18.3

0.0221

0.0354

8.4

0.0141

0.0169

20.0

0.0286

0.0312

13.4

0.0216

0.0241

17.6 5.81

0.0246 0.0069

0.0291 0.0074

4.4

0.0085

0.0115

5.2

0.0092

0.0103

5.2

0.0086

0.0105

6.6

0.0108

0.0124

5.3 0.88

0.0093 0.0011

0.0112 0.0009

5.6

0.0082

0.0094

7.9

0.0109

0.0164

5.3

0.0084

0.0104

1.9

0.0041

0.0045

4.5

0.0077

0.0086

4.0

0.0086

0.0122

4.9 1.95

0.0080 0.0022

0.0102 0.0040

Fig. 4. Relationship of and mesopore volume in coal samples from: A — Sullivan County, B — Knox County, and C — Gibson County. Note the negative correlation between and micropore volume in samples from Sullivan and Knox Counties and the positive correlation in samples from Gibson County.

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3.2.1. Mesopores and ash content In analyzing the relationships between mesopore volume and ash content, we can conclude that, in general, there is no universal trend (Fig. 4). For the Sullivan County location, with an exception of one sample, there is a general negative correlation between mesopore volume versus ash content, and the increase in ash content is accompanied by a decrease in mesopore volume (Fig. 4A). For example, when we consider two samples of very similar maceral composition (vitrinite ~86%, liptinite ~6%, inertinite ~7%, Table 2), 107.01–107.32 and 107.92–108.23, but different ash content (~7% versus 16%), the sample having the lower ash content (107.01–107.32) has a specific surface area three times as large (22.9 m2/g compared to 7.1 m2/g, Table 3), suggesting that the organic fraction is the main contributor to the surface area. The exception is sample 107.62–107.92 (Fig. 4A), which has the highest ash content of all the samples from this location (37.6%, Table 2) but which does not have the lowest surface area (11.4 m2/g, Table 3) or mesopore volume, suggesting that parameters other than ash overpower the ash influence. In the Knox County location (Fig. 4B), there is also a general tendency of a decrease in mesopore volume with an increase in ash content. Such a tendency, however, is not observed in the Gibson County (Fig. 4C) where, in fact, samples with the largest have also the largest mesopore volume. This lack of a clear relationship between ash content and BET surface area and BJH mesopore volume also becomes evident when

we compare those values on raw basis and ash-free basis (Table 3). On ash-free basis, the variations observed between individual samples in each location do not decrease, as evidenced by comparable standard deviations for raw and ash-free basis of these parameters. All these observations indicate that ash content is not the dominant control on the surface area and mesopore volume of the coal studied. 3.2.2. Mesopores and petrographic composition The maceral composition of the coal represents a set of parameters expected to influence the amount of surface area and mesopore volumes. Fig. 5A and C shows that for the locations studied there is no consistent relationship with total vitrinite content. However, when the vitrinite is split into collotelinite and collodetrinite, the latter seems to have a negative correlation with mesopore volume, while it appears to be a positive relationship with collotelinite. These correlations are not very strong, however (r2 ~0.5). The relationship between surface area and mesopore volume and inertinite content is generally positive. In general, inertinite follows the same trend as surface area and mesopore volume from the top to the bottom of the seam (Fig. 5B and D). The main exception to this trend is sample 107.62–107.92 from the Sullivan location (Fig. 5B), that has the largest inertinite content and relatively low surface area and mesopore volume. This sample also has the highest ash content (Fig. 4A), and this is likely the reason why the values of the surface area and mesopore

Fig. 5. Relationship of maceral composition, mesopore volume, and surface area in coal samples: A — relationship between collotelinite, collodetrinite, total vitrinite and mesopore volume in Sullivan County; B — trends of inertinite and liptinite contents versus those of the surface area and mesopore volume from the top to the bottom of the Seelyville Coal in Sullivan County; C — relationship between collotelinite, collodetrinite, total vitrinite and mesopore volume in Gibson County; D — trends of inertinite and liptinite contents versus those of the surface area and mesopore volume from the top to the bottom of the Seelyville Coal in Gibson County.

