Analysis of the Characteristics of Fracturing Fluid Displacement of Oil in Tight Sandstone Reservoirs and Its Influencing Factors

Hydraulic fracturing is an important method to enhance the tight sandstone reservoir oil recovery, and the fracturing fluid features oil displacement when entering the reservoir, and the oil displacement characteristics varies greatly in different types of tight reservoirs. In this study, the nuclear magnetic resonance (NMR) and microscopic visualization of fracturing fluid oil displacement measurements were launched on the Chang 8 Member of Yanchang Formation in Ordos Basin, the residual oil distribution characteristics and oil utilization degree of different types of reservoirs from microscopic scale were discussed, and the relationship between reservoir microstructure characteristics and oil mobilization characteristics is also analyzed. The results show that the reservoir could be divided into Type I and Type II, and the pore size of oil utilized in Type I reservoir is mainly pores with small pore apertures (<10 ms), forming a uniform replacement mainly due to the fracture, and the residual oil is mainly in the form of continuous piece. The displacement of oil in Type II reservoir mainly occurrences in the pores with larger pore apertures (>10 ms), forming a fracture-centered network replacement, and the residual oil is mainly in the form of clusters, patches and corners. The residual oil in both Type I and Type II reservoirs is mainly retained in small pore space. The factors influencing the effect of replacement oil utilization mainly include reservoir physical properties, pore structure and clay mineral content. The reservoir permeability is the main macroscopic control factor, pore structure is the main microscopic control factor, while clay mineral content leads to the change of pore structure.