Enhancing coalbed methane recovery using liquid nitrogen as a fracturing fluid: A coupled thermal-hydro-mechanical modeling and evaluation in water-bearing coal seam

In this study, a fully coupled thermal-hydro-mechanical (THM) model was developed considering the latent heat of phase transition and expansion strain, to investigate the water-ice phase transition and heat and mass transfer laws. The response surface methodology was adopted to examine the sensitivity of input parameters on the LN2 effective freezing radius response. The results showed that the temperature of the coal surrounding the borehole dropped rapidly and generated a low-temperature region and frozen surface during the initial stage of LN2 injection (1 h). From the water-ice interface, reservoir’s frost heaving strain and permeability can be divided into three zones: the frozen zone, the transition zone, and the unfrozen zone. As the LN2 freezing time increased, the extent of the frozen zone increases, and the corresponding low permeability range of the reservoir also gradually expands. The LN2 effective freezing radius increases exponentially, and the effective solidification radius is 1.45 m after 1000 h. Sensitivity analysis demonstrates that the significant variables affecting the LN2 effective freezing radius are the water content (0.547) and initial permeability (0.268) from the main effects.