Crude oil mobilization enhanced by nano-emulsion in the full fracturing process for tight oil reservoirs

The Chang 8 tight sandstone reservoir in the Ordos Basin is primarily developed using a nano-variable-viscosity fracturing fluid. However, the contribution of the fracturing fluid to crude oil mobilization and the mechanisms of oil–water interaction during the three stages of “fracturing–soaking–flowback” remains unclear. A full-process simulation device for “fracturing–soaking–flowback” is constructed based on nuclear magnetic resonance technology. Two types of crude oil mobilization fracturing fluids, nano-amphiphile and nano-emulsion, are compared and studied. Quantitative analysis of the distribution and changes of crude oil in core pores at different stages leads to the development of a new technology for calculating oil–water relative permeability. Results indicate that: (1) Rapid wettability modification by nano-emulsions enhances sweep efficiency, imbibition displacement, and flowback efficiency, increasing the total oil recovery factor to 87.71%. (2) A particle size of approximately 10 nm facilitates crude oil mobilization in nanopores. (3) Relative permeability movement, driven by rock wettability modification, is a key mechanism for enhanced oil recovery with residual oil saturation reduced to about 7% during the flowback stage. (4) A decrease in the degree of emulsification between the fracturing fluid and crude oil may also significantly improve crude oil mobilization. This study provides a scientific foundation for optimizing fracturing development and oil mobilization processes in tight oil reservoirs.