Experimental and Numerical Simulation of Water-Flooding Features and Remaining Oil Distribution in Water-Wet Sandstones

In this paper, two water-wet sandstone samples with similar microscopic pore structures were selected, and X-ray CT core flooding experiment under relatively low and relatively high flooding rates were performed. The distribution of oil, water, and particle phases in three-dimensional (3D) pore space at different water flooding stages was obtained by image processing technology, so as to explore the occurrence states and changes of oil phases in 3D pore space and individual pores during the water flooding process. In addition, combined with the VOF model of finite volume method, the fluid flow characteristics under the control of multiple factors (e.g., microscopic pore structure heterogeneity, displacement mode and injection rate) were also determined. The results indicate that the large and continuous oil drops were broken up and gradually separated into small oil droplets during the water flooding process, and the small oil droplets distributed in a discrete state in the 3D pore space. After water flooding, the connectivity of oil droplets becomes poorer and the geometry becomes smoother and more regular. The dominant fluid flow channels are generally well developed in the sandstones with strong microscopic heterogeneity and good pore connectivity, resulting in the development of the flow around and crossflow behaviors. Therefore, the water sweep efficiency in the sandstones with strong microscopic heterogeneity is low. The water flooding rate is also an important factor affecting the oil displacement efficiency and oil/water migration path. Increasing water displacement rate can significantly increase the number of water injection capillaries, thus enhancing oil recovery rate. This study also indicates that the oil displacement efficiency of low-permeability sandstones can be effectively improved by increasing the oil-water viscosity ratio and the injected capillary number under an appropriate interfacial tension. The research results provide an important theoretical basis for enhancing oil recovery (EOR) of low-permeability and water-wet sandstone reservoirs.