Variation Pattern of Fluid Phase Tortuosity and Its Impact on Morphology of Oil/Water Relative Permeability Curves

Summary

Oil/water relative permeability is a key parameter characterizing oil/water two-phase flow behavior in porous media, and clarifying its controlling factors is essential for deepening the understanding of oil/water two-phase flow characteristics. Based on the fractal theory of porous media, we establish a generalized oil/water relative permeability model applicable to water-wet sandstone reservoirs. This model elucidates the governing mechanism by which the phase tortuosity index regulates the morphology of oil/water relative permeability curves. Then, we validate the proposed model using oil/water relative permeability experimental data from 139 core samples and high-pressure mercury intrusion (HPMI) data from 51 core samples, and further benchmark it against conventional models, confirming its correctness and applicability. The study indicates that the phase tortuosity index is closely associated with core petrophysical properties, and its variation significantly affects the morphology of relative permeability curves. At a given water saturation, oil/water relative permeability decreases with an increasing phase tortuosity index. In low-porosity and low-permeability cores, the water-phase tortuosity index is smaller than that of the oil phase, with a pronounced disparity between the two. For high-porosity and high-permeability cores (porosity > 0.25, permeability > 50 md), the phase tortuosity indices are mainly distributed between 1 and 3, with average values of 1.92 (water phase) and 2.01 (oil phase), respectively. The proposed generalized oil/water relative permeability model provides a unified description of oil/water flow across porous media with different permeabilities, offering a theoretical basis for improving the prediction accuracy of oil/water relative permeability in complex reservoirs.