In situ SAXS study on the evolution of coal nanopore structures with uniaxial compressive stress

Abstract

In this study, an in situ synchrotron radiation small-angle X-ray scattering (SAXS) experiment is used to characterize the evolution of coal nanopore structures under uniaxial compression. The variation in the coal nanopores is measured by analyzing the obtained scattering data. From the results, the coal scattering data show a positive Porod deviation in which the deviation slope decreases with increasing stress. Smaller nanopores are more sensitive to uniaxial compressive stress. There is a positive correlation between scattering intensity I and stress at less than 30 nm. The scattering intensity I of pores larger than 30 nm is negatively correlated with uniaxial compressive stress. The structure of smaller pores is more complex. The surface fractal dimension D_S and pore fractal dimension D_P increases and decreases with increasing uniaxial compressive stress, respectively. The specific surface area is positively correlated with D_S. In the measuring range of 3–80 nm, the coal nanopores show a bimodal distribution. At the stage of below 0.6σ_p, the average diameter decreases by 1.98%, the porosity and specific surface area increases by 6.21% and 31.5%, respectively; At the stage of above 0.6σ_p, the average diameter decreases by 1.04%, the porosity and specific surface area increases by 1.18% and 5.57%, respectively. These results suggest that the variation of coal nanopores with stress have phased characteristics, and the evolution of the coal nanopores under uniaxial stress can be divided into two stages. In the nanopore fracture stage, the nanopores are deform, fracture and create new smaller pores with increasing stress, and the roughness of pore surface increase. In the nanopore closure stage, high stress intensifies the degree of pore fracture, and nanopores begin to close and disappear. This study reveals the evolution characteristics of the pore structures of coal under uniaxial compression at the nanoscale.