Mechanism Research On Increase Coal Gas Permeability By Liquid CO₂ Phase Transition Fracturing Technique

Liquid CO2 phase transition fracturing technique to increase permeability is a newstyle strengthening gas drainage technique which needs further research in terms of mechanism and reasonable drilling parameters. The paper by using theory analyses and numerical simulation and field test analyses the antireflection effect of liquid CO2 phase transition fracturing on the basic of field test of liquid CO2 phase transition fracturing. The paper also obtains the equivalent of liquid CO2 phase transition fracturing by selecting appropriate calculation method and proves the consistency between theoretical calculation and numerical simulation by fracturing test. This paper investigates the influence of coal seam gas pressure on fracturing effect using numerical simulation, and analyses the crack propagation and failure criterion by high-energy CO2 and gas from fracturing, as well as summarizes the mechanism of liquid CO2 phase transition fracturing technique.Firstly, the paper introduces the new technology of liquid CO2 phase transition fracturing on the basic of summarizing the domestic and foreign research results. By the field test in Jiulishan mine, the paper analyses the fracturing and antireflection effect and the applied range in mine about the technique, and coMPare liquid CO2 phase transition fracturing technique with high-pressure water jet reaming technique. The gas drainage quantity of former is 1.44~5.2 times than latter, which proves the superiority of liquid CO2 phase transition fracturing technique and has high economic value.Secondly, the fracturing equivalent and the stress field of fracturing coal body are numerically simulated by using ANSYS_LS-DYNA3 D. Based on the fracture mechanics theory, the paper establishes the fracturing computational model of coal body under gas pressure condition, and analyses the distribution regularity of stress field of the fracturing coal body. By analyzing the stress field of coal body and the crack propagation by high-energy CO2 and gas from fracturing, the paper proposes the mechanism of liquid CO2 phase transition fracturing.Finally, based on the rock blasting theory, the paper analyses the action of high-energy CO2 by fracturing and gas from the blasting damage and fracture process, as well as the influence of gas on main physical and mechanical properties of coal. According to the damage fracture mechanics theory, the paper calculates the effective stress intensity factor of the initial macro crack tip by fracturing stress wave, and analyses the crack expansion process by fracturing high-energy CO2 and gas, then determines the crack expansion criterion. Combined with the previous experimental results, the validity and rationality of the theoretical analysis results obtains further validation.