Effects of Supercritical Carbon Dioxide Saturation Temperature on the Multiscale Pore Structure of Coal
Wei He, Yuedu Chen, Haojie Lian, Weiguo Liang, Jiwei Yan, Xiaoxia Song
Abstract
The influence of supercritical carbon dioxide (ScCO 2 ) on the pore structure of coal at different temperatures, especially the evolution of multiscale pore structure (from nanometer scale to millimeter scale), is crucial for CO 2 sequestration in deep coal seams. Thus, multiscale measurement techniques, including proximate analysis, CO 2 adsorption, mercury intrusion porosimetry (MIP), low-field nuclear magnetic resonance (NMR), and X-ray CT scanning, were combined to investigate the pore structure evolution of anthracite after ScCO 2 saturation at different temperatures. Findings unveiled a progressive reduction in the moisture, ash, and volatile matter content of anthracite as the ScCO 2 saturation temperature increased from 40 to 100 °C. The porosity of anthracite increased by 10.48–14.4% under ScCO 2 saturation across different temperatures, and it increased gradually with the increase of temperature. The increase in anthracite porosity caused by ScCO 2 saturation at temperatures increasing from 40 to 100 °C is less than that before and after ScCO 2 saturation at 40 °C. The proportion of micropores and mesopores decreased, while the proportion of macropores increased. A CT scan showed that, due to the coupling effect of temperature and ScCO 2, the temperature increase accelerated the porosity growth of anthracite before and after ScCO 2 saturation. After saturation of ScCO 2, adsorption pores (<50 nm) did not exhibit fractal characteristics, while seepage pores (>50 nm) exhibited a favorable fractal characteristic. As the temperature increased, the fractal dimension of the seepage pores decreased, indicating that the pore roughness and complexity of the seepage pores decreased with the ScCO 2 temperature. Finally, a conceptual model was proposed to elucidate the mechanism of the pore structure evolution of anthracite under ScCO 2 saturation at different temperatures. This study is of great significance for understanding the temperature effect on coal reservoir porosity evolution during CO 2 storage in deep coal seams.