Three parallel tests were conducted in subsequent experiments using the following instruments: an LBJ-625 high-speed agitator (Guangdong Lobe), a JD90-D powerful electric agitator (Shanghai Specimen), a KQ-200VDE ultrasonic numerical control cleaning machine (Kunshan Shumei), an SHZ-B constant temperature water bath oscillator (Shanghai Hetian), a DFA-100 dynamic foam analyzer (KRUSS, Germany), a HAAKE RS6000 rotational rheometer (Germany Thermo Fisher), and a Quanta 450 field emission environmental scanning electron microscope (FEI, United States). Three parallel tests were set up in the following experiments.

A 100 ml solution was added to the measuring cup and stirred at a constant speed (18,000 r/min) for 60 s. Next, the solution was poured into a 1,000 ml measuring cylinder and we recorded the produced foam volume as V₀, including the time t_1/2 when the discharged liquid reached 50 ml in the foam. Then, the foam composite index FCI was obtained from Eq. 1. The experimental apparatus is also shown in Figure 1. F C I = 0.75 V 0 t 1 / 2 , (1) where FCI is the comprehensive index of the foam (ml·min), V ₀ is the maximum foaming volume (ml), and t _1/2 is the precipitation half-life (min).

Here, the foam solution (100 ml) was measured at a medium temperature and ventilated at a certain temperature (60–90 °C) according to certain gas–liquid ratios. Then, we recorded the foam’s maximum height H ₀ (mm) and half-life (s). Finally, the foam’s volume was calculated according to the diameter of the measuring cylinder, after which the comprehensive foam index was also calculated according to Eq. 1. The experimental apparatus is shown in Figure 2.

We used simulated formation water to prepare 100 ml of the foaming agent solutions with concentrations of 0.1%, 0.3%, 0.5%, and 0.7%. Then, at room temperature, the Waring-blender method was used to stir for 1 min, and then the foam was poured into the measuring cylinder to observe and record the foam’s maximum foaming volume and half-life. Finally, and according to Eq. 1, the foam comprehensive index of the foam system was calculated, followed by an evaluation of foam performance using the DFA-100 dynamic foam analyzer and according to the gas–liquid 1:1 ratio intake.

For this analysis, 2 g of oil sand was accurately weighed and added to the conical flask. After fully wetting with an appropriate amount of simulated mineralized water (total mineralization was approximately 4,765.6 mg/L), the aqueous foam solution was added (foam solution concentration was follows: 0.1%wt, 0.3%wt, 0.5%wt, and 0.7%wt). After shaking at 90 °C for 24 h, we subjected it to centrifugation. Then, we collected 1 ml of the supernatant and determined the main foaming agent’s content before and after adsorption by two-phase titration ^[8]. Finally, the adsorption capacity was calculated using Eq. 2: Γ = V 0 − V C M m m o l · g − 1   s a n d , (2) where V ₀ is the volume of the benzethonium chloride solution consumed by 1 ml of the foam solution before the adsorption experiment (ml), V is the volume of the benzethonium chloride solution consumed by 1 ml of the foam solution after the adsorption experiment (ml), C is the concentration of the benzethonium chloride solution (mol/L), and M is the relative molecular mass of the foaming agent (g/mol).

A steady state shear test was conducted with the HAAKE RS6000 rotary rheometer (90 °C, constant shear rate of 10 s⁻¹). The apparent viscosity of the solution at stability was measured (90 °C and the shear rate ranged from 0.001 to 1000 s⁻¹). Shear experiments were conducted on the optimal foam formula system, and logarithmic points were collected to obtain the shear rate scanning curve. The apparent viscosity curve of the foam system was obtained by scanning at a constant temperature of 90 °C and a constant shear rate of 10 s⁻¹.

This measurement was performed to obtain the energy storage modulus (G’) and the energy dissipation modulus (G”) at 90 °C and a dynamic frequency range of 0.05–10 Hz.

The foam samples of the compound system were placed in a thermostat at 90 °C. After aging for 1 day, the high-temperature and high-pressure foam working fluids were bubbled using a test device. The experimental temperature T was −180 °C and pressure p was 0–40 MPa. After the samples’ temperatures in the observation box were rapidly lowered to −60 °C by adding liquid nitrogen, the samples were observed on a quanta 450 field emission environmental scanning electron microscope.

Foam properties are usually evaluated based on the foaming volume (the foaming capacity of the surfactant) and half-life (stability of the foam).