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volumes are relatively low, in spite of the fact that inertinite content is high. Liptinite content does not show a consistent trend with respect to surface area and mesopore volume. For the coal studied, liptinite content only occasionally reaches 10% of the total volume (Table 2), and thus unlikely has significant influence on the overall mesopore volume of the samples.

Table 4 CO2 adsorption derived parameters: D–R micropore surface area and D–A micropore volume on raw and ash-free basis

3.3. Micropore characteristics

Location 1 — Sullivan County 2005-325-6 107.01– 110.3 107.32 2005-787-1 107.32– 107.4 107.62 2005-787-2 107.62– 68.7 107.92 2005-787-3 107.92– 91.8 108.23 2005-787-4 108.23– 108.0 108.54 2005-787-5 108.54– 100.6 108.84 Average 97.8 St. deviation 15.79

The low-pressure CO2 adsorption capacities for the coal samples studied are presented in Fig. 6. In the Sullivan County samples, the lowest adsorption capacity for CO2 is observed in sample 107.62–107.92, and the largest in the topmost sample 107.01–107.32, with two other samples being very close (Fig. 6A). The quantities of the adsorbed CO2 are directly related to their micropore volumes (Table 4). The difference in adsorption between the highest- and the lowest-adsorbing samples at the highest pressure is about 4 cm3/g. These relationships roughly correspond to the

CO2 adsorption Depth, m

D–R micropore sur. area, m2/g, raw basis

Location 2 — Knox County 2005-629-6 186.23– 75.8 186.53 2005-325-5 186.53– 94.8 186.84 2005-205-6 186.84– 85.4 187.15 2005-629-5 187.15– 93.9 187.45 Average 87.5 St. deviation 8.88 Location 3 — Gibson County 2005-629-1 237.98– 106.5 238.29 2005-629-2 238.29– 74.3 238.59 2005-205-8 238.59– 85.8 238.90 2005-325-3 238.90– 102.0 239.20 2005-325-4 239.20– 106.1 239.51 2005-325-2 239.51– 77.6 239.81 Average 92.0 St. deviation 14.63

Fig. 6. Low-pressure carbon dioxide adsorption isotherms in coal samples from: A — Sullivan County, B — Knox County, C — Gibson County. In contrast to N2 adsorption, differences in CO2 adsorption among the three sampling locations are not as prominent.

