Using Ground and Intact Coal Samples To Evaluate Hydrocarbon Fate during Supercritical CO₂ Injection into Coal Beds: Effects of Particle Size and Coal Moisture

Abstract

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...

Review of the impacts of leaking CO2 gas and brine on groundwater quality

“…Field based demonstration sites have shown that CO2 injection leads to greater dissolved organic carbon within sedimentary brine formations (Kharaka et al, 2006a) as well as increased concentrations of BTEX, PAHs, phenols and other toxic compounds in groundwater monitoring wells (Kharaka et al, 2009, Kharaka et al, 2010a, Kharaka et al, 2010b, Scherf et al, 2011. Laboratory based experiments using both scCO2 and organic solventsconfirmed that reservoir rocks are sources of organic compounds and that the extractable organic material was dominated by n-aliphatic and aromatics (Jarboe et al., 2015, Kolak and Burruss, 2006, Kolak and Burruss, 2014, Kolak et al, 2015, Zhong et al, 2014a, Zhong et al, 2014b. In particular, nalkanes ranging from n-C9 to n-C31 have been identified and have over 30 different aromatic compounds, including BTEX and naphthalene (Jarboe et al., 2015, Kolak and Burruss, 2014, Kolak et al, 2015, Zhong et al, 2014b.…”

“…Laboratory based experiments using both scCO2 and organic solventsconfirmed that reservoir rocks are sources of organic compounds and that the extractable organic material was dominated by n-aliphatic and aromatics (Jarboe et al., 2015, Kolak and Burruss, 2006, Kolak and Burruss, 2014, Kolak et al, 2015, Zhong et al, 2014a, Zhong et al, 2014b. In particular, nalkanes ranging from n-C9 to n-C31 have been identified and have over 30 different aromatic compounds, including BTEX and naphthalene (Jarboe et al., 2015, Kolak and Burruss, 2014, Kolak et al, 2015, Zhong et al, 2014b. These laboratory based studies have also shown that the amount and type of extractable organic compounds was influenced by the solvent type (e.g., scCO2 vs. dichloromethane), exposure duration, and the temperature and pressure of the extraction conditions as well as the source rock, water content, and geologic setting.…”

Laboratory Study on Changes in the Pore Structures and Gas Desorption Properties of Intact and Tectonic Coals after Supercritical CO2 Treatment: Implications for Coalbed Methane Recovery

“…The data also show that the porosity of the treated intact and tectonic coal samples increased by 18.90% (from 4.18% to 4.97%) and 23.14% (from 5.23% to 6.44%), respectively. Previous studies [22,23,41] showed that ScCO2 can mobilize some of the polycyclic aromatic hydrocarbons and aliphatic hydrocarbons in the coal. Additionally, CO2 will form carbonic acid when it is dissolved in the water in the coal seams, and this acid can dissolve some of the inorganic minerals in the coal, such as calcites, dolomites, and magnesites [42].…”

“…Additionally, ScCO 2 is an organic solvent and can mobilize some of the organic matter in the coal. If some of the CO 2 is dissolved in the reservoir water, the pH decreases and the CO 2 -enriched solution can dissolve some of the minerals present [22][23][24]. Studies also showed that CO 2 adsorption can cause the coal's matrix to swell [25].…”

Effects of Supercritical CO2 Treatment Temperature on Functional Groups and Pore Structure of Coals

“…The newly generated CO, the aliphatic and aromatic hydrocarbons, and the polycyclic aromatic hydrocarbons in the coal matrix extracted by the ScCO 2 are biologically toxic. These substances dissolved in gas and water will enter the coal seams, the rock formations, and the groundwater surrounding the coal seam and will become environmental hazards [23,68].…”