Study on resistivity experiments and conductive mechanism in low-permeability reservoirs with complex wettability

As clay membrane developed in low-permeability lithologic reservoirs absorbs oil, reservoirs become oil-wet, which results in abnormally high resistivity oil-water layers and water layers. Such layers bring great challenges to identification of oil and water layers. In order to make clear of the resistivity response characteristics and conductive mechanism in reservoirs with different wettability, cores were selected in Chang 8 formation, Yanchang group, Upper Triassic, the western Ordos Basin of China. Experiments were tested on these samples to simulate the process of oil displacing water, aging and water displacing oil. What's more, contact angles of thin sections after washing oil were also tested. The experimental results show that abnormally high resistivity cores are not completely water-wet after washing oil. However, the formation factors of abnormally high resistivity cores and normal resistivity cores have little difference. In the process of oil displacing water, the relationship between normal resistivity core resistivity index and water saturation is linear in log-log plot, while that of abnormally high resistivity cores is convex. The resistivity of abnormally high resistivity cores remains unchanged during aging process. Combining the analysis of Nuclear Magnetic Resonance (NMR) T-2 spectra under different conditions, it can be inferred that wettability of abnormally high resistivity cores has become less water-wet after oil displacing water. What's more, the aging process has little effect on the wettability of abnormally high resistivity cores. In the process of water displacing oil, the relationship between abnormally high resistivity core resistivity index and water saturation is almost linear, which shows that conductive structure of rock is not changed. Based on the analysis and discussion of these experimental results, an accumulation model and the corresponding conductive mechanism are proposed for low-permeability reservoirs with complex wettability. It shows that abnormally high resistivity water layers are caused by the damage of continuous conductive path under oil-wet condition. The researches and experiments are of great importance for understanding the conductive mechanism, oil layers identification and the evaluation of water flooded formation in low-permeability reservoirs with complex wettability.