Geochemical Investigation of the Potential for Mobilizing Non

The program was designed to yield a solvent/sample ratio, ∼2.4 mol CO2:1 g dry coal, comparable to that obtained in a parallel set of extractions co...
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Energy & Fuels 2006, 20, 566-574

Geochemical Investigation of the Potential for Mobilizing Non-Methane Hydrocarbons during Carbon Dioxide Storage in Deep Coal Beds Jonathan J. Kolak* and Robert C. Burruss U.S. Geological SurVey, MS 915A, 12201 Sunrise Valley DriVe, Reston, Virginia 20192 ReceiVed February 11, 2005. ReVised Manuscript ReceiVed NoVember 17, 2005

Coal samples of different rank (lignite to anthracite) were extracted in the laboratory with supercritical CO2 (40 °C; 10 MPa) to evaluate the potential for mobilizing non-methane hydrocarbons during CO2 storage (sequestration) or enhanced coal bed methane recovery from deep (∼1-km depth) coal beds. The total measured alkane concentrations mobilized from the coal samples ranged from 3.0 to 64 g tonne-1 of dry coal. The highest alkane concentration was measured in the lignite sample extract; the lowest was measured in the anthracite sample extract. Substantial concentrations of polycyclic aromatic hydrocarbons (PAHs) were also mobilized from these samples: 3.1-91 g tonne-1 of dry coal. The greatest amounts of PAHs were mobilized from the high-volatile bituminous coal samples. The distributions of aliphatic and aromatic hydrocarbons mobilized from the coal samples also varied with rank. In general, these variations mimicked the chemical changes that occur with increasing degrees of coalification and thermal maturation. For example, the amount of PAHs mobilized from coal samples paralleled the general trend of bitumen formation with increasing coal rank. The coal samples yielded hydrocarbons during consecutive extractions with supercritical CO2, although the amount of hydrocarbons mobilized declined with each successive extraction. These results demonstrate that the potential for supercritical CO2 to mobilize non-methane hydrocarbons from coal beds, and the effect of coal rank on this process, are important to consider when evaluating deep coal beds for CO2 storage.

Introduction Several possible options are available for geologic storage and sequestration of CO2, including depleted oil and gas reservoirs, coal beds, black shales, and formations containing nonpotable (e.g., saline) water. Carbon dioxide storage in depleted oil and gas reservoirs and coal beds is of particular interest because of the potential for concomitant hydrocarbon recovery (e.g., enhanced oil recovery (EOR) and enhanced coal bed methane (ECBM) recovery). Enhanced oil recovery through CO2 injection is a relatively mature technology that is economically viable. For example, CO2-EOR has been conducted at Rangely oil field (Colorado) for roughly 20 years.1 In contrast, the use of CO2 in ECBM recovery is a less mature technology. Field trials have been performed recently to evaluate the economic viability of ECBM recovery and CO2 storage in coal beds.2 Issues of economic viability notwithstanding, the extent to which CO2 storage in coal beds can be implemented as a means of mitigating greenhouse gas emissions is unresolved. Preliminary estimates have indicated that, collectively, coal beds may accommodate up to several hundred gigatons of CO2 globally.3,4 However, these estimates are poorly constrained due to a paucity * To whom correspondence should be addressed. Telephone: 703-6486750. E-mail: [email protected]. (1) Klusman, R. W. Appl. Geochem. 2003, 18, 1825-1838. (2) Reeves, S. R.; Clarkson, C.; Erickson, D. Selected Field Practices for ECBM RecoVery and CO2 Sequestration in Coals based on Experience Gained at the Allison and Tiffany Units, San Juan Basin; DOE Topical Report DE-FC26-00NT40924; U.S. Department of Energy and Advanced Resources International, 2002. Available at http://www.coal-seq.com/ Proceedings2003/40924R06.pdf. (3) Gentzis, T. Int. J. Coal Geol. 2000, 43, 287-305. (4) Herzog, H. J. EnViron. Sci. Technol. 2001, 35, 148A-153A.

10.1021/ef050040u

of quantitative measurements. Further, the extent to which coal beds can permanently (i.e., on the scale of hundreds to thousands of years) retain injected CO2 is largely unknown. Similar uncertainty exists regarding the possible effects of this CO2 storage technology on environmental quality. For example, injecting CO2 into coal-bearing units may mobilize organic and inorganic constituents from the coal matrix. In the event of CO2 leakage from the coal bed, these constituents may be transported into adjacent aquifer units and compromise water quality. Similarly, CO2 storage in coal-bearing units with concomitant ECBM recovery may result in elevated contaminant levels in the produced waters associated with gas production. This dearth of knowledge regarding CO2 storage in coalbearing units is due to, in part, an incomplete understanding of the physical and chemical interactions between CO2 and coal. Several recent studies have targeted these issues through laboratory experiments investigating gas transport and retention within coal samples.5,6 In addition, a pilot study of CO2 injection into coal beds in the San Juan basin has improved our understanding of coal bed reservoir properties.2 Despite these advances, little is known regarding the fate of CO2 injected into coal beds or the possible environmental effects stemming from CO2 sequestration or ECBM recovery. For example, the pressure-temperature conditions in deep (approximately g1km depth) coal beds may render the CO2 supercritical (Figure 1), and the fate of supercritical CO2 in this setting is poorly understood. Supercritical CO2 has physical properties intermediate between those of gaseous CO2 and liquid CO2, making it a very effective solvent that is capable of mobilizing (extracting) (5) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69-92. (6) Karacan, C. O. Energy Fuels 2003, 17, 1595-1608.

