Numerical assessment of the pressure relief effect of the hydraulic punching cavitation technique in a soft coal seam

With the increasing demand for geo-energy, the safe and efficient exploitation of deep coal resources has become increasingly important. However, the high geo-stress in the deep ground makes coal soft, which is characterised by poor permeability, high gas content, high pressure and high risk of outburst. It is difficult to drain gas from soft coal seams using traditional gas drainage methods, and thus bringing barriers to the efficient coal exploitation. Drawing lessons from successful experiences of the cave completion technology, widely used in surface well, a new permeability-enhanced method for soft coal seams, i.e. hydraulic punching cavitation, is proposed to increase the exposed area of coal bulk, release the geo-stress, and improve the coal permeability by creating lessons a large-volume cavity in the target coal seam. However, the improper layout parameters of hydraulic punching cavitation may lead to stress concentration and drainage-blind spots, increasing the gas outburst risk. For a better design of the layout parameters, numerical simulation, theoretical analysis and laboratory experiment were used to investigate the cavitation completion pressure relief mechanism and the stress path surrounding the cavity, based on the distribution characteristics of the plastic zone and geo-stress surrounding the cavitation chamber. The results show that the maximum radius of the plastic failure zone caused by the 100-mm-diameter cavity was 250 mm and the radius of the plastic zone increased to 3.25 m after punching cavitation, which is 13 times the traditional drainage radius. Besides, the hydraulic punching cavitation attains a more extensive plastic zone range and a more notable pressure relief effect than those attained by perforation cavitation. In the process of hydraulic punching cavitation, the radial stress in the coal around the cavitation chamber suddenly decreases, and the axial stress increases, inducing a crushing effect on the coal, which extends along both sides. The results can benifit to design the layout parameters of hydraulic punching cavitation for soft coal seams with low permeability and high outburst risk.

Automated summary

With the increasing demand for geo-energy, the safe and efficient exploitation of deep coal resources has become increasingly important. A new permeability-enhanced method called hydraulic punching cavitation is proposed to increase the exposed area of coal bulk, release the geo-stress, and improve the coal permeability. The study shows that hydraulic punching cavitation achieves a more extensive plastic zone range and a more notable pressure relief effect than perforation cavitation.