Experimental Investigation on Pore-Fracture Variations in Coal Affected by Carbon Disulfide

Abstract

Solvent treatment is an effective technique to stimulate pore and fracture growth in low-permeability coal seams and improve the efficiency of methane extraction. Adopting the nuclear magnetic resonance test, liquid nitrogen adsorption analysis, ultrasonic test, and CT scanning, pore variation and fracture development in lignite, bituminous coal, and anthracite after carbon disulfide treatment were analyzed. Full-scale pore size distribution characteristics were obtained. The Frenkel–Halsey–Hill model was adopted to analyze pore fractal properties. Experiment results show that carbon disulfide could increase coal porosity. Lignite showed the best pore alteration effect, with a porosity increase of 34.10%, followed by bituminous coal with a porosity increase of 14.55%, while anthracite had a slightly weaker change with only 0.91% porosity growth. The pore diameter distribution range of treated coal expanded from 0–450 to 0–1000 μm. The average pore diameter rose from 316 to 483 μm, with better connectivity between pores. After treatment, the proportions of micropore specific surface areas (SSA) in three coal samples decreased but the ratios of small pore and medium pore SSA increased. Fractal dimension D 1 of lignite and bituminous coal decreased by 5.669 and 0.054%, while D 1 of anthracite increased by 22.407%. D 2 reduced by 0.599, 3.143, and 1.262%, respectively. Raw coal had the maximum porosity near both ends of the CT section. Porosity of lignite was the largest at the ends after treatment. Surface porosity inside coal samples also rose from about 0.1 in raw coal to approximately 0.4 after treatment. Ultrasonic velocity in lignite decreased by 50.16% due to solvent treatment. Increases in ultrasonic attenuation coefficient α and ultrasonic attenuation ratio β indicate good fracture development. Furthermore, development degrees of lignite and bituminous coal are higher than that of anthracite. Results of the above methods verify with each other, indicating the effectiveness of carbon disulfide treatment in improving pore and fracture structures. The outcomes of this research could offer a theoretical basis for chemical permeability-enhancement technology.