Review on Applications of X-ray computed tomography for coal characterization: recent progress and perspectives

Characterization of microscopic structure and macroscopic physical property are the basis for better understanding of coalbed methane reservoirs. X-ray computed tomography (CT), as an nondestructive measurement, has been widely and successfully applied to characterize the internal structure of coal. In this study, we introduce the principle of CT imaging and the microstructure recognition. A summary of CT imaging-based coal microstructure characterization follows, including three- dimensional (3D) microstructure reconstruction, pore and mineral quantification, and equivalent pore network model construction. We reviewed the methods used for evaluating the macroscopic properties of coal, including porosity calculation, gas adsorption/diffusion rate test, permeability simulation, and mechanical behavior evaluation. This study discusses the application of CT to investigate the evolutionary mechanisms of microstructure and macroscopic properties during gas adsorption, temperature change, and damage deformation. We conclude this review with a summary of the challenges and application perspectives of CT. The small scanning range, limited observation accuracy, functional limitations, lengthy testing process, and high cost are some of the major hurdles in the broad application of CT for coal characterization. In the future, CT should be combined with other techniques to establish full-scale pore and fracture models, identify mineral types in microstructures, and effusively use the advantages of CT by selecting the key points in the evolutionary mechanisms of microstructure and macroscopic properties.