This article not subject to U.S. Copyright. Published 2006 by the American Chemical Society Published on Web 01/07/2006

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Table 1. Location and Context of Coal Samples sample basin ID code type

LA-1

TX-1

LA-2

IN-1

OH-1

WV-1

PA-1

Gulf Coast (Louisiana) OX-04-AB channel bench

Gulf Coast (Texas) PA-2-CN2 core

Gulf Coast (Louisiana) OX-04-BB channel bench

Illinois basin (Indiana) A3 channel bench

Appalachian basin (Ohio) 1R-SM-3(4) channel bench

Appalachian basin (West Virginia) cedar grove core

Appalachian basin (Pennsylvania) PAS-1224 channel bench

Table 2. Results of Proximate/Ultimate and Petrographic Analyses for Coal Samples sample

LA-1

TX-1

heating value, MJ kg-1 ash % moisture % fixed carbon % volatile matter %

16.6 11.15 32.26 28.29 28.30

18.6 11.11 30.87 29.44 28.58

hydrogen % carbon % nitrogen % sulfur % oxygen % free swelling index apparent specific gravity (g cm-3) heating value (M, MMF), Btu lb-1 ASTM coal rank (ASTM, 2003)

6.42 41.53 0.99 0.79 39.12 0 1.30

6.43 46.59 1.04 0.53 37.73 0 1.37

8147

9098

8796

lignite A [ligA]

subbituminous C [subC]

subbituminous C [subC]

vitrinite (%) liptinite (%) inertinite (%)

NAa NAa NAa

54. 9. 36.

Maceral Content (MMF) NAa 92.8 NAa 4.2 a NA 3.0

a

LA-2

IN-1

OH-1

WV-1

PA-1

Proximate, As-Received Basis 18.3 27.6 9.75 5.52 30.48 11.78 31.31 45.79 28.46 36.91

31.5 5.33 3.96 51.47 39.24

33.6 4.01 2.17 58.82 35.00

28.0 10.16 7.98 75.11 6.75

Ultimate, As-Received Basis 6.53 5.33 44.99 69.35 0.96 1.40 0.98 0.62 36.79 17.90 0 NAa 1.42 NAa

5.63 75.23 1.59 0.69 11.53 4 1.27

5.40 80.60 1.44 0.71 7.84 7.50 NAa

2.75 75.36 0.71 0.94 10.08 0 1.53

12684

14448

15146

13563

high-volatile C bituminous [hvCb]

high-volatile A bituminous [hvAb]

high-volatile A bituminous [hvAb]

anthracite [an]

82.5 8.4 9.1

81.4 13.2 5.4

91.7 0 8.3

NA: not analyzed.

Figure 1. Pressure-temperature diagram depicting phase behavior of pure CO2 and in situ conditions of select coal-bearing basins. Key to symbols: (O) Supercritical CO2 extraction conditions, this study; (area in solid-lined rectangle) Powder River basin; (area within dotted lines) Black Warrior basin;7 and (*) CO2 injection conditions, San Juan basin.2 (The Powder River basin pressure-temperature conditions are estimated for a select portion of the basin using data from Rice et al.8 and assuming a hydrostatic pressure gradient.)

hydrocarbons from geologic matrixes.9 The extent to which hydrocarbons may be mobilized during CO2-ECBM recovery (7) Pashin, J. C.; McIntyre, M. R. Int. J. Coal Geol. 2003, 54, 167183. (8) Rice, C. A.; Ellis, M. S.; Bullock, J. H., Jr. Water co-produced with coalbed methane in the Powder RiVer Basin, Wyoming: Preliminary compositional data; U.S. Geological Survey Open-File Report OF 00-372; U.S. Department of the Interior and U.S. Geological Survey, 2000. Available at http://pubs.usgs.gov/of/2000/ofr-00-372/.