D–R micropore sur. area, m2/g, ash-free

D–A micropore volume, cm3/g, raw basis

D–A micropore volume, cm3/g, ash-free

119.0

0.0540

0.0583

120.1

0.0510

0.0570

110.1

0.0340

0.0545

109.5

0.0430

0.0513

117.7

0.0524

0.0571

112.1

0.0457

0.0509

114.8 4.73

0.0467 0.0075

0.0549 0.0031

102.3

0.0355

0.0479

106.1

0.0431

0.0482

104.5

0.0384

0.0470

107.4

0.0437

0.0500

105.0 2.19

0.0402 0.0039

0.0483 0.0013

122.0

0.0468

0.0536

111.1

0.0315

0.0471

106.8

0.0387

0.0482

112.9

0.0439

0.0486

118.1

0.0483

0.0537

109.6

0.0354

0.0500

113.4 5.63

0.0408 0.0067

0.0502 0.0028

relationships described earlier for nitrogen adsorption. The three samples having the highest CO2 adsorption (107.01–107.32, 107.32–107.62, 108.23–108.54) also have the highest nitrogen adsorption (Fig. 2), although the sample having the lowest CO2 adsorption (107.62–107.92) does not have the lowest nitrogen adsorption. In two other locations (Fig. 6B, C), the variations in the quantities of adsorbed CO2 are of similar ranges as in the Sullivan County samples. Micropore size distribution for the Sullivan County samples is presented in Fig. 7. For the two other sampling locations, distributions of pore volumes by pore width are similar and not included here. In general, the most volume is taken up by micropore sizes of 0.4 to 0.65 nm and 0.8 to 0.9 nm. However, in each location, individual samples vary. For example, from the Sullivan County location, samples 107.01–107.32 and 107.92–108.23 have a distinct peak at 0.54 and 0.58 nm (Fig. 7), samples 107.32–107.72 and 108.54–108.84 have a broad peak within a range of 0.5 to 0.6 nm, and sample 107.62–107.92 has the lowest micropore volumes throughout the entire size range. The significance of these differences in pore size distribution is not presently known and requires further study. D–R micropore surface area, and D–A micropore volumes are shown in Table 4. Although the average values for the three sampling

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Fig. 7. Incremental micropore size distribution in coal samples from Sullivan County.

locations are not very different, except for a higher micropore volume in the shallowest location, the differences among individual samples in each location are noticeable. 3.3.1. Micropores and ash content There is a negative relationship between ash content, micropore specific surface area, and micropore volume for samples from all

locations (Fig. 8), and this relationship is much stronger than that regarding ash content and the mesopore volumes and surface areas. Correlation coefficients (r2) of micropore surface area and ash content are above 0.9 (Fig. 8), and they are also very similar for micropore volume and ash content. Such strong correlations indicate that the organic fraction of the coal dominantly contributes to the microporosity, whereas mineral-matter (as proxied by ash content) has only a diluting effect on microporosity. This dramatic influence of the on the micropore surface area and micropore volume is evident when the values on raw basis are compared to those on ash-free basis (Fig. 4). On ash-free basis, differences in micropore surface area and micropore volumes between individual samples become significantly lower, with standard deviation values decreasing dramatically in each location. 3.3.2. Micropores and petrographic composition Because of the strong influence of the on micropore surface areas volumes, in order to eliminate this overwhelming influence, the values of micropore surface area and micropore volumes on ash-free basis were used to analyze the influence of petrographic composition (Fig. 9A–D). It was observed that there were no well defined correlations, although there are some tendencies. Collotelinite shows evidence of a positive relationship with micropore volume, whereas collodetrinite appears to have a negative correlation (Fig. 9A, C). Liptinite seems to follow the same trend as these two parameters from the top to the bottom of the seam (Fig. 9B,D). Inertinite generally has a negative correlation with micropore specific surface area and micropore volume; as a rule, samples with high inertinite content have low micropore volume and micropore specific surface area. 3.4. Gas content

Fig. 8. Relationship of and micropore specific surface area for: A — Sullivan County, B — Knox County, and C — Gibson County. Note the negative correlation of ash from all three locations.

Total average gas contents of the Seelyville Coal vary from 1.4 cm3/g (44.5 scf/ton) in the Sullivan County location, 1.9 cm3/g (60.8 scf/ton) in the Gibson County location to 2.7 cm3/g (87.7 scf/ton) in the Knox County location (Table 5). There are large differences between individual canisters in all three locations. Looking qualitatively at the data, two observations could be made with regard to gas content: 1) samples having very low specific surface area and pore volume (e.g., 107.92–108.23 in Sullivan County, Table 3) have small amounts of gas (Table 5); and 2) samples with high surface area and pore volume do not necessarily have high gas contents. For example, the topmost sample from Sullivan County (107.01–107.32) has the highest surface area and pore volume (Table 3) but has very low desorbed and residual gas contents (Table 5), possibly because of gas migration into overlying, more permeable clastic sediments. Fig. 10 shows the relationships between gas content (desorbed and residual) and specific surface area (Fig.10A, D), mesopore volume (Fig.10B, E) and micropore volume (Fig. 10C, F) for the Sullivan County samples (shallowest location) and the Gibson County samples (deepest location).

