Enhancement of Carbon Sequestration Capacity of Coal through Long-Term Geochemical Reactions with scCO<sub>2</sub>–H<sub>2</sub>O
Qi Wu, Chuanjie Zhu
Abstract
The long-term interactions between coal and the scCO 2 –H 2 O system during CO 2 sequestration and CO 2 -ECBM efficiency significantly influence the safety and storage capacity of CO 2 geological sequestration, as well as the recovery efficiency of coalbed methane. In this study, coal samples of three different coal ranks were subjected to scCO 2 –H 2 O conditions at 40 °C and 9 MPa to simulate the geochemical reactions occurring during CO 2 storage in deep coal seams. A control group under N 2 –H 2 O conditions was also established for comparison. The evolution of pore structure and functional groups in coal samples was systematically analyzed at reaction durations of 5, 10, 20, 40, and 60 days using scanning electron microscopy (SEM), physical adsorption methods, mercury intrusion porosimetry (MIP), and Fourier transform infrared spectroscopy (FTIR). The results indicate that for pores with diameters below 100 nm, the specific surface area (SSA) and pore volume in the N 2 –H 2 O group exhibited a continuous decline over 60 days. In the scCO 2 –H 2 O group, SSA increased by 9 ∼ 15% within the first 5 ∼ 10 days due to the predominant effect of chemical corrosion. However, between 10 and 60 days, SSA decreased by 8 ∼ 12% and gradually stabilized, which can be attributed to pore compaction and organic matter swelling, leading to reduced connectivity with the compression effect being more pronounced in higher-rank coals. For pores larger than 100 nm, SSA in the scCO 2 –H 2 O and N 2 –H 2 O groups increased by 22 ∼ 37% and 14 ∼ 25% within 60 days. Furthermore, prolonged scCO 2 –H 2 O exposure induced significant bending and fracturing of the coal surface, with these effects being more evident in higher-rank coals. Under the scCO 2 –H 2 O system, the enhancement of adsorption capacity due to geochemical reactions is more pronounced in higher-rank coals. However, the efficiency of high-pressure effects increases while the efficiency of chemical effects decreases. The research findings can summarize CO 2 coal seam sequestration into three stages: the fluctuation stage, the increasing stage, and the moderately increasing stage.