projects, and the subsequent effect on dissolved hydrocarbon concentrations in produced waters, is unknown. Coal rank, that is, the collective physicochemical properties of coal that reflect changes due to the extent of burial and thermal maturation, may affect coal-CO2 interactions. For example, a recent study has shown that CO2 storage capacity measured in coal samples varies with coal rank.10 An improved understanding of the effects of coal rank on these interactions would provide information essential to national and global studies of CO2 storage opportunities in coal-bearing units. In this study, we investigated the influence of coal rank on the potential for mobilizing hydrocarbons from coal-bearing units during CO2 injection, and the possible relationships among coal rank, CO2 storage in deep coal beds, and environmental quality. Coal samples were extracted under pressure-temperature conditions corresponding approximately to the shallowest depth (∼1 km) at which CO2 would exist as a supercritical fluid within a coal bed. The amount and type of hydrocarbons mobilized during simulated CO2 injections were characterized to evaluate the potential for adversely affecting environmental quality during CO2 storage and ECBM recovery. Experimental Section Sample Collection and Preparation. For this study, seven coal samples in total were collected from the Appalachian and Illinois basins and the Gulf Coast region of the United States (Table 1). These samples were taken from fresh exposures either at mine faces or from freshly drilled coal core. All coal samples were sent to a commercial lab for grinding, sieving, splitting, and proximate/ (9) Monin, J. C.; Barth, D.; Perrut, M.; Espitalie M.; Durand B. Org. Geochem. 1988, 13, 1079-1086. (10) Gluskoter, H.; Stanton, R. W.; Flores, R. M.; Warwick, P. D. EnViron. Geosci. 2002, 9, 160-161.

568 Energy & Fuels, Vol. 20, No. 2, 2006 ultimate analyses (Table 2). Gas adsorption isotherms (CO2 and CH4) were conducted to determine gas storage capacities; these results are reported elsewhere.10,11 Petrographic analyses were performed on polished plugs of ground coal (-60 mesh) mounted in epoxy. Coal rank was determined according to the American Society for Testing and Materials (ASTM) Test Method D388-99(2004)e1,12 which makes use of the Parr formula to calculate the moist, mineral-matter free (M,MMF) heating value in Btu lb-1.13 Samples LA-1 and LA-2 were collected from discrete facies within the same coal bed, but were found to have slightly different ranks of lignite A and subbituminous C, respectively. These two facies were separated by a 5-cm-thick ash parting. The LA-1 sample was collected from the middle of a 0.45-m-thick facies lying atop the ash parting. This facies contained woody bands up to 3-cm thick at the base and was finely laminated to the top. The LA-2 sample was collected below the parting from a 1.20-m-thick facies characterized by scattered bright bands, finely to medium banded, and was more blocky and woody in nature than the overlying facies. Supercritical CO2 Extraction. Portions of the ground, sieved coal samples were dried overnight at 40 °C under air in a gravity convection oven to remove moisture. Three replicate splits of each dried coal sample were extracted with supercritical CO2 (40 °C, 10MPa), simulating CO2 injection into deep (∼1-km depth) coal beds. Supercritical fluid extraction-grade CO2 (no helium headspace) was used in all the experiments. The extractions were carried out using an ISCO 260D syringe pump coupled to an ISCO SFX 220 extraction unit. (Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.) For each experiment, approximately 1.0 g of dried coal sample was weighed into an extraction vessel and spiked with 10 µL each of aliphatic and PAH surrogate solutions. The extraction program consisted of a 15-min static (no-flow) step followed by a 60-min dynamic (flow) step at a flow rate of approximately 1.8 mL of CO2 min-1. The program was designed to yield a solvent/sample ratio, ∼2.4 mol CO2:1 g dry coal, comparable to that obtained in a parallel set of extractions conducted using dichloromethane in Soxhlet extraction apparatuses.14 The hydrocarbons that the supercritical CO2 mobilized from a given coal sample were collected in a chilled (0 °C) hexane solvent trap. One replicate from each coal sample was extracted multiple (3-4) times in succession to determine the effect of continued supercritical CO2 injection on hydrocarbon release. A fresh hexane solvent trap was installed prior to each extraction so that changes in the amount and type of hydrocarbons mobilized could be monitored. Hydrocarbon Fractionation and Analysis. The extracted hydrocarbons were fractionated into two compound classes, aliphatics and aromatics, using preparative liquid chromatography. Glass columns (11-mm i.d.) were packed in dichloromethane (DCM) with 1 mL of activated copper, 2.5 g of neutral alumina (5% water-deactivated), 2.5 g of silica 62 (100% activated), and 5.0 g of silica 923 (100% activated). The packed columns were then flushed with hexane and charged with the sample extracts. The aliphatic and aromatic fractions were consecutively eluted from each column using 100% hexane and a 30:70 (v/v) DCM/hexane mixture, respectively. Both fractions were subsequently evaporated under N2 to a final volume of 1 and 2 mL, respectively, and stored in the dark at 4 °C until analysis. (11) Kolak, J. J.; Burruss, R. C. In Greenhouse Gas Control Technologies; Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies; Vancouver, Canada, Sept 5-9, 2004; Wilson, M., Morris, T., Gale, J., Thambimuthu, K., Eds.; Elsevier: Oxford, U.K., 2005; pp 2233-2237. (12) Standard Classification of Coals by Rank; ASTM Test Method D388-99(2004)e1; ASTM International: Philadelphia, PA, 2003. (13) Parr, S. W. The Chemical Composition of Illinois Coal; Illinois State Geological Survey Bulletin No. 16; Illinois State Geological Survey: Springfield, Illinois, 1910; pp 203-243. (14) Kolak, J. J.; Burruss, R. C. An Organic Geochemical Assessment of CO2-Coal Interactions during Sequestration; U.S. Geological Survey Open-File Report 03-453; U.S. Geological Survey, 2003. Available at http:// pubs.usgs.gov/of/2003/of03-453/.

