Coal Geology & Exploration


Due to low permeability, fast gas-rate decline, uneven fracturing, easy fracture closure, it is difficult to perform advance gas extraction in broken-soft coal seams. Aimed at these issues, this study has been proposed a new idea to enhance the coalbed permeability and eventually the gas extraction, which is based on roof-supporting, long-reach directional drilling, and hydraulic fracturing. Corresponding equipment including fracturing pump unit, directional sandblasting perforation assembly, sand control packer and tools have been developed. Specifically, the pumping unit is capable of a pumping rate up to 90 m3/h, a pumping pressure up to 70 MPa, and a proppant concentration up to 20%, and a proppant particle size up to 1 mm; The directional sandblasting perforation assembly is driven by hydraulic pressure, with a maximum rotation angle of 180°; And the sand control packer can uphold up to 70 MPa with a maximum expansion factor of 2. In addition, we applied such technologies as long-reach directional drilling, continuous sandblasting perforation, and multistage hydraulic fracturing to a field pilot test in Xinjing coal mine, Yangquan, Shanxi Province. A total of 80 directional sandblasting shots at a pressure of 22.6-28.6 MPa, and 16 fracturing stages were carried out in two boreholes at an average depth of 609 m. And 19.8 t silica sand (a concentration of 2-3 vol.%), 2808.5 m3 fracturing fluid (153.8-235.1 m3 for a single fracturing stage), and 36.47 t walnut shell (a concentration of 2.02%-2.56%) were injected. The tests achieved a fracture half-length of 20-38 m in estimation, and average gas rates of 1025 and 2811 m3/d in 100 days after the treatment. Besides, it is found that the fracturing equipment has a large sand injection capacity, high pumping rate, and continuous operation. The coal seam permeability was significantly enhanced after fracturing, which led to great improvement in gas concentration and purity. This research provide new ideas for permeability enhancement in broken-soft coal seams and serves as a good reference for advance treatment in similar mining areas in China.


staged sand fracturing, roof directional long drilling, directional sand blasting perforation device, hydraulic sand fracturing pump set, directional sand blasting perforation




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