Variation in the Pore Structure of Coal after Hydraulic Slotting and Gas Drainage

Abstract

ABSTRACT:The integration of hydraulic slotting and gas drainage techniques has become a mainstream technique for enhancing permeability in coal seams with low permeability. However, the mechanism of action of this process is unclear. In this paper, field experiment and laboratory tests are described that aim at elucidating this process. Given the sensitivity and accuracy of test methods and their corresponding determination principles, a combination of mercury intrusion porosimetry and nitrogen gas adsorption was proposed as a complementary technique and the pore-size distribution (PSD) was obtained. It is shown that the proportion of minipores decreases remarkably, whereas that of the macropores gradually increases with the decrease in the distance from the slotted borehole. By contrast, the mesopores and micropores present insignificant changes. Meanwhile, the adsorption pore and the seepage pore show a similar variation in tendency with the minipores and macropores, respectively. Moreover, the specific surface area decreases substantially with the decrease in borehole distances. The integration of hydraulic slotting and gas drainage can lower the gas-adsorption properties and enhance the gas-seepage capacity within the disturbed zone significantly. The paper highlights the guiding factors for improving the enhanced coal bed methane recovery.

Microscopic mechanism for enhanced coal bed methane recovery and outburst elimination by hydraulic slotting: A case study in Yangliu mine, China

“…For the same coal, it is assumed that after hydraulic slotting, the coal rank, ash content, and elemental and total organic carbon contents of coal remain unchanged. According to Zou et al , the hydraulic slotting process has some effect on the moisture content of the coal samples, and with an increase of the distance between the sampling site and the slotted borehole, the moisture content decreased gradually on the whole . However, since coal samples were dewatered before performing the CH 4 isothermal adsorption test, the effect of moisture content on adsorption capacity of coal was not considered.…”

“…In this section, we investigated the pore structure characterizations of coal around the slotted borehole by scanning electron microscope (SEM), nitrogen gas adsorption (N 2 GA), and mercury intrusion porosim-etry (MIP). Because these measurement methods have been explained in detail in our previous work, 27 here we only briefl y describe the test results. Figure 8 depicts the pore volume distributions of micropore, minipore, mesopore, and macropore in samples with diff erent distances from the slotted borehole.…”

Influence of coupled effect among flaw parameters on strength characteristic of precracked specimen: Application of response surface methodology and fractal method

“…In addition, coalbed methane (CBM) is a type of fuel that is characterized by its high calorific value and low environmental impact, i.e., clean energy (Zou et al, 2014a(Zou et al, , 2014bNi et al, 2014). Moreover, CH 4 , the primary component of CBM, is a greenhouse gas that has a greenhouse effect 21 times greater than that of CO 2 (Wang and Cheng, 2012;Cheng et al, 2011;Zhang et al, 2014).…”

“…This technique has been widely used in many coal mines in China to improve the gas drainage efficiency (Yan et al, 2015;Lin et al, 2014, Lin andShen, 2015;Lu et al, 2009Lu et al, , 2011Zou et al, 2014aZou et al, , 2014bShen et al, 2014Shen et al, , 2012aShen et al, , 2012b. Fig.…”

“…These researchers' study results showed that the existence of a flaw is favorable for the propagation of cracks and an increase in coal porosity, both of which improve the gas drainage efficiency. Zou et al (2014aZou et al ( , 2014b investigated the variations in the pore structure of coal after hydraulic slotting and gas drainage. Based on the analysis, the gas adsorption properties of coal around a slotted borehole were also discussed.…”