Kolak and Burruss Table 3. Roster of PAH Compounds and Corresponding Target Ions Used in GC-MS Analysesa analyte

abbreviation

target ion (m/z)

naphthalene 1-benzothiophene 1-methylnaphthalene and 2-methylnaphthalene biphenyl 1-ethylnaphthalene 2-ethylnaphthalene 2,6-dimethylnaphthalene acenaphthylene acenaphthene 2,3,5-trimethylnaphthalene fluorene 1,2,5,6-tetramethylnaphthalene dibenzothiophene phenanthrene anthracene 1-methylphenanthrene 3,6-dimethylphenanthrene fluoranthene pyrene 1,2,4-trimethylphenanthrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene dibenz[a,h]anthracene benzo[ghi]perylene

N0 1BT ∑N1

128 134 142

Bip 1EtN 2EtN ΣN2 Acey Acen ΣN3 F0 ΣN4 DBT P0 Ant ΣP1 ΣP2 Flu Pyr ΣP3 B[a]a C0 B[b]f B[k]f B[e]p B[a]p Per Icp Dba Bgp

154 141 141 156 152 153 170 166 184 184 178 178 192 206 202 202 220 228 228 252 252 252 252 252 276 278 276

a Bold entries indicate where the calibration curve for a single analyte was used to determine the total concentration (∑) of all corresponding isomers.

Figure 2. Total alkanes mobilized with supercritical CO2 (single extraction) versus coal rank. Graph bars denote the mean of three replicate analyses; error bars represent one standard deviation about the mean.

The fractionated sample extracts were analyzed via gas chromatography-mass spectrometry (GC-MS) using an Agilent 6890 gas chromatograph interfaced with an Agilent 5973 mass selective detector. Prior to analysis, perdeuterated n-nonadecane was added to the aliphatic fractions as an internal standard to correct for variations in instrument response. Likewise, an internal standard solution containing naphthalene-d8, phenanthrene-d10, chrysene-d12, and perylene-d12 was added to each aromatic sample. For both aliphatic and aromatic fractions, 1 µL of sample extract was injected

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Figure 3. Distributions of alkanes mobilized with supercritical CO2 (single extraction) from: (A) ligA, (B) subC, (C) hvCb, (D) hvAb, and (E) an rank coal samples. Note the different y-axis scale for Figure 3A.

in the splitless mode and separated on an HP5-MS column (30 m × 0.25 mm × 0.25 µm) using He as a carrier gas (0.9 mL min-1). The following GC oven program was used during the aliphatic hydrocarbon analyses: initial temperature held at 50 °C for 1.5 min, followed by a 10 °C min-1 ramp to 315 °C, and held at the final temperature for 15 min. The following GC oven program was used for the aromatic hydrocarbon analyses: the initial oven temperature (50 °C) was held for 4.0 min, followed by a 10 °C min-1 ramp to 150 °C, followed by a 6 °C min-1 ramp to 230 °C, followed by a 3 °C min-1 ramp to 300 °C, followed by a 10 °C min-1 ramp to 310 °C, and held at final temperature for 5 min.

External standards were analyzed with each batch of samples to generate five-point, concentration-response calibration curves. The response of the m/z 57 ion fragment was used to quantitate the concentrations of the nC9-nC31 n-alkanes, pristane (Pr), and phytane (Ph) in the aliphatic fractions. Concentrations of approximately 30 PAHs, including both parent compounds and alkylsubstituted homologues, were determined using the responses of appropriate target ions (Table 3). The abbreviations NX and PX are used to indicate members of the naphthalene and phenanthrene homologous series, respectively, where X is the number of methyl groups (0-4) substituted into the parent compound. The bold entries

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in Table 3 denote instances where the total concentration of all isomers associated with a given homologue, for example, all dimethylnaphthalenes (ΣN2), was determined from the concentration-response curve of a single isomer (e.g., 2,6-dimethylnaphthalene).

