Microstructural response of coal fracture surface induced by ScCO2 injection measured with AFM

Abstract

Fractures in coal serve as the principal channels for the migration of coalbed methane (CBM). The microscopic traits of coal fracture surface are of significance for evaluating reservoir permeability and directly influence the outcome of CO₂ storage and displacement. To comprehend the impact of CO₂ on the coal fracture surface, the alterations in the coal fracture surface before and after CO₂ injection are investigated vis atomic force microscopy. As CO₂ gradually transforms into a supercritical state, the fracture density and aperture on the coal fracture surface gradually escalate. The mean pore size and porosity of the coal fracture surfaces rose from 5.45 nm to 6.55 nm and from 4.66% to 6.85%, respectively. The proportion of micropores (<2 nm) gradually diminished. The proportions of mesopores (2–50 nm) and macropores (>50 nm) proportions gradually. Swelling, extraction and mineral dissolution are the predominant reasons for the alteration in the pore structure of the coal fracture surface. CO₂ injection modifies the roughness and fractal characteristics of the coal fracture surface. The mean roughness R_a dropped from 6.54 nm to 4.03 nm, and the fractal dimension D_s declined from 2.45 to 2.28. With the fixed scanning range, the injection of CO₂ has been shown to reduce the roughness of coal fracture surface and lower their fractal dimension, thereby has a positive influencing permeability enhancement. Therefore, in CO₂-ECBM engineering, it is essential not only to analyze the physical properties of the target reservoir, but also to conduct a comprehensive evaluation of permeability with a suitably defined area of the reservoir.