CO₂ Storage Behavior in Nanopores: Implications for CO₂ Sequestration in Ultra-Tight Geological Formations

Abstract

The fatal challenge that human beings are currently facing is global warming as a result of excessive CO 2 emission in the atmosphere. CO 2 sequestration, gaseous CO 2 injection into ultra-tight geological sites, is regarded as a promising approach to achieve CO 2 reduction substantially. In this work, emphasis is paid to CO 2 storage potential inside depleted shale or coal seam where the presence of nanopores is rich, and CO 2 molecules store in both bulk and adsorption states in nanopores. The microscopic characterization on CO 2 behavior in the nanospace, particularly quantitative description on the difference between CO 2 in the adsorption and bulk states, is still lacking. With the intention to shed light on nanoconfined CO 2 behavior, a simple yet robust theoretical work rooting in the chemical potential equilibrium of each CO 2 molecule in the entire system is implemented, and the shift of critical properties due to the nanoconfinement effect is coupled. Then, the CO 2 density can be described as a function of distance away from the nanopore wall; the CO 2 molecule is found to accumulate more densely while approaching the nanopore wall, suggesting an adsorption behavior from microscopic perspective. Results show that (a) the CO 2 adsorption-phase thickness is insensitive to nanopore size, ranging from 0.58 to 0.64 nm, and the ratio of adsorption density over bulk density could reach 1−2 orders of magnitude; (b) the CO 2 amount the 2 nm nanopore is able to store could reach over 7.2 times that in macropores, displaying the unique advantage of shale and coal formations on CO 2 sequestration over conventional oil/gas reservoirs; (c) increasing pressure can improve the total CO 2 geological sequestration performance, and the improvement of magnitude at the lowpressure range could be as great as 2.9 times that at a high-pressure range. This work provides a doable framework to investigate the CO 2 existence behavior in nanopores, enriching the theoretical basis to identify favorable geological sites for CO 2 sequestration.

Surface-Molecule Interaction Strength on CO2 Adsorption Capacity in Nanopores: Implications to Advance CO2 Sequestration Performance

“…In order to fill the knowledge gap, this work puts emphasis on the correlation between the surface-molecule interaction strength and adsorption performance. Thus, the difference between this work and Wu et al (2023) is evident as well. In short, the mentioned papers focus on three different aspects of nanoconfined CO 2 behavior; results from these research works are totally different.…”

“…It is clear that Wu et al (2022) focus on the CO 2 transport inside nanopores, which is obviously different in comparison with the research on CO 2 adsorption in this work. Meanwhile, Wu et al (2023) focus on the adsorption structure from a microscopic viewpoint, particularly, basic properties of the surface are fixed and unchanged, indicating surface wettability that manifests the surface-molecule interaction strength is not considered in the work. In order to fill the knowledge gap, this work puts emphasis on the correlation between the surface-molecule interaction strength and adsorption performance.…”