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Fig. 9. Relationship of maceral composition, micropore volume, and micropore surface area in coal samples: A — relationship between collotelinite, collodetrinite, total vitrinite and micropore volume in Sullivan County; B — trends of inertinite and liptinite contents versus those of the micropore surface area and micropore volume from the top to the bottom of the coal bed in Sullivan County; C — relationship between collotelinite, collodetrinite, total vitrinite and micropore volume in Gibson County; D — trends of inertinite and liptinite contents versus those of the micropore surface area and micropore volume from the top to the bottom of the Seelyville Coal in Gibson County.

The best correlation is observed between gas content and micropore volume in the Sullivan County samples (Fig. 10C); the correlation is especially strong between micropore volume and residual gas content (r2 =0.95). Relationships among gas content and mesopore volume and specific surface area are also positive, but weaker. These relationships are not observed at greater depths in Gibson County samples (Fig. 10D–F). In the Knox County location (intermediate depth, graph not included), there is a tendency for higher gas contents to correspond with higher specific surface area and micropore volumes, however, these relationships are not as strong as in samples from the shallowest location (Sullivan County). 4. Discussion 4.1. Local versus regional variations in mesopore and micropore characteristics Our data provide an opportunity to analyze variations of coal properties in iso-rank high volatile bituminous coals. The Seelyville Coal was sampled in three locations, separated by a distance of up to 50 km, at

depths increasing from Sullivan County (~107 m) through Knox County (~186 m) to Gibson County (~238 m). Analyses of multiple samples from each location and also at different depths allowed us to separate influences of depth from very local depth-unrelated differences in coal properties in iso-rank high-volatile C/B bituminous coals (R0 ranging from 0.57 to 0.61%). One of the first observations from this study is that specific surface area (measured by nitrogen adsorption), and mesopore pore volumes, change significantly with depth. At the shallowest depth (Sullivan County), average specific surface areas and mesopore volumes are the highest (15 m2/g and 0.0246 cm3/g, respectively, Table 3), and at the greatest depth (Gibson County) they are the lowest (3.8 m2/g and 0.0080 cm3/g, respectively. This demonstrates that hydrostatic pressure plays a large role in defining the surface area and mesopore volume. In contrast to the observed changes in mesopore characteristics with depth, micropore characteristics (determined by CO2 adsorption) do not seem to be significantly affected by depth (Table 4). Our results also show that one can expect significantly larger variations in specific surface area and mesopore volumes at shallower

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Table 5 Gas content (obtained by canister desorption) of the coal samples Desorbed + lost cm3/g, air dry

Residual cm3/g, air dry

Total cm3/g, air dry

Location 1 — Sullivan County 2005-325-6 107.01–107.32 2005-787-1 107.32–107.62 2005-787-2 107.62–107.92 2005-787-3 107.92–108.23 2005-787-4 108.23–108.54 2005-787-5 108.54–108.84 Average St. deviation

0.1 1.9 1.0 0.4 0.8 2.0 1.0 0.78

0.0 0.5 0.4 0.3 0.4 0.5 0.4 0.19

0.2 2.4 1.5 0.6 1.1 2.5 1.4 0.96

Location 2 — Knox 2005-629-6 2005-325-5 2005-205-6 2005-629-5 Average St. deviation

2.4 3.1 1.0 2.8 2.3 0.92

0.4 0.6 0.1 0.6 0.4 0.25

2.9 3.7 1.1 3.3 2.7 1.17

3.2 1.3 0.6 1.4 0.6 2.5 1.6 1.04

0.4 0.3 0.0 0.3 0.1 0.4 0.3 0.16

3.6 1.6 0.6 1.7 0.8 2.9 1.9 1.18

Depth, m

County 186.23–186.53 186.53–186.84 186.84–187.15 187.15–187.45

Location 3 — Gibson County 2005-629-1 237.98–238.29 2005-629-2 238.29–238.59 2005-205-8 238.59–238.90 2005-325-3 238.90–239.20 2005-325-4 239.20–239.51 2005-325-2 239.51–239.81 Average St. deviation

