Pore structure characterization and its significance for gas adsorption in coals: A comprehensive review

The coal reservoir has multifaceted and hierarchical characteristics influenced by heterogeneity of the pore and fracture networks. The coal pore including micropore, mesopores and macropores determines the critical migrate processes of gas molecules, especially for adsorption. Normally, coal pore structure refers to the pore shape, size distribution and interconnection of variable pores. Together with pore volume, pore size distribution, specific surface area and porosity. For coalbed methane, pore structure is the storage place, which largely controls the methane adsorption capacity of coal. Fluid injection, microscope observation and X-ray and spectroscopic analysis can be used to explore coal pore structure. Understanding the pore structure in varied rank coals is very challenging mostly due to the complex pore system leading to ambiguous petrophysical properties and gas performance. Among several important factors controlling the performance of coalbed methane, adsorption is the most crucial one, which affected by the pore aspects including pore volume, specific surface area, pore size distribution, pore shape and porosity. Therefore, accurate characterization of pore structure is coupled with advanced technologies, which resulted in the development of a wide range of methods. In this work, all the necessary technologies from imaging, segmentation, and modeling of the pores to various methods of gas adsorption evaluation are reviewed. This paper presents a critical review of all of the existing relevant and significant techniques and compares their performances together with their limitation with special reference to pore structure characterization. And the impacts of pore structure are thus reviewed. Finally, this review work proposes the current challenges and possible future research.