Study on characterization and fractal features of adsorption pores of coal reservoirs in Jixi Basin

In order to investigate coal reservoir pore structure characteristics which is favor of coalbed methane exploration and development,we collected 8 kinds of coal from different coal mine areas in Jixi Basin,based on low temperature nitrogen adsorption experiment,we used the nitrogen adsorption fractal geometry analysis method to quantitatively characterize the coal reservoir adsorptive pore structural features in Jixi Basin .The adsorptive pore fractal dimensions D1(relative pressure 0～0.5) and D2 (relative pressure 0.5～1.0) were obtained from the nitrogen adsorption isotherm using the FHH equation discussed relationship between fractal dimension and pore structure,material composition,as well as the degree of coal metamorphism. Results showed that pore morphology in the study area can be partitioned into 3 types,mainly including slit shaped pores,better openness parallel plate-like pores and ink bottle or narrow neck shaped pores. Fractal dimensions D1 was 1.38～2.74 and the average fractal dimensions D1 was 2.32; fractal dimensions D2 was 2.30～2.84 and the average fractal dimensions D2 was 2.59. D1representing the surface roughness of adsorption pores and D2 the irregularity of the pore structures. As D1 gets bigger,the pore specific surface area increases,whereas the bigger D2 gets,the smaller the average pore size becomes,the higher the micropore content is,and the larger the pore heterogeneity is. Pore fractal dimension D1 of coal samples with different adsorption and desorption curve types presents a pattern of typeⅢ＞typeⅡ＞typeⅠ,while D2 presents a pattern of type Ⅲ＞typeⅠ＞type Ⅱ. Between D1and moisture,ash yield,vitrinite content all present polynomial distribution,but between D2 and moisture content,vitrinite content present linear negative correlation,correlation with coal Ro is not apparent.