Abstract
Carbon dioxide (CO2) has been used to replace coal seam gas for recovery enhancement and carbon sequestration. To better understand the alternations of coal seam in response to CO2 sequestration, the properties of four different coals before and after supercritical CO2 (ScCO2) exposure at 40 °C and 16 MPa were analyzed with Fourier Transform infrared spectroscopy (FTIR), low-pressure nitrogen, and CO2 adsorption methods. Further, high-pressure CO2 adsorption isotherms were performed at 40 °C using a gravimetric method. The results indicate that the density of functional groups and mineral matters on coal surface decreased after ScCO2 exposure, especially for low-rank coal. With ScCO2 exposure, only minimal changes in pore shape were observed for various rank coals. However, the micropore specific surface area (SSA) and pore volume increased while the values for mesopore decreased as determined by low-pressure N2 and CO2 adsorption. The combined effects of surface property and pore structure alterations lead to a higher CO2 adsorption capacity at lower pressures but lower CO2 adsorption capacity at higher pressures. Langmuir model fitting shows a decreasing trend in monolayer capacity after ScCO2 exposure, indicating an elimination of the adsorption sites. The results provide new insights for the long-term safety for the evaluation of CO2-enhanced coal seam gas recovery.
Highlights
IntroductionConcerns about increasing carbon dioxide (CO2 ) concentration in the atmosphere have driven research into the technical reduction of the emission of CO2 from fossil fuel use [1]
Concerns about increasing carbon dioxide (CO2 ) concentration in the atmosphere have driven research into the technical reduction of the emission of CO2 from fossil fuel use [1].Carbon capture and storage (CCS) is an effective approach for CO2 mitigation currently under consideration [2,3]
The effect of supercritical CO2 (ScCO2) exposure on surface property of various rank coals was examined by Fourier Transform infrared spectroscopy (FTIR)
Summary
Concerns about increasing carbon dioxide (CO2 ) concentration in the atmosphere have driven research into the technical reduction of the emission of CO2 from fossil fuel use [1]. Carbon capture and storage (CCS) is an effective approach for CO2 mitigation currently under consideration [2,3]. Un-minable coal seams have been targeted as safer sites for CO2 sequestration because the sequestrated CO2 is predominantly stored as a relatively stable adsorption phase in coal seams. The replacement of coal seam gas (CSG) offsets some proportion of sequestration costs [4,5,6]. An understanding of the physical and chemical alterations of coal reservoirs in response to CO2 sequestration would offer a scientific foundation on which to base long-term storage predictions.
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