Characterization of the Porosity and Permeability of Gasified Coal in UCG Process: An Experimental and Simulation Study

Under the environment of energy transformation in the world, underground coal gasification (UCG) is an important means to realize the green and clean development and utilization of deep coal resources. Due to a series of complex chemical reactions, the porosity and permeability of coal have changed significantly. Accurately characterizing the porosity and permeability of gasified coal is of great significance to the field screening, production control, and numerical simulation of the UCG project. In this study, the porosity and permeability of coal samples are studied by means of experiment and numerical simulation, respectively. Subsequently, a predictive model for porosity–permeability relationships is established with its feasibility verified by comparison to previous experimental results. In the experimental phase, a nitrogen atmosphere simulates pyrolysis conditions, while an air atmosphere replicates gasification environments. Scanning electron microscopy (SEM) is employed to characterize the porosity and permeability changes in heat-treated coal samples. For the numerical simulation component, through the analysis of UCG physical and chemical processes, the UCG three-dimensional numerical simulation model at the field scale is established by using CMG-STARS simulation software, and the changes in porosity and permeability during coal gasification are analyzed. The result shows that following heat treatment experiments, the average equivalent pore diameter of the coal sample increases to 0.748 μm, an increase of 493.65%, with general expansion observed across pore diameters. Additionally, the average shape factor decreases from 3.20 to 2.584, suggesting enhanced roundness in the porosity characteristics. Post-heat treatment observations reveal an increase in pore quantity alongside expanded diameters, thus indicating significant alterations in pore structure. Through 3D UCG numerical simulation, the evolution of coal porosity and permeability is categorized into three distinct stages: 25–320, 320–750, and 750–1000 °C. This categorization reflects the evolutionary model of the pore structure during coal gasification. In the predictive model, the permeability of coal during pyrolysis and gasification is represented as an exponential function of the porosity. As the porosity increases, the growth rate of permeability initially rises slowly before gradually accelerating. Furthermore, it is observed that in the gasification process, the increase in coal permeability with respect to porosity is less pronounced than that observed during pyrolysis. The findings presented in this paper hold significant implications for field screening, process design, and optimization of UCG applications.