We prepared simulated formation water based on the reservoir conditions of Shengli Oilfield. The mineralization degree reached 19,334 mg/L, with calcium and magnesium ions constituting 514 mg/L. Composition of chemical agents in simulated formation water: 17.786 g/L NaCl, 1.14 g/L CaCl₂, 0.86 g/L MgCl₂·6H₂O. We obtained all analytical grade chemical reagents from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). To prepare the experimental crude oil, we diluted dehydrated crude oil with kerosene until its viscosity reached 42 mPa·s at ambient temperature (25°C).

Shandong Noel Biotechnology Co., Ltd. (Shandong, China) provided the hydrophobically associating water-soluble polymer (HAWP) for this study. To achieve stable dispersion of SiO₂-nanoparticles in the composite system, SiO₂-nanoparticles modified by surface silanization were selected (later referred to as SiO₂), with particle sizes ranging from 20 to 30 nm.

To obtain the polymer stock solution, we dissolved HAWP powder in the simulated formation water to achieve a mass concentration of 5,000 ppm and allowed the solution to age for 24 h. By diluting the stock solution, we prepared HAWP solutions at 1,000 ppm, 1,500 ppm, and 2,000 ppm. The addition of varying amounts of SiO₂ to these solutions got composite systems containing 0.05 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt%, and 1.0 wt% SiO₂ at each HAWP concentration.

To observe microscopic morphology more clearly, we prepared two additional composite systems by diluting 5,000 ppm HAWP solution to 3,000 ppm(0.3 wt%) with simulated formation water and adding SiO₂ to achieve concentrations of 0.5 wt% and 1.0 wt%, that is, the ratios of HAWP to SiO₂ were 3:10 (0.3 wt%:1 wt%) and 3:5 (0.3 wt%:0.5 wt%) respectively. Then we examined the surface microstructure of the HAWP solution, SiO₂, and the composite systems using scanning electron microscopy.

We selected four composite systems to test the interfacial tension: 2,000 ppm HAWP with SiO₂ concentrations of 0.05 wt%, 0.1 wt%, 0.3 wt%, and 0.5 wt%. Before measurement, the composite systems underwent ultrasonic dispersion. Then we measured their interfacial tension with oil using an interfacial tensiometer at reservoir temperature (75°C) with a rotation speed of 6,000 r·min⁻¹.

We measured the viscosity, temperature-viscosity relationships, and rheological behavior of different systems using an Anton Paar M302 high-temperature and high-pressure rheometer. To accurately simulate reservoir conditions, the following tests were conducted: The influence of different formulations on composite system viscosity was tested at reservoir temperature (75°C) and wellbore shear rate (7.34 s⁻¹); The effect of temperature on composite system viscosity was investigated from 25°C–90°C at wellbore shear rate (7.34 s⁻¹); The impact of shear rate on composite system viscosity was examined at shear rates of 3.5–100 s^-1 at reservoir temperature (75°C).

The microscopic oil displacement experiments utilized a microfluidic device comprising a microinjection pump, microfluidic chip, microscope, and computer (Figure 1). The microfluidic chip (45 mm × 45 mm) with pore diameters ranging from 20–80 μm. We investigated four displacement systems: formation water, 0.1 wt% SiO₂, 2,000 ppm HAWP, and 2,000 ppm HAWP +0.1 wt% SiO₂, each stained with eosin.

The experimental oil was injected into the microfluidic chip at an injection rate of 0.5 mL/min until saturation was achieved. Subsequently, a displacement agent was injected at a rate of 0.5 mL/min to perform displacement until the water cut reached 98%, at which point injection was stopped. The process was repeated four times using different oil displacement systems to observe the displacement effects.

Core flooding experiments employed the same displacement systems as the microscopic studies. The experimental setup (Figure 2) incorporated an ISCO pump (Teledyne ISCO, United States), three containers, a core holder, a six-way valve, a pressure transducer, and a hand pump.

