Abstract
Coal seam permeability is one of the key factors influencing the gas extraction efficiency, which is of great significance to reduce coal and gas dynamic disasters in gassy coal mines. Hydraulic slotting technique is an effective method to stimulate the coal reservoir, but the selection of slotting key parameters has great impact on gas extraction efficiency. For this reason, the hydraulic slotting model was established by using FLAC3D software to analyze the stress distribution before and after slotting. Then, the influence of borehole diameter, slotting width, and slotting length on coal seam stress relief is also discussed. The results show that the slotting width has a great influence on the stress relief of the coal seam, while the borehole diameter and slotting length have no obvious influence on that. Based on the results of numerical simulation, field tests were carried out in Sangshuping NO.2 coal mine. The results show that the coal seam stress can be fully released, resulting in the improvement of coal seam permeability. The gas extraction efficiency can be highly enhanced by hydraulic slotting. This research achievement provides the guidance basis for high-stress water jet slotting technology with adaptive selection of slotting parameters in different geological conditions.
Highlights
As the most important energy in China, coal plays an important role in promoting China’s economic development [1]
China’s coal reservoir has common characteristics such as high gas content, high gas stress, and low permeability, which results in low production of coalbed methane and difficulties in large-scale development
To improve the efficiency of gas extraction, experts and scholars at home and abroad have been putting lots of effort in permeability improvement in the coal reservoir. e widely applied techniques to improve the permeability of the coal seam include deep-hole split blasting [6,7,8], hydraulic slotting [9,10,11,12], hydraulic fracturing [13, 14], hydraulic flushing [15,16,17], CO2 fracturing [18,19,20], and gas injection [21, 22]
Summary
As the most important energy in China, coal plays an important role in promoting China’s economic development [1]. The selection of slotting parameters has a significant effect on the stress relief effect of the coal seam. Wei [31] simulated the formation of the stress relief area by hydraulic slotting utilizing PFC2D software and concluded that the effective slotting radius of the coal seam was 0.5 m and optimal hole spacing was 5 m. Ge et al [39] used COMSOL software to study the drainage influence radius of the slotted borehole in different coal seam conditions, and field tests were carried out. How to preserve the high permeability of the coal seam and how to extend the period of high gas extraction efficiency are the core issues of the development of hydraulic slotting technology. Based on the guidance of numerical simulation results, the hydraulic slotting field test is carried out to verify the improvement of gas extraction by hydraulic slotting. is study provides the theoretical support for the optimization of slotting process and high-efficiency gas extraction
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