Article pubs.acs.org/EF
Using Ground and Intact Coal Samples To Evaluate Hydrocarbon Fate during Supercritical CO2 Injection into Coal Beds: Effects of Particle Size and Coal Moisture Jonathan J. Kolak,* Paul C. Hackley, Leslie F. Ruppert, Peter D. Warwick, and Robert C. Burruss †
U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia 20192, United States S Supporting Information *
ABSTRACT: To investigate the potential for mobilizing organic compounds from coal beds during geologic carbon dioxide (CO2) storage (sequestration), a series of solvent extractions using dichloromethane (DCM) and using supercritical CO2 (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 CO2 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 CO2 extractions. Concentrations of aliphatic and aromatic compounds measured in supercritical CO2 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 CO2 extractions of core plug samples appear to reflect the transition from a water-wet to a CO2-wet system. Changes in coal core plug mass during supercritical CO2 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 CO2 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 CO2 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 CO2 injection into coal beds.
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INTRODUCTION The 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 (fractures) and matrix porosity (pores).1−3 In this context, coal beds can serve both as sources for and sinks of fluids and other substances. Changes to moisture (water) content, matrix porosity, and pore surface chemistry during coalification affect these exchanges. During thermal maturation (coalification), coal beds can serve as sources of water, organic acids, or CO2, as a result of reactions involving the polymer network.4−6 The occurrences of gas with high levels of CO2 in some coal beds7,8 demonstrate the ability of coal beds to act as CO2 sinks, and constitute natural analogs that facilitate investigation of physicochemical processes affecting CO2 storage in coal beds. Characterizing the effects of the physical properties of coal on coal-fluid interactions is especially pertinent in the context of CO2 storage in coal beds. Sorption isotherm measurements This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
on ground coal samples are used to evaluate CO2 storage capacity in coal beds.9,10 Several interlaboratory studies have measured CO2 sorption on ground coal samples to evaluate the effects of variations in physical properties, e.g., coal rank, sorption models, and CO2 phase on analytical results.11−13 Coal particle size also has been shown to influence gas sorption isotherms measured on coal samples.14,15 In addition to laboratory studies using ground coal samples, several studies have conducted pilot-scale CO2 injection tests into coal beds, collectively spanning coal rank from lignite through anthracite.16−23 Several of these pilot studies16,17,21 targeted deep, low-permeability coal beds (∼1 km below land surface) into which supercritical CO2 could be injected and where existing subsurface pressure−temperature (P-T) conditions were sufficient to maintain the injected CO2 in a supercritical state. A primary focus of these pilot studies has been to evaluate the potential for coal beds as a CO2 sink; however, the potential exchange of fluids (water) and other constituents during this process remains an important consideration for both the Received: November 20, 2014 Revised: June 23, 2015
A
DOI: 10.1021/ef502611d Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Location, Description, and Characteristics of Coal Samples Used in This Study sample LA-1
LA-2
LA-3
OH-1
OX-04-BB Red River Parish, Louisiana block from channel bench lower Wilcox Group Paleocene−Eocene
LA-04-B3-22 Catahoula Parish, Louisiana
1R-SM-3(4) Carroll County, Ohio
type group/bed name age
OX-04-AB Red River Parish, Louisiana block from channel bench lower Wilcox Group Paleocene−Eocene
core lower Wilcox Group Paleocene−Eocene
block from channel bench Middle Kittanning Pennsylvanian
total moisture (%) residual moisture (%) equilibrium moisture (%) ash yield (%) fixed carbon (%) volatile matter (%)
32.26 3.80 33.14 11.15 28.29 28.30
Proximate Analysis, As-Received Basis 30.48 12.68 3.14 2.55 31.71 11.49 9.75 14.70 31.31 35.86 28.46 36.76
3.96 0.74 3.89 5.33 51.47 39.24
hydrogen (%) carbon (%) nitrogen (%) sulfur (%) oxygen (%) free swelling index apparent specific gravity (g cm−3) heating value (Btu lb−1) [M,MMF] ASTM coal rank38
6.42 41.53 0.99 0.79 39.12 0 1.30 8147
Ultimate Analysis, As-Received Basis 6.53 5.75 44.99 57.02 0.96 1.36 0.98 0.63 36.79 20.54 0 0.5 1.42 1.44 8796 12 233
5.63 75.23 1.59 0.69 11.53 4 1.27 14 448
vitrinite (%) liptinite (%) inertinite (%) mean reflectance, R0
na na na 0.34−0.37a
identification code location
lignite A [lig A]
subbituminous C [subC]
high volatile C bituminous [hvCb]
Petrographic Analysis, MMF Basis na 79.0 na 7.2 na 13.8 0.34−0.37a 0.46b
high volatile A bituminous [hvAb]
68.3c 8.1c 23.5c 0.68c
a Values for mean vitrinite reflectance are from Dennen et al.,39 and measured on samples collected from the same location (Oxbow mine, Red River Parish, Louisiana) during another site visit. bThis value corresponds to an Rmax measurement, which, at this level of thermal maturity, should approximate an R0 value because there is no significant anisotropy in the vitrinite. cValues from Hackley and Kolak.40 Other studies36,41 report the following results (MMF basis) from a separate petrographic analysis of sample OH-1: 82.5% vitrinite, 8.4% liptinite, 9.1% inertinite. Abbreviations: na = not analyzed; M,MMF = moist, mineral matter-free.
