Study on the Influence of Different Factors on Spontaneous Oil Recovery of Nanosurfactants in a Tight Reservoir

[Image: see text] The surface of a tight reservoir appears to be oil-wet or mixed-type wet upon soaking in crude oil for a long time, and the yield decreases rapidly after fracturing under the influence of capillary force. The oil sweep efficiency affected by many factors such as formation water dilution, salinity, crude oil type, temperature, and pressure can be enhanced by adding nanosurfactants into the fracturing fluid, so it is necessary to study the influence of different factors on the spontaneous imbibition replacement efficiency of nanosurfactants. In this study, the basic properties of nanosurfactants such as particle size, oil–water interfacial tension (IFT), and the wetting modification effect were tested, and the influence of surfactant type, concentration, temperature, and pressure on imbibition replacement efficiency was studied. The main conclusions are as follows: (1) The particle size of the nanosurfactant that was synthesized by a microemulsion method is 12–21 nm, which indicated good injectability in tight cores. Moreover, the IFT values between the crude oil and five kinds of 0.30 wt % nanosurfactants were all lower than 0.15 mN/m, and nanosurfactant C had the best wetting modification effect with increasing the contact angle by 100.30°. (2) The type and concentration of surfactant have a certain influence on imbibition replacement efficiency, and appropriate concentration of anionic nanosurfactant is beneficial to enhancing the imbibition replacement efficiency. The imbibition replacement efficiency of 0.30 wt % anionic surfactant C solution is higher than that of nonionic and cationic surfactant solutions, and the imbibition replacement efficiency is as high as 33.386% under NTP. (3) The nanosurfactant in brine is prone to forming fine emulsified oil droplets with crude oil and activates the oil droplets in the small pores to enhance the imbibition replacement efficiency. The crude oil type, temperature, and pressure can influence imbibition replacement efficiency, and the influence of crude oil type and temperature is greater than that of pressure. This work further studies the influencing factors of imbibition replacement efficiency.