Evolution of anthracite nanopores with CO2 adsorption at different pressures by synchrotron radiation small angle X-ray scattering
Yixin Zhao, Xiaodong Guo, Zhenyu Tai, Yirui Gao, Siqi Li
Abstract
As a porous medium, coal has a strong adsorption capacity for CO 2 . Nanopore is the main adsorption space of coal reservoir. To study the evolutionary characteristics of nanopores in coals with CO 2 adsorption, in situ small angle X-ray scattering experiments of anthracite at different CO 2 adsorption pressures were carried out. The change of scattering intensity ratio R I 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 CO 2 pressure, while the scattering phenomenon in the 15 nm pores changes most obviously at lower CO 2 pressure. The deformation sensitivity of 35–75 nm pores to CO 2 adsorption is consistent. The scattering data in anthracite show a positive Porod deviation. CO 2 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 CO 2 and free CO 2 , 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 CO 2 pressure. In the process of CO 2 adsorption, the dynamic deformation of anthracite nanopores is obvious, resulting in synchronous changes of porosity and specific surface area. CO 2 adsorption pressure is negatively correlated with porosity and specific surface area, but positively correlated with the average aperture. The decrease of CO 2 pressure leads to the gradual recovery of nanopore deformation. However, at different evolution stages of the same CO 2 pressure, the porosity, specific surface area and average aperture of anthracite are inconsistent. Irreversible damage to anthracite nanopores is produced with CO 2 adsorption. The evolution of nanopores is the result of the superposition of CO 2 compression and adsorption.