amounts of water, which can affect the physical and chemical behavior of coal.28 Water is known to affect the solvation properties of supercritical CO2, and the potential for supercritical CO2 to extract, or mobilize, organic compounds.29−31 Conversely, the dissolution of supercritical CO2 into water can lower the pH of the resulting solution.32 This acidification can hydrolyze kerogen33 and affect the exchange of organic compounds with surrounding fluids, and other processes, e.g., CO2 uptake in coals and shales.34 The objective of this study is to investigate processes that could occur during supercritical CO2 injection into coal beds, with a focus on the potential for mobilization and transport of hydrocarbons and other compounds from coal. Given the influence of coal rank on the mobilization of hydrocarbons with supercritical CO2,35−37 we collected four samples, ranging in rank from lignite A to high volatile A bituminous from locations in Louisiana and Ohio (Table 1). The inclusion of low-rank coal samples is important given recent studies of the potential feasibility of CO2 storage in low-rank Gulf Coast coals42 and North Dakota lignites.19 In this study, the sample set consisted of three coal block samples and one coal core. These samples
physical (mechanical) and the chemical response of the coal bed. A common challenge encountered during these pilot studies is coal swelling and loss of coal matrix permeability, with a corresponding reduction in CO2 injectivity. In a laboratory study using an intact coal core sample, Karacan and Mitchell24 documented a complex response of coal to a simulated CO2 injection, highlighted by spatial variations in CO2 uptake and by redistribution of CO2 within the coal sample over time. These outcomes underscore the need for additional measurements using intact coal samples to augment the understanding of in situ processes that might occur during CO2 injection into coal beds. The potential for hydrocarbon mobilization, and the processes that affect hydrocarbon partitioning between solid and fluid phases over different temporal and spatial scales, are important considerations for CO2 storage in coal beds.25,26 Recent studies have used larger, or intact, coal samples as a means of scaling-up laboratory experiments to better simulate in situ processes affecting coal beds.27 The use of fresh coal samples that have not undergone desiccation nor weathering also is important, given that low-rank coals contain considerable B
DOI: 10.1021/ef502611d Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 1. Flowchart summarizing the approach used for collection and analysis of coal samples. EOM = extractable organic matter. SFE = supercritical fluid extraction. After desorption, the canister was opened and the core sample was split in half vertically. One half of the coal core was sent to a commercial laboratory (Geochemical Testing, Inc.) for grinding, sieving, and proximate and ultimate analyses; the other half was archived. Block samples of the Middle Kittanning coal bed were collected in 2004 from a newly exposed working face at the underground Carroll Hollow mine, located near Bergholz, Ohio. The block sample (OH-1) was collected and sealed in multiple plastic bags in accordance with ASTM D4596-99(2004).44 Core Plug Subsampling. At the USGS laboratory, core plugs were subsampled in 2004 from the coal blocks (Table 2) or the core sample (LA-3) using a diamond core bit attached to a drill press outfitted with a water swivel.47 The LA-3 core plug sample was drilled from the archived half of the coal core, about 1 cm below the top of the core sample. Organic-free, deionized water (Milli-Q; 18 MΩ) was used during the coring process. The sample orientationperpendicular or parallel to beddingvaried depending on coal block thickness and ability to obtain an intact core plug. Both the core bit and supercritical fluid extraction (SFE) vessel are approximately 15 mm inner diameter. Immediately after coring, a metal piston was used to transfer intact core plugs directly from the core bit into an extraction vessel. The potential influence of coal lithotype on core plug integrity, coupled with findings from previous studies48,49 documenting variations in physical and chemical properties among coal lithotypes, suggest that core plug sample analyses might not be representative of the original whole-coal block samples. Coal Sample Preparation and Petrographic Analysis. After coring, the remaining block samples were split and a portion was sent to a commercial laboratory (Geochemical Testing, Inc.) for grinding, sieving, and proximate and ultimate analyses according to ASTM D3172-89(2002) and ASTM D3176-89(2002), respectively.50,51 The remaining portion was sent to a commercial laboratory (RMB Earth Science Consultants Ltd.) for sorption isotherm analyses and companion measurements of ash yield and equilibrium moisture (Figure 1). Sample preparation for petrographic analysis followed ASTM D2797-04,52 wherein samples ground to a top size of 20 mesh (841 μm sieve opening) were set in a 2.54 cm (1 in.) circular thermoplastic briquette and ground with successively finer grit abrasives to a final polish with 0.05 μm colloidal silica. Vitrinite
are not representative of whole coal beds, but do provide a rank suite of samples with which to investigate geochemical processes that could occur during CO2 injection into coal beds. The block samples and the core sample were drilled to obtain intact core plugs, measuring approximately 1.5 cm (diameter) by 1−4 cm (length). Subsplits from the block and core samples were taken for additional analyses (Figure 1), and one of the splits was dried and ground (−60 mesh;
