Development of Anionic-Modified Black Nanosheet and Its Oil Imbibition Performance in Ultralow-Permeability Reservoirs

Ultralow-permeability oil reservoirs are characterized by small pore throats, ultralow permeability, and high fluid flow resistance, which often lead to challenging injection during water flooding processes. Black nanosheet, as an emerging sheet-like nano oil displacement material, can alter the wettability of reservoir rocks, effectively improving water injection performance. However, its dispersion stability in an aqueous solution is insufficient, leading to probable aggregation during water flooding and affecting the oil recovery efficiency. In this study, anionic-modified black nanosheet was developed using a chemical method. The dispersion stability of the anionic-modified black nanosheet was evaluated by using a multiple light stability analyzer and a ζ-potential analyzer. Additionally, the interfacial properties were assessed with a contact angle measurement instrument and emulsification experiments. Finally, the imbibition performance in ultralow-permeability oil reservoirs was evaluated through imbibition experiments combined with nuclear magnetic resonance technology. Results showed that sulfonate groups can be grafted onto black nanosheets by chemical methods. Anionic-modified black nanosheet exhibited a black sheet-like structure, showing potential applicability in ultralow-permeability oil reservoirs. The transmittance stability index (TSI) value of the anionic-modified black nanosheet decreased from 14.26 to 3.33, and the absolute value of ζ-potential increased from 10.4 to 27.5 mV, indicating significantly improved dispersion stability. The effect of anionic-modified black nanosheet on altering the surface wettability of oil-saturated rocks was more pronounced, and the crude oil emulsification effect was also enhanced. In addition, solutions of modified black nanosheets showed a better imbibition oil recovery performance. The nuclear magnetic resonance (NMR) T 2 spectrum revealed at the microscopic scale that the modified black nanosheet could facilitate the better imbibition from the small pores of ultralow-permeability reservoir core samples, thereby enhancing the efficiency of microscopic flooding.