depths than at greater depths (Table 2). The observed variations of these parameters are remarkably large at the Sullivan County location (~107 m depth). Differences in specific surface area (from 7 to 22 m2/g), and mesopore volume (from 0.0141 to 0.0339 cm3/g) between 1-ft sample

intervals are of the same magnitude as the variations in average values between the shallowest and the deepest location. One implication of these differences is that the entire coal thickness must be used to obtain a representative sample for testing mesopore characteristics. Using only a partial sample may underestimate or overestimate specific surface areas and mesopore volumes. Another implication is that surface area and mesopore volumes as measured by nitrogen adsorption should not be viewed as a function of coal rank because they show large variations within iso-rank coals. 4.2. Influence of mineral-matter and maceral composition on mesopore and micropore characteristics The influence of mineral-matter content (indicated by ash content) on the amount of surface area and mesopore volume is rather complex. In the two shallower locations (Sullivan and Knox Counties), a negative correlation between ash and mesopore volume contrasts with a positive correlation between these parameters in the deepest (Gibson County) location (Fig. 4). This positive correlation between ash content and pore volume (if not coincidental) suggests that, at certain pressure conditions, mineral matter can provide relatively more mesopore volume than the organic fraction. In contrast to surface area and mesopore volume, micropore volume is uniformly dependent on the mineral-matter content. The strong negative correlation for all the locations indicates that mineral matter contributes only minimally to the micropore volume. The influence of maceral composition on surface area, mesopore volume, and micropore volume is complex and difficult to determine with certainty unless pure macerals are available for analysis. In this study, no pure maceral fractions were available. Taking into account proportions of macerals, our results indicate that, for the coals studied, collotelinite and collodetrinite are different from one another with regard to both mesopore and micropore characteristics, with collotelinite positively influencing both meso- and micropore volume, whereas

Fig. 10. Relationship of gas content and specific surface area, mesopore volume, and micropore volume for Sullivan County (A–C) and Gibson County (D–F). Note the positive relationships in samples from Sullivan County (shallowest location) and the lack of correlation in samples from Gibson County (deepest location).

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Fig. 11. Relationship of desorbed and residual gas for the three locations studied.

collodetrinite contributes to the reduction of these volumes. Because of the different behavior of these two macerals of the vitrinite group, no consistently good correlation between pore volumes and total vitrinite content could be expected, which was the case in many studies (e.g., Gürdal and Yalçin, 2001). Inertinite seems to correlate positively with mesopore volumes because it contributes pores larger than 20 nm (Fig. 3). Inertinites are often macroporous (pores N50 nm), and are known to contribute significantly to macropore volume (Harris and Yust, 1976). In this study, macropore volumes are not quantified (pores up to 75 nm are measured using nitrogen adsorption, Fig. 3), but the largest volumes of pores N20 nm in the samples having the highest inertinite content are consistent with these characteristics. Liptinite macerals occur in our coals in small quantities and their influence on the overall pore sizes is limited. 4.3. Coal bed gas content and coal properties Coal bed gas content (as determined by canister desorption) shows large variations between individual adsorption canisters containing samples from each location. Canisters were checked for leaks frequently, and results from leaky canisters were not used in this discussion. Leaky canisters usually lost their gas within the first few days. The remaining canisters desorbed over long periods of time (usually about 3 months). There was a good correlation between the desorbed and residual gas content from samples from all three locations (Fig. 11), supporting the absence of leaks (it is expected that leaky canisters would have a high proportion of residual gas in the total gas volume). The reasons for these inter-canister variations are not well understood. Potter (1993), studying medium volatile bituminous coals from Canada, and Faiz and Cook (1993) studying Australian coals, noticed that the greatest gas yields came from coals rich in inertinite. Clarkson (1994) showed that gas content generally increased with mesopore volume, using several Canadian coals. For the coal studied, variations in gas content may result from differences in the activity level of microbial consortia, and consequently, different gas generation and/or distraction in different portions of the coal bed. Recent studies by Strąpoć et al. (2007, 2008) demonstrated biogenic origin of the gas and recent (including present-day) gas generation in the coal studied. Regardless of the reasons, these variations have large practical implications. First, they indicate that in order to get representative gas content for a coal bed, the whole thickness must be desorbed and the average value of all canisters calculated. If only selected portions are desorbed, gas contents can be over- or underestimated. The same holds true for the calculation of the degree of saturation. Calculations based on desorption of only partial coal thickness may be totally different from that for the seam as a whole.

