Research on the prediction of liquid CO₂ phase transition cracking radius in coal seams

Predicting cracking radius is a prerequisite for determining the holes spacing of gas extraction technology by liquid CO₂ phase transition cracking and permeability improvement, which directly affects the gas extraction effect. Most existing prediction methods are based on single factor analysis. In order to grasp the influence of multiple factors on the radius of liquid CO₂ phase transition cracking and effectively predict the spacing between holes, ANSYS/LS-DYNA numerical simulation software is used to carry out the research on predicting the radius of coal seam liquid CO₂ phase transition cracking combing with orthogonal experiments. The numerical simulation results indicate that the order of factors affecting the radius of liquid CO₂ phase transition cracking is ground stress>gas pressure>coal solidity coefficient. The cracking radius decreases with the increase of stress, and increases with the increase of gas pressure and coal solidity coefficient with a linear relationship. A multiple regression analysis is conducted on the numerical simulation results. A prediction model for the radius of liquid CO₂ phase transition cracking is established based on three different coupling conditions of ground stress, gas pressure, and coal solidity coefficient. Industrial experiments are conducted on the coal mine site. Extraction boreholes are set up based on the predicted model calculation results. The pressure index method is used to test and analyze the gas extraction effect. The results show the following points. The gas pressure in the observation holes on both sides of the liquid CO₂ phase transition cracking hole shows a decreasing trend with time. The farther away from the cracking hole in the initial stage of extraction, the greater the gas pressure. It is consistent with theoretical analysis and numerical simulation results. The effective cracking range of liquid CO₂ phase transition is basically consistent with the predicted results. The gas volume fraction in the observation hole is 73.4% higher than that in the natural extraction hole, and the gas extraction efficiency is significantly improved.