Chemical flooding, as one of the most promising EOR techniques in both laboratory research and field practice has been widely applied to enhance residual oil displacement in conglomerate and sandstone reservoirs. To fully understand the displacement mechanism of chemical flooding in reservoirs with different lithologies, it is critical to understand the way residual oil is displaced from the pore-scale perspective.

In this study, three cores with similar permeability but different lithologies (sandstone, sandyconglomerate and poorly-sorted conglomerate) were selected for unclear magnetic resonance (NMR) experiments. Deuteroxide was used to replace water as aqueous phase to distinguish signals of water phase and oil phase. The lower size limit of workable pores was determined based on the relationship between mercury injection curves and NMR T₂ spectrums. The distribution and migration patterns of residual oil in different flooding processes were evaluated by quantitatively analyzing the change of the relaxation time. The amounts of oil displaced from pores of different sizes after water flooding, polymer flooding, and surfactant/polymer (SP) flooding were calculated, respectively.

The ultimate oil recovery by chemical flooding in cores with different lithologies decreased in the order of sandstone, sandy conglomerate, and poorly-sorted conglomerate cores. Specifically, SP flooding achieved much higher oil recovery than polymer flooding, especially in the conglomerate core. It has been demonstrated that polymer solution could effectively mobilize residual oil in medium pores (1-10μm) in the sandstone core. Meanwhile, the incremental oil recovery during SP flooding was mostly attributed to the small and medium pores (0.25-1.6μm). But for the conglomerate core, polymer flooding was only to some extent effective in the pores with diameters ranging from 4.3μm to 25μm. Moreover, it was found that the lower size limit of workable pores by SP flooding decreased with the increase of heterogeneity. Generally, oil trapped in pores smaller than 4μm was hard to be mobilized.