Characterization of Pores and Fractures in Soft Coal from the No. 5 Soft Coalbed in the Chenghe Mining Area

Abstract

The characteristics of the pore structure and gas migration in soft coalbeds are the premise of evaluating gas discharge in soft coalbeds. To explore the pore structure characteristics of soft coal masses, the No. 5 soft coalbed in the eastern zone of Chenghe Mining Area, was investigated and compared with the No. 5 hard coalbed in the western zone. By using a mercury intrusion method, low-temperature liquid nitrogen adsorption, and scanning electron microscopy (SEM), the pore structure characteristics of the No. 5 coalbed were explored. Moreover, based on fractal theory, the pore structure of coal was characterized. The results showed the pores in soft coal mainly appeared as small pores and micropores in which the small pores accounted for nearly half of the total pore volume. Mesopores and macropores were also distributed throughout the soft coal. The mercury-injection and mercury-ejection curves of soft coal showed significant hysteresis loops, implying that pores in coal samples were mainly open while the mercury-injection curve of hard coal was consistent with its mercury-ejection curve, showing no hysteresis loop while having an even segment, which indicated that closed pores occupied the majority of the pore volume in the coal samples. The curves of low-temperature nitrogen adsorption of soft coal all follow an IV-class isotherm. Moreover, the fractal dimensions of soft coal are respectively larger than the fractal dimensions of hard coal. It can be seen that the characterization of pores and fractures of the soft coal was different from the hard coal in the western distinct of the old mining area. The gas prevention and control measures of soft coal should be formulated according to local conditions.

Numerical simulation of boreholes for gas extraction and effective range of gas extraction in soft coal seams

“…-2017 accounted for about 45% of the total number of accidents in coal mines. 1 Gas in coal mines is also a green energy source, showing multiple advantages including efficiency and low-pollutant emitting. [2][3][4] China is rich in gas reserves, ranking second in the world; however, in the process of coal mining, the vast majority of gas cannot be effectively utilized.…”

Experimental Analysis of Pore Structure and Fractal Characteristics of Soft and Hard Coals With Same Coalification

“…The shape of this kind of adsorption branch of the hysteresis loop is caused by the adsorption of a monolayer on the surface. Then, multilayer and capillary condensation of gas molecules occur (Thommes et al, 2015;Wei et al, 2019). The signi cant rise of adsorption isotherms at low pressure (P/P 0 ) region corresponds to microspores lling, while at the beginning of isotherm, indicating the completion of monolayer adsorption and starting of multilayer adsorption (Jin et al, 2016;Thommes et al, 2015).…”

“…The seam thickness of soft coal (varies from several centimeters to the height of the whole bed) changes signi cantly, and most of them are unstable. Thus, investigating pore and fracture structure characteristics is vital to understand the adsorption/desorption phenomenon (Cheng et al, 2011;Wei et al, 2019). In general, coal is considered an organic rock consisting of pores and fractures, which act as a gas reservoir and migration channels for methane and other gases (Ren et al, 2019a;Durucan, 2005, 2003).…”

Experimental analysis of pore structure and fractal characteristics of soft and hard coals with same coalification

“…Liu et al (2021) studied the variation of the pore structure and adsorption capacity of deformed and undeformed coal (soft and hard) with the particle size. The structural characteristics of pores and fractures in soft coal masses were explored and compared with those of hard coal from the same mining area (Wei et al 2019). Guo et al (2017) studied the effect of structural characteristics of deformed coal with different degrees of deformation on methane adsorption.…”