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 low-pressure 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.