Coal samples of different rank (lignite to anthracite) were extracted in the laboratory with supercritical CO 2 (40 °C; 10 MPa) to evaluate the potential for mobilizing non-methane hydrocarbons during CO 2 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 CO 2 , although the amount of hydrocarbons mobilized declined with each successive extraction. These results demonstrate that the potential for supercritical CO 2 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 CO 2 storage.

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To investigate the potential for mobilizing organic compounds from coal beds during geologic carbon dioxide (CO 2 ) storage (sequestration), a series of solvent extractions using dichloromethane (DCM) and using supercritical CO 2 (40°C and 10 MPa) were conducted on a set of coal samples collected from Louisiana and Ohio. The coal samples studied range in rank from lignite A to high volatile A bituminous, and were characterized using proximate, ultimate, organic petrography, and sorption isotherm analyses. Sorption isotherm analyses of gaseous CO 2 and methane show a general increase in gas storage capacity with coal rank, consistent with findings from previous studies. In the solvent extractions, both dry, ground coal samples and moist, intact core plug samples were used to evaluate effects of variations in particle size and moisture content. Samples were spiked with perdeuterated surrogate compounds prior to extraction, and extracts were analyzed via gas chromatography−mass spectrometry. The DCM extracts generally contained the highest concentrations of organic compounds, indicating the existence of additional hydrocarbons within the coal matrix that were not mobilized during supercritical CO 2 extractions. Concentrations of aliphatic and aromatic compounds measured in supercritical CO 2 extracts of core plug samples generally are lower than concentrations in corresponding extracts of dry, ground coal samples, due to differences in particle size and moisture content. Changes in the amount of extracted compounds and in surrogate recovery measured during consecutive supercritical CO 2 extractions of core plug samples appear to reflect the transition from a water-wet to a CO 2 -wet system. Changes in coal core plug mass during supercritical CO 2 extraction range from 3.4% to 14%, indicating that a substantial portion of coal moisture is retained in the low-rank coal samples. Moisture retention within core plug samples, especially in low-rank coals, appears to inhibit accessibility of supercritical CO 2 to coal matrix porosity, limiting the extent to which hydrocarbons are mobilized. Conversely, the enhanced recovery of some surrogates from core plugs relative to dry, ground coal samples might indicate that, once mobilized, supercritical CO 2 is capable of transporting these constituents through coal beds. These results underscore the need for using intact coal samples, and for better characterization of forms of water in coal, to understand fate and transport of organic compounds during supercritical CO 2 injection into coal beds. ■ INTRODUCTIONThe physical properties of coal have important applications to quantifying the potential for hydrocarbon generation, retention, and expulsion from coal beds, understanding interactions of injected CO 2 with coal during geologic CO 2 storage (sequestration), and recovering hydrocarbons, such as during enhanced coalbed methane recovery. Coal generally is described as a three-dimensional, cross-linked polymer containing fluid phases within network porosity, i.e., cleats (fractu...

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