Influence of geological factors on coal permeability in the Sihe coal mine

Permeability of coal reservoirs influence the extraction of coal gas from coal seams.Twelve coal samples were collected at an anticline and a syncline of the No.3 coal seam in the Sihe coal mine.Porosity,permeability,pore size,vitrinite reflectance,and liquid nitrogen adsorption of the samples were evaluated.Structural curvatures at the sample locations,and the distance between the sampling locations and the nearest faults were calculated based on seismic data.The influences of the evaluated parameters on permeability were analyzed.Major factors that influence permeability of the No.3 coal seam were extracted using principal component analysis(PCA).Based on the porosity–permeability model derived from the Archie formula and classic Kozeny–Carman equation,we deduced that the permeability of coal increased with an increase in porosity.With an increase in average vitrinite reflectance,permeability decreases first and then increases.PCA results showed that coal permeability was regulated by three key components representing three modes.The first component included pore size,depth,and pore complexity accounting for 52.59%of the variability indicating that it was the most important in controlling permeability.The second component included specific surface area,structural curvature,and porosity,and the third component comprised of specific surface area,porosity,and average vitrinite reflectance.Overall,pore diameter and complexity had significant effects on coal permeability.The results show that researchers and stakeholders must consider the interactions among multiple factors rather than single factors to understand the influences on permeability to facilitate efficient utilization of coalbed methane resources.

Two-dimensional SEM image-based analysis of coal porosity and its pore structure

A quantitative analysis of the porosity,pore size distribution,and fractal dimensions of pores is significant for studying the pore structure characteristics of coal.This study utilized 12 anthracite coal samples from the Sihe mining area to explore the pore structure characteristics of the coal therein.Hundred randomly selected points on each sliced coal sample were imaged via scanning electron microscopy,and a total of 1200 images were used for the analysis.The porosity and fractal dimensions of the coal samples were analyzed via digital image processing and box-counting dimension methods.This method is characterized by extensive graphical analysis,and the results are based on statistical methods.These were also used to analyze the structural and development characteristics of the microscopic pores in the coal.The results reveal that the surface porosity obtained via digital image processing was 16.11%lower than that measured experimentally.The fractal dimension and porosity of the pore surface were fitted to a natural logarithmic curve.The rate of change in the pore fractal dimension depends on the porosity such that,to some degree,a greater porosity is associated with more complex pore structures,a higher degree of micropore development,and improved pore connectivity.