Series of numerical simulations were done to quantify the effects of pore structure approximations on micro-scale displacement mechanisms. Coupled Cahn–Hilliard phase field and Navier–Stokes equations were solved using finite element method to simulate two-phase flow in a heterogeneous model -having a real pore network- as the base case, compared to two other approximated models; in one of those, the complex grain geometries were simplified to circular shapes; while in the other one, only the sizes of the real-shaped grains were gently (less than ±10%) modified. In both approximated models, the medium porosity and absolute permeability were kept unchanged, compared to the base case. The simplification of the grain shapes changed the morphologies of the formed fingers and the trapped oil volumes. The general trends of the oil recovery factor and macro-scale capillary pressure variations were similar during displacement at different medium wettabilities for the base and the simplified models. However, the simplified model showed higher displacement efficiency at water-wet conditions and lower capillary pressures at oil-wet conditions; due to less complexity of its pore network geometry. Different micro-scale events were captured similarly in both models, including reverse displacement, interface coalescence, water bursting and stick-slip motion. But the oil trapping mechanisms were totally different. Slight modification of the grain sizes resulted in different displacement profiles, especially at neutral and oil-wetting conditions and low capillary numbers (log Ca < -3.69), due to modification of capillarity in the medium, hence variation of the preferred paths of the displacing phase finger(s).

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