Structural and fractal characterization of adsorption pores of middle–high rank coal reservoirs in western Yunnan and eastern Guizhou: An experimental study of coals from the Panguan syncline and Laochang anticline

Abstract

To better understand the structural characteristic of adsorption pores (pore diameter < 100 nm) of coal reservoirs around the coalbed methane production areas of western Yunnan and eastern Guizhou, we analyzed the structural and fractal characteristics of pore size range of 0.40–2.0 nm and 2–100 nm in middle–high rank coals ( R o,max = 0.93–3.20%) by combining low-temperature N 2 /CO 2 adsorption tests and surface/volume fractal theory. The results show that the coal reservoirs can be divided into three categories: type A ( R o,max < 2.15%), type B (2.15% < R o,max <2.50%), and type C ( R o,max > 2.15%). The structural parameters of pores in the range from 2 to 100 nm are influenced by the degree of coal metamorphism and the compositional parameters (e.g., ash and volatile matter). The dominant diameters of the specific surface areas are 10–50 nm, 2–50 nm, and 2–10 nm, respectively. The pores in the range from <2 nm provide the largest proportion of total specific surface area (97.22%–99.96%) of the coal reservoir, and the CO 2 -specific surface area and CO 2 -total pore volume relationships show a positive linear correlation. The metamorphic degree has a much greater control on the pores (pore diameter less than 2 nm) structural parameters than those of the pore diameter ranges from 2 to 100 nm. D v1 and D v2 can characterize the structure of 2–100 nm adsorption pores, and D v1 (volume heterogeneity) has a positive correlation with the pore structural parameters such as N 2 -specific surface area and N 2 -total pore volume. This parameter can be used to characterize volume heterogeneity of 2–10 nm pores. D v2 (surface heterogeneity) showed type A > type B > type C and was mainly affected by the metamorphism degree. D s2 can be used to characterize the pore surface heterogeneity of micropores in the range of 0.62–1.50 nm. This parameter has a good correlation with the pore parameters (CO 2 -total pore volume, CO 2 -specific surface area, and average pore size) and is expressed as type C < type B < type A. In conclusion, the heterogeneity of the micropores is less than that of the meso- and macropores (2–100 nm). D v1 , D v2 , and D s2 can be used as effective parameters to characterize the pore structure of adsorption pores. This result can provide a theoretical basis for studying the pore structure compatibility of coal reservoirs in the region.