Abstract
Self-oscillating Water Jet (SOWJ) slotting in Coalbed Methane (CBM) is proposed to overcome low gas permeability, high gas desorption, and difficult mining in deep coal beds. SOWJ slotting excitation expands the fracture network, increases coal permeability, and strengthens gas desorption. The coupled effect of these three processes increases CBM extraction. Analysis of the characteristics of SOWJ, the effect of coal slotting, and changes in coal permeability shows that (1) SOWJ impacts on coal-rock mass, forming the erosion–peeling zone, fragmentation zone, and distal conical crack zone in the rock. The jet impact and cavitation sonic vibrating effect generate coal vibration; (2) The slots and fractures formed by the jets release the coal’s elastic energy, depressurising the coal and changing the stress field. The stress redistribution further expands the fractures and the subsequent perforative fracture network; (3) Slot formation increases the coal’s exposed area, changing the gas flow pattern. The decrease of effective stress increases coal permeability; the vibration characteristics of the jets and the cavitation sonic vibrating effect enhance gas desorption, which increases gas emission; (4) Extraction field tests showed that single-hole extraction of CBM from conventional boreholes was 1606 m3 and the average standard scalar volume was 0.01 m3/min, compared to 7081 m3 and 0.042 m3/min for SOWJ slotting boreholes, 4.41 and 4.2 times, respectively, of the conventional boreholes. Thus, SOWJ slotting can significantly improve CBM mining.
Highlights
Coalbed methane (CBM) is an unconventional gas stored in coal matrix as free and adsorptive state types [1]
The decrease of effective stress increases coal permeability; the vibration characteristics of the jets and the cavitation sonic vibrating effect enhance gas desorption, which increases gas emission; (4) Extraction field tests showed that single-hole extraction of CBM
38.2% and 54.5% of the total CBM resources, respectively, while the resources within a 1000-m depth make up only 7.3% of the total [5]; this indicates that China’s CBM resources are mainly found in deep coal seams
Summary
Coalbed methane (CBM) is an unconventional gas stored in coal matrix as free and adsorptive state types [1]. As coal seams are being mined at increasingly deeper layers, mining conditions become more complex These conditions include increased in-situ stress, decreased coal gas permeability, and high levels of gas desorption. Many scholars have developed methods to increase coal permeability, including protective layer mining, hydraulic slotting, hydraulic fracturing, and presplitting blasting. A protective mining layer [6,7] increases the permeability of the coal seam, mainly by depressurising the coal; the hydraulic slotting, hydraulic fracturing, and presplit blasting methods increase the permeability mainly through forming fracture networks [8,9,10]. The Self-oscillating Water Jet (SOWJ) slotting method, proposed by the author, can be applied in high-efficiency CBM mining by controlling and expanding the fracture network, improving the permeability of the coal, and strengthening gas desorption. By means of theoretical analysis, laboratory experiments, and numerical simulation, the rock-breaking characteristics of the SOWJ slot effects and the resulting change of permeability are investigated, revealing the mechanism of the SOWJ slotting technology which improves CBM production efficiency
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