Hydraulic cutting cooperative pressure relief and permeability enhancement technology in low permeability outburst coal seam

Abstract

The effect of a single hydraulic cutting is not very ideal for thick coal seam with low permeability or coal seam with gangue to increase the permeability and release the pressure. In order to solve the above problem, taking the 21212 working face of Dongpang Coal Mine of Jizhong Energy Co. as an example, this paper proposes hydraulic cutting cooperative pressure relief and penetration enhancement technology in the upper and lower coal seam. According to the specific structure of coal seam, the hydraulic cutting construction is carried out once in the upper and once in the lower coal seam. The fractures generated by the 2 slots in the upper and lower coal seam continue to crack, expand and extend, resulting in the full development of coal fissures between the hydraulic cutting holes, forming a three-dimensional fissure network connected with each other. The network generates cooperative pressure relief and penetration enhancement effects, and expands the effective extraction radius. In order to arrange the test hole spacing of hydraulic cutting more reasonably, FLAC 3D software is used to establish a physical model of hydraulic cutting cooperative pressure relief. The hydraulic cutting water pressure is 30 MPa. One coal output is 4.5 m3 (cutting width is 0.3 m, seam depth is 1.57 m, double cutting in the upper and lower coal seams). Another coal output is 6 m3 (cutting width is 0.3 m, seam depth is 1.78 m, double cutting in the upper and lower coal seams). The theoretical effective extraction radii of these two types of coal rock are 4.5 m and 4.8 m respectively. In order to determine the effect of hydraulic cutting, the extraction effect of test hole S1 with coal output of 4.5 m3 and test hole S2 with coal output of 6 m3 and non-cutting extraction hole. The results show that the average volume fraction of gas extraction in the non-cutting extraction hole is 22.38% and the average pure volume of gas extraction is 0.206 m3/min. The average volume fraction of gas extraction in the cutting extraction test hole S1 is 75.73% and the average pure volume of gas extraction is 0.382 m3/min. The average volume fraction of gas extraction in the cutting extraction test hole S2 is 86.91%, and the average pure volume of gas extraction is 0.454 9 m3/min. Compared with the hole without cutting, after adopting hydraulic cutting to increase permeability, the permeability of coal seam is greatly improved. The gas extraction volume fraction is increased by about 4 times, the gas extraction volume is increased by about 2 times, and the gas extraction effect is good. According to the actual measured effective extraction radius of gas pressure drop method, the relationship between the effective extraction radius and extraction time of the coal output of 4.5 m3 and 6 m3 hydraulic cutting can be obtained. When the extraction time is 30 d, 60 d, 120 d and 180 d, the effective extraction radii of the 4.5 m3 hydraulic cutting process are 4.9 m, 5.5 m, 6.1 m and 6.5 m. And the effective extraction radii of the 6 m3 hydraulic cutting are 5.1 m, 5.6 m, 6.3 m and 6.7 m. After comprehensive consideration, the construction parameters of hydraulic cutting technology suitable for Dongpang Coal Mine are finally obtained. The coal output is 4.5 m3, the extraction time is 60 days, the effective extraction radius is 5.5 m, and the hole spacing is 7.7 m.

Keywords

• low permeability outburst coal seam; gas extraction; hydraulic cutting; pressure relief and penetration enhancement; effective extraction radius