Evolution of anthracite nanopores with CO2 adsorption at different pressures by synchrotron radiation small angle X-ray scattering

Abstract

As a porous medium, coal has a strong adsorption capacity for CO2. Nanopore is the main adsorption space of coal reservoir. To study the evolutionary characteristics of nanopores in coals with CO2 adsorption, in situ small angle X-ray scattering experiments of anthracite at different CO2 adsorption pressures were carried out. The change of scattering intensity ratio RI in nanopores with different sizes were analyzed. The porosity, specific surface area and average aperture were quantitatively characterized. The research shows that the 5 nm pores is more susceptible to higher CO2 pressure, while the scattering phenomenon in the 15 nm pores changes most obviously at lower CO2 pressure. The deformation sensitivity of 35–75 nm pores to CO2 adsorption is consistent. The scattering data in anthracite show a positive Porod deviation. CO2 will become dense with increasing pressure, and its filling of nanopores makes some scattering phenomena become invisible to X-ray. There will be a new boundary between dense CO2 and free CO2, which affects the fluctuation of electron density between two-phase systems. There is a positive correlation between the Porod deviation of the scattering data and the CO2 pressure. In the process of CO2 adsorption, the dynamic deformation of anthracite nanopores is obvious, resulting in synchronous changes of porosity and specific surface area. CO2 adsorption pressure is negatively correlated with porosity and specific surface area, but positively correlated with the average aperture. The decrease of CO2 pressure leads to the gradual recovery of nanopore deformation. However, at different evolution stages of the same CO2 pressure, the porosity, specific surface area and average aperture of anthracite are inconsistent. Irreversible damage to anthracite nanopores is produced with CO2 adsorption. The evolution of nanopores is the result of the superposition of CO2 compression and adsorption.