Heat-mass transfer coupling effects in water-ice phase transformation of water-bearing coal frozen with liquid nitrogen

Liquid nitrogen (LN2) fracturing, as a waterless and environmentally friendly fracturing technology, has a broad application prospect in coalbed methane (CBM) exploitation. Currently, there is a lack of sufficient understanding of the heat mass transfer mechanism of LN2 fractured coal mass. In this paper, the heat-flow coupling problem of LN2 frozen coal mass was systematically studied by means of numerical simulation. A fluid-solid coupled multiphase module was mathematically modeled on LN2 flow, coupling between LN2 and coal mass temperature, and the variation law of coal temperature and water-ice phase with time and geometric position during LN2 flow was obtained under the coal seam freeze-thaw simulation analysis program by FORTRAN language. The results showed that the most obvious heat mass transfer occurred in the coal mass area close to the LN2 inlet and the fluid-solid interface. Along the direction of coal burial depth, the changes of temperature and water-ice phase are broadly divided into rapid change zone, slow change zone and stable zone, and along the horizontal direction of LN2 flow, the temperature and unfrozen water volume first peaked, then decreased and gradually stabilized, while the ice content was diametrically opposed, which indicating that the change of temperature and water-ice phase have significant anisotropic characteristics, and the hydrothermal coupling effect out to be one of the most important factors affecting the effect of LN2 fracturing coal. The main findings of this study are the keys to the research of LN2 fracturing physical mechanisms in CBM reservoirs.