Results Aliphatic Hydrocarbons. The average, total extracted alkane concentrations mobilized with supercritical CO2 during the single extractions ranged from 3 to 64 g tonne-1 of dry coal (Figure 2). In general, the amount of alkanes extracted with supercritical CO2 exhibited a negative relationship with respect to coal rank. The highest alkane concentration was obtained from the lignite A sample. The subbituminous C and highvolatile bituminous coal samples yielded smaller amounts of alkanes, and the anthracite coal sample yielded the fewest. The disparity in extracted alkane concentrations between samples LA-1 and LA-2 is noteworthy, given that these two samples represent discrete coal facies collected from the same mine face. The apparent relationship between amount of extracted alkanes and coal rank does not fully describe observed variations within the sample set as the two high-volatile A bituminous coal samples yielded markedly different alkane concentrations. The alkane distributions in the coal sample extracts exhibited pronounced variations with coal rank. Higher molecular weight n-alkanes, particularly nC27, nC29, and nC31, dominated the alkane profile from the lignite (LA-1) sample extract (Figure 3A). For comparison, the subbituminous coal sample extracts contained lower concentrations of these three n-alkanes, but slightly higher concentrations of alkanes from nC13-nC23 (Figure 3B). Although supercritical CO2 mobilized comparable total amounts of alkanes from the subbituminous and highvolatile bituminous coal samples (excluding the WV-1 coal sample), the alkane distributions extracted from these two coal ranks were markedly different. For example, the alkane distributions from the IN-1 and OH-1 sample extracts did not exhibit the enrichment in the nC27, nC29, and nC31 n-alkanes that was present in extracts from the subbituminous coal samples. Instead, the alkane distributions extracted from these two high-volatile bituminous coal samples were relatively enriched in the lower molecular weight alkanes (nC21 but had a relatively uniform distribution among the lower molecular weight n-alkanes. Continued (multiple) extraction with supercritical CO2 yielded additional hydrocarbons from each coal sample, regardless of rank (Figure 4). However, for each sample/rank, the greatest amounts of alkanes were mobilized during the first CO2 extraction (Figure 4). There was a sharp decline in measured alkane concentrations between the first and second extractions, followed by more gradual declines in successive extractions. This general pattern was observed for all coal samples regardless of either rank or the total amount of alkanes mobilized during extraction. In the fourth extraction, all coal samples (excluding the anthracite sample, PA-1) yielded similar total alkane concentrations. Polycyclic Aromatic Hydrocarbons. Total amounts of PAHs mobilized during a single extraction with supercritical CO2 ranged from 3.1 to 91 g tonne-1 of dry coal. Total extracted PAH concentrations varied with coal rank, with the highest concentrations mobilized from the high-volatile bituminous coal

Figure 4. Amounts of alkanes mobilized during successive extractions with supercritical CO2. A fourth successive extraction was not performed for samples LA-1, LA-2, and OH-1.

Figure 5. Total PAHs mobilized with supercritical CO2 (single extraction) versus coal rank. Graph bars denote the mean of three replicate analyses; error bars represent one standard deviation about the mean.

samples (Figure 5). Supercritical CO2 mobilized comparable amounts of PAHs from the lignite, subbituminous, and anthracite coal samples. The extracted PAHs consisted largely of parent and alkyl-substituted members of the naphthalene and phenanthrene homologous series. Higher molecular weight PAHs (e.g., chrysene and indeno[1,2,3-cd]pyrene) were detected in some of the sample extracts, but at levels below reporting limits. The PAH distributions among the sample extracts also exhibited considerable variation with coal rank. The lignite, subbituminous, and anthracite coal sample extracts contained relatively uniform distributions of parent and methylated naphthalenes and phenanthrenes (Figure 6). In contrast, the high-volatile bituminous coals yielded much higher concentrations of naphthalenes than phenanthrenes (Figure 6C,D). The N3 homologues were the most abundant naphthalenes in the IN-1 and WV-1 extracts, whereas the N1 homologues were the most abundant members of the naphthalene series in the OH-1 extract. Continued (successive) extraction of coal samples with supercritical CO2 yielded additional PAHs. Polycyclic aromatic hydrocarbon concentrations generally declined with each successive extraction (Figure 7), although the most substantial declines were observed for the high-volatile bituminous coal samples IN-1 and OH-1. However, these two coal samples continued to yield higher PAH levels during successive extractions than the remaining coal samples. The WV-1 high-volatile

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Figure 6. Distributions of PAHs from the naphthalene and phenanthrene homologous series in the supercritical CO2 extracts from: (A) ligA, (B) subC, (C) hvCb, (D) hvAb, and (E) an rank coal samples. Note different y-axis scale for Figure 6D.

A bituminous coal sample exhibited a slightly different behavior: after the initial release of PAHs, the PAH levels in successive extracts declined to levels comparable to those measured in the low-rank and anthracite coal sample extracts. Discussion Effect of Coal Rank. Both the amount and type of hydrocarbons that supercritical CO2 mobilized from ground coal samples were found to vary considerably with coal rank. These variations may reflect a combination of the physical and chemical changes that occur during coalification. For example, the elevated temperatures encountered during burial promote the thermal maturation of a coal bed, resulting in the generation of discrete hydrocarbons from the organic matter present in coal.15 Coal can be described generally as a three-dimensional, network solid consisting of aromatic units cross-linked by alkyl chains.15 As the extent of maturation increases (and rank (15) Taylor, G. H.; Teichmu¨ller, M.; Davis, A.; Diessel, C. F. K.; Littke, R.; Robert, P. Organic Petrology; Gebru¨der Borntraeger: Berlin, Germany, 1998.

