Micro-Cleat and Permeability Evolution of Anisotropic Coal During Directional CO2 Flooding: An In Situ Micro-CT Study

Cleat and permeability altered by CO 2 injection are key issues for enhanced coalbed methane production and CO 2 geo-sequestration into unminable coal seams. As the main channel for CO 2 flowing through, pre-existing macro- and micro-cleats in coal directly determine permeability. Both permeability and cleats are pressure-dependent. However, few researches have been made to observe directly changes in micro-cleats as a function of injection pressure during CO 2 flooding. In this study, using a novel in situ micro-CT core flooding apparatus, we observed the gradual closure of micro-cleats in small anthracite coal at CO 2 injection pressures from 1 to 4 MPa. Quantitative micro- aperture size variations were analyzed further. An increasing-then-decreasing trend of the mean aperture size of the micro-cleat with increasing injection pressures was obtained, and the rebound in mean micro-cleat aperture size measured parallel to cleat directions was higher than that perpendicular to bedding plane direction. The permeability decreased drastically first and then slowly with increase in saturation time. Relatively weak coal permeability anisotropy (