Supercritical CO2 Interaction Induced Pore Morphology Alterations of Various Ranked Coals: A Comparative Analysis Using Corrected Mercury Intrusion Porosimetry and Low-Pressure N2 Gas Adsorption

Quanlin Yang; Wei Li; Kan Jin

doi:10.1021/acsomega.0c00134

Supercritical CO₂ Interaction Induced Pore Morphology Alterations of Various Ranked Coals: A Comparative Analysis Using Corrected Mercury Intrusion Porosimetry and Low-Pressure N₂ Gas Adsorption

ACS Omega. 2020 Apr 13;5(16):9276-9290. doi: 10.1021/acsomega.0c00134. eCollection 2020 Apr 28.

Authors

Quanlin Yang¹, Wei Li², Kan Jin³

Affiliations

¹ Academic Journal Center, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, China.
² Key Laboratory of Gas and Fire Control for Coal Mines, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China.
³ College of Quality & Safety Engineering, China Jiliang University, Hangzhou, Zhejiang 310018, China.

Abstract

Enhanced coal bed methane (ECBM) recovery via CO₂ geological storage (CGS) involves CO₂ adsorption and CH₄ desorption from coal pores, gas diffusion, and flow through coal matrix and cleats, which are closely related to coal pore morphology. To characterize and quantify the alterations of pore morphology of different ranked coals treated by supercritical CO₂ (ScCO₂), CGS was simulated with a custom-built high-pressure ScCO₂ geochemical reactor at 10 MPa and 35 °C for sub-bituminous coal, bituminous coal, and anthracite, respectively. The bituminous coal was collected from a ScCO₂-rich reservoir to examine the effect of second exposure to ScCO₂ on pore morphology. Mercury intrusion porosimetry (MIP) after data correction using the Tait equation (TE) and low-pressure N₂ gas adsorption (LPN₂GA) were employed. The decrease of coal matrix compressibility and increase of pore volume compressibility indicate that sub-bituminous coal becomes more resistant to microfracture closure and pore shrinkage after ScCO₂ treatment. Both the thermodynamics fractal model after MIP data correction and Frenkel-Halsey-Hill (FHH) model, which are strongly scale-dependent and meaningful from a geometric viewpoint, reveal a downward trend of the irregularity and heterogeneity of pore structures for sub-bituminous coal. The effects of hydrocarbon mobilization and inorganic matter dissolution by ScCO₂ play an important role in pore size distribution (PSD), pore volume (PV), and pore shape (PS) alterations, whereas these changes are quite small for bituminous coal. The above observed alterations of sub-bituminous coal are all positive, such as closed pore reopening, pore volume enlargement, pore roughness decline, pore connectivity enhancement, and gas desorption improvement, which makes sub-bituminous coal a desired option for CO₂-ECBM recovery.