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.
Highlights
CO2 sequestration has been identified as the most viable and effective option to mitigate the adverse effect of greenhouse gases on global warming.[1]
Afterward, combined with the low-pressure N2 gas adsorption (LPN2GA) method, we provided a richer picture of the changes of fractal dimension, pore volume, pore size distribution (PSD), and pore shape before and after supercritical CO2 (ScCO2) treatment comprehensively and systematically
The decrease of coal matrix compressibility indicates that subbituminous coal becomes harder to compress after ScCO2 treatment, which might prevent microfracture closure, pore shrinkage, or connectivity loss due to bond breakage between interconnected pores
Summary
CO2 sequestration has been identified as the most viable and effective option to mitigate the adverse effect of greenhouse gases on global warming.[1]. The presence of matrix and cleat within coal structure creates favorable conditions for gas adsorption, desorption, diffusion, and flow.[2] The higher affinity of coal substance toward CO2 than CH4 and the denser state of CO2 (supercritical state) guarantee the long-term isolation of CO2.3,4 The injected CO2 could displace the original adsorbed CH4 and be sealed perpetually while enhancing the CH4 recovery (referred to as CO2-enhanced coal bed methane, ECBM), thereby offsetting the cost of CO2 sequestration and providing additional clean energy for sustainable development of society.[5,6] As the critical point of CO2 is 30.97 °C and 7.38 MPa, in most cases, CO2 is likely to be present in a supercritical state under the conditions of high reservoir temperature and pressure after being injected into deep coal seams.[7,8] Compared with subcritical CO2 (SubCO2), many properties of supercritical CO2 (ScCO2), such as density, viscosity, diffusivity, and permittivity, change significantly. ScCO2 exhibits higher adsorption capacity, greater dissolution, extraction, and expansion effect on coal than SubCO2.9,10
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