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

Cleat and permeability altered by CO2 injection are key issues for enhanced coalbed methane production and CO2 geo-sequestration into unminable coal seams. As the main channel for CO2 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 CO2 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 CO2 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 (< 50%) was observed in this test and the permeability measured in the bedding plane direction was not necessarily smaller than that in the cleat directions for a small coal body.

Automated summary

Cleat and permeability in coal play a crucial role in enhanced coalbed methane production and CO2 geo-sequestration. This study used a novel in situ micro-CT core flooding apparatus to observe the closure of micro-cleats in small anthracite coal at different CO2 injection pressures. The results showed that the mean aperture size of the micro-cleat increased and then decreased with increasing injection pressures, with a higher rebound in mean aperture size parallel to cleat directions. The permeability decreased drastically initially and then slowly with an increase in saturation time.