This system is mainly targeted at medium-high permeability and high water-cut oil reservoirs. We conducted experiments using standard sandstone cores (permeability: 400 mD, diameter: 25 mm, length: 100 mm). After the core was saturated with crude oil, water flooding was carried out. Water flooding was carried out until the water content reached 98%, and the corresponding chemical flooding was carried out until the water content reached 98% for subsequent water flooding. Following the completion of each experimental run, the oil displacement systems were replaced, and the aforementioned procedures were repeated. Throughout the flooding process, injection pressure variations were continuously monitored, and effluent samples were collected every 3 min for oil recovery and water content analysis.

Figure 3 shows scanning electron microscopy (SEM) images of (a) HAWP, (b) SiO₂, (c) 3,000 ppm HAWP +1.0 wt% SiO₂ (3:10), and (d) 3,000 ppm HAWP +0.5 wt% SiO₂ (3:5) systems.

SEM image observations revealed the microstructural morphology of the polymer solutions and the SiO₂ surface. The polymer molecules formed network structures. The SiO₂ particles showed a spherical shape with diameters ranging from 20–30 nm and maintained a specific surface area of ≥120 m²/g.

Furthermore, SiO₂ combined with HAWP to form stable composite systems, which increased the system viscosity through adsorption and bridging effects. As shown in Figure 3C, When the ratio of HAWP to SiO₂ is 3:10, SiO₂ aggregates were distributed throughout the polymer network structure, showing aggregate sizes between 2–3 μm, with a few aggregates reaching 4–5 μm. In comparison, when the ratio of HAWP to SiO₂ is 3:5, SiO₂ particles formed smaller aggregates of approximately 1–2 μm and displayed a more uniform distribution within the polymer network structure, indicating better compatibility between the nanoparticles and polymer.

Figure 4 shows the dynamic interfacial tension curves of HAWP/SiO₂ composite systems at different SiO₂ concentrations. The addition of nano-SiO₂ reduced the interfacial tension between the HAWP solution and crude oil. The polymer chains adsorbed onto the nano-SiO₂ surface through hydrogen bonding. The nano-SiO₂ particles became encapsulated within the free polymer chains. Both components adsorbed at the oil-water interface, leading to lower interfacial tension.

The oil-water interfacial tension continuously decreases as SiO₂ mass concentration increases, exhibiting the most significant reduction at lower SiO₂ concentrations. The interfacial tension drops to its minimum value at a SiO₂ concentration of 0.3 wt%. The system maintains good stability when the SiO₂ mass fraction remains below 0.3 wt%, and increasing the SiO₂ mass fraction enhances its interfacial tension reduction capability. However, when the SiO₂ mass fraction exceeds 0.3 wt%, the system stability declines, causing partial aggregation and failure of SiO₂ particles, which weakens their ability to reduce interfacial tension. However, compared with surfactants, the magnitude of its interfacial tension reduction is relatively small, only decreasing by 2–3 mN m⁻¹. While surfactants can reduce the interfacial tension to 10^–2 – 10^–3 mN m⁻¹ (Negin et al., 2017). Therefore, reducing the interfacial tension is not the main mechanism for the composite system to improve the oil recovery rate. Considering factors such as cost comprehensively, a lower SiO₂ concentration should be chosen.

Figure 5 shows the viscosity of HAWP at 75°C under a shear rate of 7.43 s⁻¹. The viscosity increased with higher polymer concentration. This increase occurred because polymer molecules formed network structures. At higher concentrations, polymer chains became more entangled, resulting in more stable network structures. Therefore, polymer viscosity increased with concentration at the same shear rate.