increases), this network structure undergoes thermal decomposition (i.e., cracking), resulting in the generation of bitumen, or free hydrocarbons, within the coal matrix. For example, the abundance of n-alkanes in coal bitumen has been attributed to the catalytic dissociation of n-alkanoic acids derived from cuticular waxes.16 The maximum bitumen content of the coal matrix typically corresponds to the high-volatile bituminous rank.17,18 This production of bitumen within the coal matrix yields free hydrocarbons that may be easily mobilized from the coal at relatively low temperatures.19 Further increases in rank are met with a corresponding decline in bitumen content as the bitumen is decomposed largely into methane. (16) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969, 33, 1183-1194. (17) Hood, A.; Gutjahr, C. C. M. Abstracts with Programs, Geological Society of America Annual Meeting, Minneapolis, MN, 1972; Geological Society of America: Boulder, CO, 1972; pp 542-543. (18) Teichmu¨ller, M. AdVances in Organic Geochemistry 1973; EÄ ditions Technip: Paris, 1974; pp 379-407. (19) Jaffe´, R.; Diaz, D.; Hajje, N.; Chen, L.; Eckhardt, C.; Furton, K. G. Org. Geochem. 1997, 26, 59-65.

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Figure 7. Amounts of PAHs mobilized during successive extractions with supercritical CO2. A fourth successive extraction was not performed for samples LA-1, LA-2, and OH-1.

The amount of hydrocarbons potentially mobilized during supercritical CO2 injection into coal beds may vary in response to the amount and type of bitumen present within the coal matrix. Using organic solvents, Allan et al.20 found that the extractable n-alkane content of bitumen in coal samples initially increased with increasing coal rank, then declined as rank increased further. The lower rank coal samples showed enrichment in the odd numbered n-alkanes. However, this enrichment disappeared as rank increased and was replaced by a broad, smooth distribution of n-alkanes in the higher rank coal samples.20 This shift from high molecular weight alkanes to lower molecular weight alkanes and the loss of odd-even predominance as coal rank increases have been described previously.21 In this study, the amount and distribution of n-alkanes extracted with supercritical CO2 (Figures 2 and 3) mirrored these earlier findings. A comparable study conducted on coal samples using different supercritical CO2 extraction conditions (120 °C and ∼20.3 MPa) found similar variations in the relative intensities of extracted n-alkane distributions with coal rank.22 Nelson et al.22 inferred that the change in n-alkane distributions with coal rank derived from mineral-catalyzed cracking of constituents rather than temperature-controlled thermolysis. Similar to the n-alkanes, the amount of solvent-extractable PAHs has been found to increase with rank up through the highvolatile bituminous coal samples and subsequently to decrease as coal rank increased further.23 In their study, Leythauser and Welte21 noted that the relative proportion of the alkane content in the total sample extract decreased with increasing coal rank, whereas the aromatic hydrocarbon proportion increased with coal rank. Because the polyaromatic units within the lowerrank coal samples have not yet undergone dissociation, there are apparently relatively fewer PAHs available for extraction or mobilization from these coals. The results presented here are consistent with the findings from these earlier studies: the most (20) Allan, J.; Bjorøy, M.; Douglas, A. G. AdVances in Organic Geochemistry 1975; Empresa Nacional Adaro De Investigaciones Mineras: Madrid, 1977; pp 633-654. (21) Leythauser, D.; Welte, D. H. AdVances in Organic Geochemistry 1968; Pergamon Press: Oxford, 1969; pp 429-442. (22) Nelson, C. R.; Li, W.; Lazar, I. M.; Larson, K. H.; Malik, A.; Lee, M. L. Energy Fuels 1998, 12, 277-283. (23) Radke, M.; Schaefer, R. G.; Leythauser, D. Geochim. Cosmochim. Acta 1980, 44, 1787-1800.

