Improving the temperature and salt resistance of hydrolyzed polyacrylamide (HPAM) is crucial for its effective application in enhanced oil recovery (EOR). Due to the abundance, nanoscale, high oil-water interfacial adsorption efficiency of nanocellulose, it has attracted significant attention in EOR applications. In this study, a new kind of surface-functionalized amphiphilic tunicate cellulose nanocrystals (TCNCs-M2) was successfully prepared by sulfonate and alkylated modification, which was synergistically used with HPAM to formulate a hybrid flooding system (0.2 wt% HPAM +0.1 wt% TCNCs-M2). Benefiting from the structure of TCNCs-M2, the hybrid system exhibited stronger performance of thickening ability (viscosity increased by 48.94 % at 65 °C in brine with salinity 8044 mg.L−1), temperature resistance (25–90 °C), salt tolerance (salinity 8044 mg.L−1), viscoelasticity and aging stability compared to that of HPAM solution. These enhancements were attributed to the hydrophobic association in addition to strong hydrogen bonding and electrostatic repulsion in the hybrid system. Furthermore, the hybrid system exhibited a higher oil recovery factor (22.8 %) than HPAM solution (16.4 %). These results indicate that the newly formulated amphiphilic nanocellulose/HPAM hybrid system could be an effective oil-displacing agent for harsh condition reservoirs.

In recent years, there has been a growing emphasis on research and innovation in oilfield exploitation technology in response to the increasing petroleum resource extraction and the decline in production . Enhanced oil recovery (EOR) treatments, such as chemical flooding (polymer, surfactant, alkali, and combination of the above), gas injection, thermal methods, etc., have undergone significant improvements and been widely applied in oilfields after continuous practical exploration. Due to the simplicity and being low-cost, polymer flooding is the most extensively utilized EOR method to date in oilfields especially in China. However, partially hydrolyzed polyacrylamide (HPAM), the most widely used polymer, is very susceptible to harsh reservoir conditions, which severely influences its EOR efficiency, especially in elevated temperature and salinity formations . Although the derivatives of HPAM (hydrophobic polyacrylamide, etc.) could improve the temperature and salt resistance of HPAM by copolymerization of acrylamide and other functional monomers, there are still some shortcomings of these copolymers, such as long dissolution time, low molecular weight, and complex preparation, which inhibit their application in the oilfield .Recently, combined flooding system of polymer and nanoparticles has attracted more and more attention as a novel EOR method . Nanoparticle suspensions possess the ability of a reduced injection pressure and an improved EOR performance in low-permeability reservoirs. They can alter the wettability of the rocks' surface by forming a wedge film between the crude oil and the rock surface, due to its nanosize, large surface area, and high heat transfer capability . When nanoparticles are introduced into a polymer solution, the synergisms between polymer and nanoparticles can further improve the EOR performance via the cross-linking between the polymer molecules and nanoparticles through hydrogen bonds, which reinforces the molecular network structure and enhances the rheological properties of the polymer solution . Moreover, nanoparticle and polymer molecules compete for attracting cations, so the degradation of polymer molecules is avoided to a certain extent in salt solution at high temperature . In addition to conventional aspherical nanoparticles such as SiO2 , CaCO3 , TiO2 particles etc., anisotropic nanoparticles such as nanosheets, nanofibrils and nanorods also exhibit excellent EOR performance, which can be oriented at the oil-water interface with their long axis parallel to the interface, generating networks in the continuous phase with multi-layers of nanoparticles onto the droplet interface .

Nanocellulose, as a renewable, green, and environmentally friendly nanomaterial, has attracted significant attention in emulsion stabilization and EOR, due to their rod-like shape and high aspect ratio, which contributes to higher adsorption energy . The physicochemical properties of the nanocellulose dispersions including rheology, stability, and oil/brine/rock interfacial behaviors have been thoroughly investigated by Wei et al. before . The experimental results showed that the stability and EOR relevant properties (salt and temperature resistance) of the nanocellulose fluids can be significantly improved via surface-modification to introduce excessive negative charges and steric hindrance. It is worth noting that tunicate cellulose nanocrystals (TCNCs), isolated from the mantles of tunicates, are comprised by more stable cellulose Iβ form, and exhibit higher aspect ratio, Young's modulus and more hydroxyl functional groups compared with the cellulose nanocrystals from other bioresources . These features facilitate the formation of stable cross-linked networks with polymer molecules and provide an excellent foundation for surface modification, thus enhancing their potential utility in oilfield applications. However, TCNCs exhibit poor compatibility with polymers. The hybrid tends to agglomerate in electrolyte due to the weak repulsive forces, which may render this combination flooding problematic in injection and deep migration in porous media. In our previous work, we found that the addition of anionic surfactant sodium dodecyl sulfate (SDS) to the hybrid system of polymer and TCNCs could obviously improve the stability and EOR performance of the system due to the synergistic effect of SDS, polymer and TCNCs. SDS can interact with polymer molecules through hydrophobic interactions between hydrophobic groups and polymer backbones, which in turn enhanced the strength of the three-dimensional network formed by TCNCs and polymer chains. SDS can also reduce the mobility ratio between oil and hybrid system due to its excellent emulsion performance [26]. With these advantages all together, we decided to synthesize a novel surface modified TCNCs, which has both hydrophilic negative groups and hydrophobic groups, and introduce this amphiphilic TCNCs to the HPAM flooding system for high-temperature, high salinity reservoir. As far as we know, this work has not been done before.

In this study, a two-step modification process was employed to prepare amphiphilic TCNCs (TCNCs-M2) using 3-chloro-2-hydroxypropyl sulfonate (CHPS-Na) and dodecyl trimethoxysilane (WD-10). The structure of TCNCs-M2 was characterized using FT-IR, TG, XRD, XPS, TEM and AFM, respectively. Then, TCNCs-M2 was mixed with HPAM to obtain a homogeneous hybrid system (HPAM/TCNCs-M2). The interaction mechanism between HPAM and TCNCs-M2 was analyzed. Subsequently, the dispersion stability, rheological properties such as temperature resistance, salt tolerance, shear resistance, and viscoelastic modulus of the HPAM/TCNCs-M2 were evaluated, which showed excellent thickening ability, temperature resistance, salt tolerance, shear resistance as well as viscoelastic property in simulated oil reservoir circumstance. Finally, a core flooding test was conducted and the obtained data showed that the oil recovery ratio of HPAM/TCNCs-M2 hybrid system was 22.8 %, which is higher than that of HPAM solution (16.4 %), confirming that the HPAM/TCNCs-M2 hybrid system have practical application prospects.