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The controls on the gas content vary between locations. As described in the Results section, at the shallowest location (~ 107 m) in Sullivan County, gas content showed a strong positive relationship with micropore volume and a somewhat weaker, but still distinct relationship with mesopore volume and specific surface area. These relationships indicate that in this location micropores play the dominant role in gas retention, but they also suggest that mesopores play an important role in methane retention. Together these relationships indicate that both volume filling and adsorption on the surface area are important mechanisms in gas retention in the coals studied. At greater depths, the relationships of gas content with micropore and mesopore characteristics become weaker (Knox County) or disappear (Gibson County; Fig. 10). We speculate that this lack of a relationship of the gas content with micropore and mesopore volumes may indicate a significant contribution of free (not adsorbed) gas to the total gas content. It has been suggested by Bustin and Clarkson (1999) that for coals of high volatile bituminous rank, the free gas present in the coal matrix porosity makes up ~50% of the total reservoir gas capacity at pressures of 1 MPa. This pressure is comparable to the subsurface pressure conditions of the coals studied. Our studies have recently documented present-day biogenic gas generation (Strąpoć et al., 2007), and perhaps this newly generated gas occupies matrix porosity as well as the cleat and fracture systems not quantified by nitrogen or CO2 adsorption. At the shallowest location (Sullivan County), the contribution of free gas may be smaller, because of easier gas migration to the surface. Therefore, at this location, the gas content of the coal correlates well with pore parameters calculated from nitrogen and CO2 adsorption. 5. Conclusions This study leads to the following conclusions: 1. In coals of high volatile bituminous rank, significant variations exist in the specific surface area and the distribution of mesopore and micropore volumes in different parts of a coal bed in a single location. These variations are of the same magnitude as those between this isorank coal from different locations. For the coals studied, the range of variations changes with depth, being greatest at the most shallow and least at the deepest location. 2. Mineral matter can cause an increase or decrease of specific surface area and mesopore volume of a bulk coal sample, depending upon the magnitude of specific surface area of the accompanying organic fraction. In comparison, mineral matter contributed to a decrease in micropore volume of the bulk coal. 3. Collotelinite and collodetrinite are different with regard to both mesopore and micropore characteristics, whereas collotelinite influencing positively both meso- and micropore volume, and collodetrinite contributes to the reduction of these volumes. Because of such a different behavior of these two macerals of the vitrinite group, no consistently good correlation between pore volume and total vitrinite content could be expected. Inertinite seems to contribute significantly to the mesopore volume. 4. Gas content (as determined by canister desorption) shows a large amount of variability through the vertical section of a coal bed in a single location. The relationships of gas content with surface area and mesopore and micropore volumes of the coal are site specific. In the Sullivan County location, there was a strong correlation between gas content and micropore volume (r2 = 0.79), and a weaker but still significant correlation with specific surface area and mesopore volume (r2 = 0.40 and 0.55, respectively). In the Gibson County location, there was no correlation, possibly because of the contribution of free gas to the total gas. 5. Large variations of coal properties, pore characteristics, and gas content suggest caution when calculating coal gas reserves. This also indicates the necessity of analyzing the entire coal thickness, rather than selected coal seam intervals.

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