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 ( Ro,max = 0.93–3.20%) by combining low-temperature N2/CO2 adsorption tests and surface/volume fractal theory. The results show that the coal reservoirs can be divided into three categories: type A ( Ro,max < 2.15%), type B (2.15% < Ro,max <2.50%), and type C ( Ro,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 CO2-specific surface area and CO2-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. Dv1 and Dv2 can characterize the structure of 2–100 nm adsorption pores, and Dv1 (volume heterogeneity) has a positive correlation with the pore structural parameters such as N2-specific surface area and N2-total pore volume. This parameter can be used to characterize volume heterogeneity of 2–10 nm pores. Dv2 (surface heterogeneity) showed type A > type B > type C and was mainly affected by the metamorphism degree. Ds2 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 (CO2-total pore volume, CO2-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). Dv1, Dv2, and Ds2 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.
Highlights
As an unconventional reservoir rock, the storage space of a coal reservoir is dominated by the defect space between the surface molecules of the coal matrix and the nanometer level of micropores
Chen et al (2017), Heister (2016), and Zhao et al (2016) showed that
Summary
As an unconventional reservoir rock, the storage space of a coal reservoir is dominated by the defect space between the surface molecules of the coal matrix (solid solution state) and the nanometer level of micropores. Pore and fracture systems of coals have been comprehensively studied by constantly updated methods in recent years, including experimental techniques such as photoelectric observation, gas adsorption, and fluid intrusion, and reconstruction of three-dimensional structures (Chalmers et al, 2012; Clarkson et al, 2013; Li et al, 2017; Liu and Nie, 2016; Yao et al, 2010). The pores of coal can be classified into those 50 nm (macropores) in size according to the International Union of Pure and Applied Chemistry (IUPAC). Another widely accepted classification of 1000 nm was provided by Hodot (Hodot, 1966; IUPAC, 1994). Chen et al (2017), Heister (2016), and Zhao et al (2016) showed that
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