Tight Sandstone Reservoir Wettability Alteration and Enhanced Fracturing Fluid Flowback by a Gel-like Hydrophobic Nano-Emulsion

In unconventional reservoirs, the flowback efficiency of fracturing fluid is relatively low, resulting in the lower recovery rate of hydrocarbons. This work proposes a new method to improve the flowback of fracturing fluid by using cross-linked polymer nano-emulsion. Using the semi-continuous emulsion polymerization process, a core–shell polymer nano-emulsion was prepared with cross-linked core layer, namely FRACFLOW-WA. Through dynamic light scattering and transmission electron microscopy analysis, it was found that the average particle size of the nano-emulsion was 91.7 nanometers, with a narrow particle size distribution and excellent dispersion. The product can quickly adsorb on the pore surface and spontaneously form a uniform and stable hydrophobic film. The results showed that the contact angle of the core sample treated by the nano-emulsion at 1% concentration can increase to 108°. The hydrophobic film structure was confirmed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). The core gas flooding experiment showed that introducing 1% FRACFLOW-WA into the fracturing fluid increases the flowback efficiency to 72.3%, and the permeability recovery rate reaches 70.2%, which is better than the commonly used commercial surfactants. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analysis results showed that the thickness of formed hydrophobic film is approximately only 2.34 nm, suggesting that this is a “lower damage” method to the pore structure. Based on the analysis of the mechanism, it is believed that the hydrophobic modification changes the surface of the pores from strong hydrophilicity to hydrophobicity, thereby converting the capillary pressure from resistance to a driving force that promotes fluid discharge.