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PAHs were extracted from the high-volatile bituminous coal samples; smaller amounts were extracted from the lignite, subbituminous, and anthracite coal samples. The mass ratio of PAHs/n-alkanes in the lignite and subbituminous coal sample extracts ranged from 0.06 to 0.15. This ratio was significantly higher in the high-volatile bituminous and anthracite sample extracts (ranging from 0.45 to 1.92), signifying that the aromaticity of the sample extract increased with coal rank. These results indicate that the total amount of PAHs mobilized during supercritical CO2 injections likely varies in response to the maturation (rank)-controlled changes in bitumen type. Although the amount and type of hydrocarbons mobilized from coal samples with supercritical CO2 appeared to exhibit a dependence on coal rank, other parameters, such as solubility, diffusion rate, steric factors, and coal composition (maceral content), may also have affected the degree to which hydrocarbons were mobilized from the coal samples. The amount and types of PAHs present in the supercritical CO2 extracts afford some insight into the possible roles of these other factors. Phenanthrene structures are prominent constituents of the average coal sample and extractable bitumen in bituminous coals.24 A parallel set of dichloromethane extractions14 on several of the same coal samples used in this study demonstrated a similar prominence of phenanthrenes, particularly relative to the naphthalenes, in the bitumen. In contrast, the supercritical CO2 extracts from the bituminous coal samples contained higher concentrations of naphthalene homologues than phenanthrene homologues. Published PAH solubilities document that naphthalenes are typically more soluble in supercritical CO2 than phenanthrene.25-27 The influence of hydrocarbon solubility on the coal-supercritical CO2 interaction may be apparent given the PAH compositions of the supercritical CO2 extracts. Hydrocarbon partitioning between coal and supercritical CO2 may also affect the release of hydrocarbons during CO2 injection. For example, the n-alkanes and PAHs present in the coal bitumen may be dissolved or physically trapped within asphaltenes,28 a prominent constituent of bitumen. Asphaltenes are polar compounds with nitrogen, sulfur, and oxygen (NSO) functionalities and are relatively insoluble in supercritical CO2.29 Indeed, neither NSO compounds nor asphaltenes were detected in the supercritical CO2 extracts of this study, consistent with previous findings involving extractions using pure (unmodified) supercritical CO2.9 Because supercritical CO2 would be unable to significantly disrupt these polar associations present within bitumen, hydrocarbons present initially within the bitumen phase may partition between supercritical CO2 and bitumen phases during injection, with the distribution reflecting the hydrocarbon solubilities in the respective phases. This process may account for why the coal samples in this study continued to yield hydrocarbons even after multiple extractions with supercritical CO2. Variations in coal maceral composition may also play a significant role in CO2-coal interactions. Work by Karacan6 and Karacan and Mitchell30 has demonstrated that CO2 uptake (24) Radke, M.; Leythauser, D.; Teichmu¨ller, M. Org. Geochem. 1984, 6, 423-430. (25) Bartle, K. D.; Clifford, A. A.; Jafar, S. A.; Shilstone, G. F. J. Phys. Chem. Ref. Data 1991, 20, 713-756. (26) McHugh, M.; Paulaitis, M. E. J. Chem. Eng. Data 1980, 25, 326329. (27) Kurnik, R. T.; Holla, S. J.; Reid, R. C. J. Chem. Eng. Data 1981, 26, 47-51. (28) Price, L. C.; Clayton, J. L. Geochim. Cosmochim. Acta 1992, 56, 1213-1222. (29) Guiliano, M.; Boukir, A.; Doumenq, P.; Mille, G.; Crampon, C.; Badens, E.; Charbit, G. Energy Fuels 2000, 14, 89-94.

Hydrocarbons Mobilized during Carbon Dioxide Storage

(storage) rates in coal samples varied significantly depending on maceral type. Likewise, variations in maceral content may account for the large disparity in the amount of PAHs mobilized from the WV-1 and OH-1 coal samples (Figure 5). These two coal samples have similar heating values and, hence, the same rank. However, nearly an order of magnitude fewer PAHs were mobilized from the WV-1 coal sample than from the OH-1 sample. Both samples contain similar proportions of vitrinite (Table 2), but the WV-1 sample contains a higher proportion of liptinite (13.2 vs 8.4%) and less inertinite (5.4 vs 9.1%) than the OH-1 sample. The disparity in liptinite contents may seem counterintuitive, given that liptinite macerals have been proposed as hydrogen-rich macerals with good oil-generating potential.31 However, the abundance of the liptinite group does not necessarily translate directly into hydrocarbon generation potential.32 Further delineations of macerals within each group are necessary to ascertain whether the observed disparity in PAH yields can be attributed to differences in maceral composition. Environmental Ramifications for CO2 Sequestration in Deep Coal Beds. These experiments have shown that injection of supercritical CO2 into coal samples can mobilize organic constituents from the coal matrix. For example, the coal samples, particularly the high-volatile bituminous coals, yielded appreciable amounts of hydrocarbons, including PAHs, which are known to be hazardous to biota and can adversely affect environmental quality even at relatively low concentrations. The two- and three-ringed members of the naphthalene and phenanthrene homologous series were the most extensively mobilized PAHs, whereas the more toxic PAHs (e.g., the five-ringed members such as chrysene and indeno[1,2,3-cd]pyrene) tended to remain associated with the coal matrix. How this observed behavior will translate to field-scale CO2 sequestration or ECBM projects depends on several parameters, including coal rank, geologic setting, water content, and time. The findings from this study indicate that the amount of hydrocarbons, especially PAHs, mobilized during supercritical CO2 injection varies with coal rank, likely due to the relationship between coal rank and available hydrocarbons (i.e., bitumen). The potential influences of other factors, such as water content and time, require consideration to fully evaluate the fate of injected CO2 and of hydrocarbons associated with the coal matrix. The water content of coals is an important consideration due to its influence, for example, on fluid polarity and on the solvating power of supercritical CO2. The moisture content of coals varies considerably with rank, ranging from as much as 75 wt % in lignite to less than 2 wt % in higher rank coals.15 The drying process used in this study (overnight at 40 °C) likely removed most of the inherent moisture from coal samples, such that typical moisture variations with coal rank were obscured. If these experiments were repeated on as-received coal samples, the total amount of water present (Table 2) in each of the 1-g coal samples relative to the amount of solvent (CO2) used would, at most, reduce the mole fraction of CO2 in the extracting fluid by less than 1%. This small variation would not significantly affect either the amounts or types of hydrocarbons extracted from the coal samples. In addition to inherent moisture, coal beds typically contain appreciable quantities of excess surface, or “free”, moisture (30) Karacan, C. O.; Mitchell, G. D. Int. J. Coal Geol. 2003, 53, 201217. (31) Powell, T. G.; Boreham, C. J. Coal and Coal-bearing Strata as Oil-prone Source Rocks?; The Geological Society: London, UK, 1994; pp 11-29. (32) Isaksen, G. H.; Curry, D. J.; Yeakel, J. D.; Jenssen, A. I. Org. Geochem. 1998, 29, 23-44.

