Effects of multiple hydraulic punching for ultra-deep buried high ground stress powdered coal seam with low permeability

Abstract

The high ground stress and poor gas permeability of the ultra-deep buried highland stress coal seam make it difficult for conventional unloading and permeability-increasing techniques, such as fracturing or cutting, to achieve the goal of coal seam pressure relief. This article proposes a multi-stage hydraulic punching permeability increasing scheme for the M1-7∼M1-9 gas drainage project of the powdered coal seam with highland stress (44.42 MPa) in tunnel No. 2 of Zhaotong, and studies the effect of cavity formation and gas changes caused by multiple hydraulic punching through theoretical analysis and numerical simulation methods. The gas drainage effect after multiple hydraulic punching was also monitored on-site and systematically compared with natural gas drainage and single hydraulic punching permeability-increasing techniques to determine the optimal solution for multi-stage hydraulic punching. Using the COMSOL numerical simulation software, a coal body model was established based on theoretical analysis. Boundary pressures, ideal gas pressures, and high-pressure water flow pressures were set as boundary conditions to simulate the process of hydraulic punching. The theoretical analysis and numerical simulation were then used to guide on-site experiments. The research results shows that for ultra-deep buried highland stress powdered coal seams, natural gas drainage efficiency is extremely low, and the initial effect of single hydraulic punching permeability increasing is significant, but the fast decay rate of short gas is still due to the time that the cavity formed by hydraulic punching exists. Multiple hydraulic punching effectively alleviates the gas decay rate, and the gas drainage efficiency can reach three times and 22 times that of single hydraulic punching and natural drainage techniques, respectively. When the hydraulic punching interval period is five days and the hydraulic punching frequency is three times, the gas drainage effect is optimal. Multiple hydraulic punching permeability-increasing technologies can provide reference for gas extraction in similar low permeability coal seams with highland stress. The research holds great significance in promoting efficient gas extraction and ensuring the safety construction of gas tunnels.