Abstract:

The pore structure of low-permeability sandstone is complex. The three-dimensional (3D) quantitative characterization of micro-nano scale fluid flow characteristics and the analysis of occurrence mechanisms are of great significance for fine reservoir description and enhanced oil recovery. Two water-wet sandstone samples with similar micro-pore structure characteristics from the upper Es₄ Member in G Oilfield were selected, and X-ray CT coreflooding experiments under relatively low and relatively high flooding rates were performed. The distribution of oil, water and particle phases in 3D pore space at different water flooding stages was obtained by image processing technology. The occurrence states and changes of oil phases in 3D pore space and individual pores during the water flooding process were discussed. In addition, combined with the finite volume method, the fluid flow characteristics under the control of multiple factors (e.g. microscopic pore structure heterogeneity, displacement mode and waterflooding 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.

Key words: low-permeability sandstones')">

low-permeability sandstones, pore structure, oil-water two-phase displacement, finite volume method, fluid flow characteristics, recovery rate, oil reservoir