Asphaltene precipitation and reservoir damage characteristics of CO2 flooding in different microscopic structure types in tight light oil reservoirs

• Asphaltene precipitation and reservoir damage during CO 2 flooding are studied by NMR. • The amount of asphaltene precipitation increases with increasing injection pressure. • The permeability damage is significant when the MMP is reached. Asphaltene precipitation is often accompanied by CO 2 injection in tight light oil reservoirs. In tight oil reservoirs, the traditional studies of reservoir damage by asphaltene precipitation after CO 2 flooding have limitations that the influence of reservoir microscopic pore structure characteristic parameters is not considered. In order to accurately evaluate the asphaltene precipitation characteristics and reservoir damage degree of different microscopic structure types, SEM, high pressure mercury injection and NMR have been used. In this study, the asphaltene precipitation characteristics under different CO 2 flooding conditions have been clarified. The damage degree of asphaltene precipitation on the pore throat blockage characteristics, porosity, permeability and wettability of different types of reservoirs and the effect on oil displacement efficiency have been evaluated. The results show that the Chang 8 reservoir can be divided into type I, type II, and type III. The percolation capacity and storage space of the three pore structure reservoirs decrease successively. The asphaltene precipitation of three types of reservoirs increases with the increase of threshold pressure and injection volume. The blockage rate of large and small pores is the highest in type III reservoirs, followed by type II reservoir, and type I reservoir is the lowest. The oil displacement efficiency is the highest for type II reservoir and the lowest for type I reservoir. Asphaltene precipitation has little effect on porosity of three types of reservoirs, but has high damage on permeability. Asphaltene precipitation easily causes wettability reversal to lipophilicity and the reversal index increases with the injection pressure. The wettability reversal index of type I reservoir is the highest and that of type III reservoir is the lowest. The surfactant can be injected in reservoir to reduce the wettability reversal index caused by asphaltene precipitation. This paper has evaluated the asphaltene precipitation characteristics of CO 2 flooding in tight light oil reservoirs, which further supplements the mechanism of CO 2 flooding and provides reference and guidance for the optimization of field parameters.