Coalbed Methane Extraction Using the Self-Oscillating Water Jet Slotting Method

摘要

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.