Influence of Multi-field Coupling Effect on Coal Porosity in the Process of Liquid Nitrogen Fracturing in Watery Coal

The aim was to further investigate the influence of heat transfer in phase transition on gas-liquid-solid multiphase and multiphysics fields during liquid nitrogen (LN₂) fracturing watery coal, and to explore the role of the coupling effect of multiple factors (initial moisture content, initial temperature, LN₂ injection pressure, and triaxial stress) on the coal porosity. A multiphase multi-field coupling model of LN₂ fracturing watery coal was derived, and the multiphase energy-mass transport law and porosity change characteristics during the LN₂ fracturing watery coal process were explored by finite element analysis. The results indicate that the initial moisture content of the coal is positively correlated with the maximum increase in coal porosity, while the initial temperature of the coal has a smaller correlation with it. The decrease rate of coal’s porosity in the radial direction of the LN₂ injection hole becomes higher with an increase in the coal’s initial moisture content and initial temperature. The greater the LN₂ injection pressure, the larger the overall porosity of the coal around the hole. The more the coal is subjected to an increase in the horizontal stress difference, the more the coal’s pore dilatancy tends to move in the direction of greater horizontal stresses. The greater the total coal stress, the smaller the overall porosity of the coal around the hole.