Impact of Secondary and Tertiary Floods on Microscopic Residual Oil Distribution in Medium-to-High Permeability Cores with NMR Technique

In order to explore the impact of various flood schemes and pore throat heterogeneity on oil recovery efficiency in porous media, core-flood experiments and nuclear magnetic resonance (NMR) tests are conducted to quantitatively determine the initial oil distribution and the residual oil distribution in medium-to-high-permeability cores subjected to these various flood schemes. Multiple experimental runs are conducted with four field core samples to cover the various flood schemes: the secondary water flood, CO2-foam flood, and water-alternating-CO2 flood (WAG). Experimental results show that, relatively speaking, at the initial oil saturation condition, the moderate pore throats contain the highest amount of oil. The water flood recovery degree is higher from larger pore throats (average recovery degree of 98.57%) than that from moderate pore throats (average recovery degree of 78.29%). The water flood efficiency in different cores is found to be dependent on the degree of heterogeneity in pore throat distribution. After water flood, the residual oil is mainly located in smaller pore throats. CO2-foam flood shows good performance in tapping the residual oil contained in smaller pore throats, while the WAG can recover more oil from larger pore throats. Furthermore, it is found that the combination of CO2-foam flood and WAG provides the highest recovery efficiency since it is effective in reducing the oil saturation in pore throats with varied sizes. Based on this investigation on the residual oil saturation in pore throats subjected to secondary and tertiary floods, it is possible to design an optimum flood scheme which suits the microscope pore throat characteristics for a given reservoir.