Response of anthracite microcrystalline structure due to multi-phase CO2 injection

Abstract

CO2 injection in coal causes permeability reduction, but also affects the microcrystalline structure. To quantify the different response of microcrystalline structure of anthracite after multi-phase CO2 injection, anthracite samples from the Qinshui basin, the samples were demineralized by HF-HCl before CO2 injection, then subjected dry, demineralized samples to CO2 injection experiments at 4 MPa (gaseous), 6 MPa (subcritical) and 8 MPa (supercritical) pressure, the microcrystalline quantification was obtained using X-ray diffraction and image analysis of HRTEM micrographs, the morphological characteristics of ultramicropore were quantitatively characterized by HRTEM image analysis. With increasing injection pressure, the interplanar spacing (d002) and aromaticity (fa) of the coal gradually increased, while the average number of aromatic layers (Nc) and unit stacking height (Lc) gradually decreased; the variation in the unit lateral size (La) also became more complex. Thus, the coal crystal structure was altered after CO2 treatment and the degree of damage increased with pressure. Further, the average length of coal lattice fringes becomes shorter with the increase of pressure, while the length, width and area of ultra-micropores in subcritical and supercritical SH and ZZ coal increase. In addition, the average curvature of the lattice fringes became larger, and the directional ordering decreased after CO2 intrusion. Thus, CO2 injection led to the transformations of crystal structure, generating intermolecular structural defects, and a reduction in order in the striation direction. This study presents the microcrystalline structure and ultra-micropore response of anthracite due to CO2 injection, which also helps with in-depth understanding of the impact of CO2-ECBM.

Keywords

• Coal
• Multi-phase CO2
• XRD
• HRTEM
• Microcrystalline structure