Nanoscale coal deformation and its geomechanics effect on pore structure evolution under hydrostatic and uniaxial compression conditions

Nanoscale coal deformation will affect geomechanics response of coal with gas injection coupled with the external stress condition. In this study, in situ small-angle neutron scattering (SANS) is used to characterize the nanopore structure evolution of San Juan coal under hydrostatic gas injection and Illinois coal under uniaxial compression, respectively. Argon pressurization can induce pore contraction in San Juan coal. Both methane and carbon dioxide injection can induce pore shrinkage due to the combined effects of hydrostatic pressurization, adsorbed molecule occupation, and sorption-induced matrix swelling. An apparent anisotropic nanopore structure is shown in Illinois coal cut perpendicular to bedding, in which the degree of anisotropy generally increases with increasing pore size. Uniaxial compression perpendicular to the bedding can cause a “two-stage” pore contraction in both the directions perpendicular and parallel to the bedding. The degree of anisotropy tends to decrease with increasing uniaxial stress during the nanopore deformation but with variations among different deformation stages and pore sizes.