Nanoscale Coal Deformation and Alteration of Porosity and Pore Orientation Under Uniaxial Compression: An In Situ SANS Study

Nanoscale coal deformation affects the geomechanics response of coal structure under external stress conditions. In this study, in situ small-angle neutron scattering (SANS) is used to characterize the evolution of nanopore structures under uniaxial compression on an Illinois coal specimen. Porod invariant mapping is used to estimate apparent porosities at different azimuthal angles. Structure-free orientation factors, including the alignment factor and Hermans’ orientation factor, are used to characterize orientation degree in nanopores quantitatively. The nanoscale pore deformation and evolution with stress have not been directly measured before, and the in situ SANS technique offers the unique capability to probe the nanoscale rock deformation. From the results, higher values of apparent porosities are shown near the coal bedding direction. At different azimuthal angles, the apparent porosities increase with increasing uniaxial stress in the elastic deformation range (54–575 bar) for the measured coal. The pore-scale degree of orientation is a size-dependent parameter, and pore orientation along the coal bedding generally increases as pore size increases. The degree of orientation in nanopores on average (in a size range between ~ 2 × 10³ Å and ~ 15 Å) tends to increase first and then decreases with increasing stress loading, although different from that of macropores, indicating higher stresses on the nanopore edges along the bedding direction and then on the nanopore body along the normal direction. There is no distinct permanent change of porosity within the measured stress range (0–575 bar) after the stress relief.