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occupying fractures or macropores within the coal matrix. Coal beds may be dewatered prior to CO2 injection, particularly in ECBM projects, where the concomitant decrease in reservoir pressure is necessary to induce methane desorption from the coal bed. However, following an initial dewatering stage, a coal bed still contains a substantial amount of free moisture, as evidenced by the volumes of water that are produced during coal bed methane extraction. Subsurface injection of supercritical CO2 would likely displace this free moisture from coal beds, especially in the vicinity of the injection borehole. The ability of supercritical CO2-water mixtures to mobilize hydrocarbons from coal beds is poorly understood, but the amount of volatile matter mobilized from coal samples with supercritical CO2 (∼40 °C and 148 bar) has been found to increase as water content of the coal increased from 0 to 11 wt % water.33 The presence of other dissolved organic compounds can greatly enhance PAH solubility in water.34 As a result, there exists the potential for PAHs mobilized by supercritical CO2 and supercritical CO2-water mixtures to remain in a mobile fluid phase, even following conclusion of CO2 injection and sequestration activities. The supercritical CO2 extractions conducted in this study involved very short contact times (∼1 h) and may only reflect CO2-coal interactions encountered during the earliest stages of CO2 injection into deep coal beds. In contrast, the injection phase of CO2 sequestration-ECBM field projects will likely be much longer in duration, (e.g., years2), and successful CO2 sequestration projects may need to “permanently” retain injected CO2 on the scale of hundreds to thousands of years. Supercritical CO2 can displace 83-96% of the moisture content from lignite and bituminous coal samples in a relatively short time period,35 but the long-term fate of stored CO2 in coal beds is poorly understood.36 Recent work has documented significant variations in CO2 uptake and redistribution among coal macerals over 4-5 days.6,30 Carbon dioxide stored in coal may initially dissolve into the structure and plasticize the coal. Subsequent rearrangement of the coal structure may result in expulsion of CO2 from the coal matrix.36 On a larger-scale CO2 storage project, such expulsion of CO2 may provide a vector by which hydrocarbons are mobilized from the coal matrix and transported within the subsurface. Knowledge of CO2-coal interactions at these longer time scales is beyond the scope of this study but is essential to address the ultimate fate and environmental ramifications of injecting supercritical CO2 into deep coal beds. Taking into account both the temporal extent of CO2-coal interactions and the fact that coal samples were dried prior to extraction, the experiments conducted in this study are constrained such that the findings may best represent hydrocarbon behavior near the injection site (borehole) shortly after CO2 injection has commenced. Conclusions Experiments simulating CO2 injection into deep coal beds have shown that supercritical CO2 is capable of mobilizing hydrocarbons from the coal matrix. The amount and type of (33) Ng, S. H.; Bhattacharya, S. N. In 1987 International Conference on Coal Science; Proceedings of the 1987 International Conference on Coal Science; Maastricht, The Netherlands, Oct 26-30, 1987; Moulijn, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier: Amsterdam, The Netherlands, 1987; pp 491-494. (34) Gordon, J. E.; Thorne, R. L. Geochim. Cosmochim. Acta 1967, 31, 2433-2443. (35) Iwai, Y.; Amiya, M.; Murozono, T.; Arai, Y.; Sakanishi, K. Ind. Eng. Chem. Res. 1998, 37, 2893-2896. (36) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63-70.

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hydrocarbons mobilized varied in response to coal rank, with the highest concentrations of hydrocarbons mobilized from the high-volatile bituminous coal samples. The hydrocarbon distributions in sample extracts indicate that this influence of coal rank is likely derived from the chemical changes associated with bitumen generation and cracking during increasing degrees of coalification and thermal maturation. The coal samples continued to yield hydrocarbons during consecutive supercritical CO2 extractions, indicating that the extent of hydrocarbon mobilization is not controlled solely by the solubility of hydrocarbons in supercritical CO2, but may also be affected by other factors,

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including hydrocarbon partitioning between bitumen-supercritical CO2 phases. Acknowledgment. We thank Nick Fedorko, Hal Gluskoter, Maria Mastalerz, Leslie Ruppert, and Peter Warwick for providing the coal samples used in this study. We greatly appreciate the thoughtful comments and suggestions that the anonymous reviewers and USGS reviewers provided on earlier drafts of this manuscript. The USGS Mendenhall Postdoctoral Research Program provided financial support for this work. EF050040U