Study on Distribution Characteristics of Plastic Zone and Stress Field of Coal with Different Ranks by Liquid Nitrogen Phase Change Expansion Fracturing

Based on the low-temperature impact effect of liquid nitrogen, liquid nitrogen phase change expansion fracturing (LN-PCEF) leverages the high phase change expansion rate of liquid nitrogen to generate a pressure of 43.3 MPa in sealed boreholes, extending primary fractures and demonstrating significant potential for enhancing coal seam permeability. Due to the immaturity of this technology and limited field application experience, this study employs FLAC3D numerical simulations to investigate LN-PCEF effects on coals with varying ranks, aiming to provide theoretical guidance for future engineering practices. A permeability enhancement method combining liquid nitrogen jet slotting and phase change expansion fracturing is proposed. The plastic zone and stress field distribution characteristics under three configurationssingle borehole, single-slot borehole, and double-slot boreholeare systematically analyzed. Results indicate that the range of the plastic zone of fractured coal increases with decreasing coal strength and grows as the number of horizontal slots increases, with shear failure being the predominant failure mode. Higher coal strength corresponds to elevated maximum stress values during fracturing, while the critical stress threshold for plastic zone formation in such coal also increases. Stress concentration zones are observed in the stress field, whose regional area expands with the number of horizontal slots. The stress superposition area is found between the horizontal slots, and the area enlarges as coal strength diminishes.