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Abstract: The empirical rule show that the relative permeability ratio curve is characterized by a straight line in the middle, and bending at both ends in most rocks. However, the microscopic mechanism of relative permeability ratio varies with saturation is not clear at present. Based on the two phase flow law and flow state of capillary, the microscopic percolation mechanism of different flow patterns is studied. Then, the microscopic mechanism model of the relationship between the relative permeability ratio and the fluid saturation is established, and the micro mechanism of the relative permeability ratio with the fluid saturation is explained in theory. Research shows that the microscopic mechanism model is consistent with the rule of empirical rule in porous media; Droplet flow and annular flow cause the curve to be upwarped or downwarped; The curve of the theoretical model of the slug flow is basically consistent with the fitting curve of the empirical formula (1); Slug flow is the fundamental reason for the emergence of straight lines in curve, and is also the theoretical basis of empirical formula (1).
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