When the HAWP concentration increased from 1,000 ppm to 1,500 ppm, the viscosity increased by 191.7%; however, as the concentration increased from 1,500 ppm to 2,000 ppm, the viscosity increment was only 5.7%. These results suggest that the polymer viscosity is more sensitive to concentration changes at lower concentrations, whereas the viscosity increment becomes less significant above a certain concentration threshold. This is mainly because the high-valence ions (such as Ca²⁺ and Mg²⁺) in high salinity water compress the electric double layer through the charge screening effect, causing the molecular chains to curl up prematurely. The hydrophobic groups preferentially undergo intramolecular association (rather than intermolecular association). At low concentrations, the molecular chains form a dynamic intramolecular association-dissociation cycle through instantaneous contact, and a slight change in concentration can significantly affect the viscosity. High salinity enhances the polarity of the solution and weakens the ability of the molecular chains to stretch, resulting in a significant increase in the critical association concentration. Even if the concentration increases, the intermolecular association network is difficult to form due to salt inhibition, and the viscosity cannot show the conventional exponential growth.

Figure 6 shows the viscosity variations of composite systems at 75°C, where we added different mass fractions of SiO₂ into HAWP solutions at various concentrations. The results demonstrate that the viscosity of the composite systems increases with rising SiO₂ content. This increasing trend is more pronounced at higher polymer concentrations. The figure shows that in the 2,000 ppm HAWP + SiO₂ composite system, the maximum slope occurs at 0.1 wt% SiO₂ concentration. This indicates that SiO₂ nanoparticles at 0.1 wt% demonstrate the most significant tackifying effect on 2,000 ppm HAWP. In practical applications, considering that nanoparticles may still be adsorbed in the formation, a higher concentration can be selected.

Figure 7 shows the viscosity variations of composite systems with different HAWP-to-SiO₂ ratios as a function of temperature (25°C–90°C) at a constant wellbore shear rate of 7.34 s⁻¹. The experimental results demonstrated that the viscosity of all composite systems exhibits a consistent declining trend with increasing temperature.

However, the viscosity of the HAWP/SiO₂ composite system remained higher than that of HAWP. Furthermore, the viscosity increased with increasing SiO₂ concentration. This phenomenon occurred because SiO₂ could form a stable composite system with HAWP, enhancing the polymer chain network structure through adsorption and bridging effects, thereby increasing its viscosity. As shown in Figure 7C, when the HAWP concentration was 2,000 ppm, the temperature-viscosity curves of systems containing 0.8wt% and 1.0wt% SiO₂ nearly overlapped. This indicated that beyond a certain threshold of SiO₂ concentration, the viscosity-enhancing effect became insignificant, accompanied by decreased system stability, and SiO₂ nanoparticles aggregated and lost their effectiveness.

Figure 8 shows the viscosity variations of composite systems with different ratios as a function of shear rate (3.5–100 s⁻¹) at reservoir temperature (75°C). The experimental results demonstrated that all composite systems exhibited a decreasing trend in viscosity with increasing shear rate. Systems containing SiO₂ show higher viscosity compared to HAWP solutions of the same concentration, and the viscosity increased with increasing SiO₂ concentration.

However, the thickening effect of SiO₂ varies across different HAWP concentrations. As shown in Figures 8A, B, the viscosity-shear rate curves of composite systems with 1,000 ppm and 1,500 ppm HAWP demonstrate uniform distribution with increasing SiO₂ mass concentrations. Figure 8C reveals that in the 2,000 ppm HAWP composite system, the addition of 0.1 wt% SiO₂ leads to a rapid increase in the viscosity-shear rate curve. Further increases in SiO₂ concentration result in the rising speed of the viscosity slows down. This indicates that SiO₂ at 0.1 wt% concentration exhibits the most pronounced tackifying effect on 2,000 ppm HAWP.

While the addition of SiO₂ did not fully address the viscosity reduction of HAWP solutions at high temperatures and shear rates, it significantly enhanced the overall viscosity of the composite systems. The viscosity increased with rising SiO₂ concentrations, with 0.1 wt% SiO₂ showing the most significant thickening effect on 2,000 ppm HAWP. Considering cost-effectiveness and performance, the 0.1 wt% SiO₂ + 2,000 ppm HAWP composite system was selected as the optimal flooding system.The viscosity of this system at 75°C under a shear rate of 7.43 s⁻¹ is 1.73 times that of the polymer with the same concentration under the same conditions, which can effectively reduce the usage amount of the polymer.