It was evident from Figures 3A,B that within the concentration range of 0.1%–0.7%, the foaming volume and half-life gradually increased with a concentration increase. However, when the concentration of the foaming agent increased to 0.5%, the foaming volume no longer grew as fast, because with the increase in foam concentrations (Nguyen et al., 2003), the surfactant molecules were enriched on the surface of the solution and formed a dense surface film. As a result, the liquid film’s surface strength increased, the liquid film drainage was blocked, and the foam surface’s viscosity increased, which increased the foaming volume and made the foam more stable. When the foam concentration continued to increase (Barbian et al., 2005), the foaming agent on the foam liquid-membrane surface concentration became too large, the density increased, and the formation of the foam liquid content was reduced. However, with an increase in “rigidity,” liquid film drainage increases and ultimately decreases the stability of the foam.

Next, we comprehensively analyzed the foam performance. The performance order for several foams was anionic > amphoteric > nonionic, which is consistent with the determination results of a previous article (Ma et al., 2014). However, of the anionic foams, CQS-1, CST-1, and CST-2 had better overall performance with the comprehensive foam index at 0.5% for the CQS-1 solution and reached greater than 1,000 ml·min, which made it the best foaming agent. In contrast, while the comprehensive foam index of 0.7% CST-1 and CST-2 solutions was approximately 700 ml·min, the CST-2 was slightly better than CST-1, but there was a big gap compared with CQS-1. For the anionic–nonionic OS and CCT-1 foams, while the foam-integrated index was low, only 1/5 of CQS-1, the foam grade was relatively small. Finally, the amphoteric LH-2 foam with a comprehensive foam index close to CST-1 at the 0.7% concentration (680 ml·min) was selected as a compounding agent. Interestingly, the selected foam had a consistent optimal concentration range that was 0.5%.

Foam stability improvements are mainly achieved by adding foam stabilizers (Jiang et al., 2011), where the surface adsorption strength or foam liquid phase viscosity is increased to form an elastic film. Therefore, we chose a 0.5% CQS-1 solution and added a DQL thickening stabilizer and the NS-2 nanoparticle liquid-film enhancer to prepare a ternary composite reinforced foam system. Under the same concentrations of DQL and NS-2, the effect of the tackifier and stabilizer concentrations on the performance of the foam system was then investigated. Figure 4 shows the experimental results.

It is evident from Figure 4 that with an increase in the concentrations of DQL and NS-2, the foam performance of the 0.5% CQS-1 foam system was significantly improved. However, when the concentrations of DQL and NS-2 were both at 0.15%, the half-life sharply increased and the comprehensive foam index suddenly changed. Generally, studies have confirmed that the stability of foam depends on the liquid film discharge rate (Wang et al., 2008; Yu et al., 2014; Guo and Aryana, 2016; Nazari et al., 2017). Similarly, we recorded that as the addition of two foam stabilizers increased, the foaming solution’s viscosity and the liquid film’s surface strength, which delayed the drainage time, reduced the liquid film’s thinning rate. This enhanced the mechanical strength of the foam film. This finding was proposed because the hydroxyl groups on the surface of the NS-2 nanoparticles combined with the carboxyl and amide groups in the DQL molecule through hydrogen bonds and Vander Waals forces to form a more stable spatial network structure.

Conversely, when the concentrations of the two foam stabilizers were 0.2% each, the comprehensive foam indices reached 11,672 ml·min, and the half-life and foaming volume results were 28.82 min and 540 ml, respectively. Moreover, compared with the single agent with a concentration of 0.5% CQS-1, we also observed that the half-life increased eight times, the stability significantly increased, the foam level of the foam system was very strong, and it was part of the best three-component reinforced foam system. However, considering the stratum injection, the foaming system with 0.5% CQS-1/0.15% DQL/0.15% NS-2 should be selected.

Four thickening foam stabilization accelerators (TEA-18, TEA-20, TEA-22, and TEA-24) were selected and compounded with the 0.15% CQS-1/0.1% DQL/0.1% NS-2 system (noted as 1#) to further enhance the foam stabilization performance of the foaming system. Then, the performances of the foam system were investigated when the accelerator concentrations were 0.05%, 0.1%, 0.15%, and 0.2%, followed by the selection of a thickening and stabilizing agent with the best foam stability. Figure 5 shows the experimental results.

According to the comparison in Figure 5, except for TEA-20, the foaming volume and half-life increased by varying degrees with increasing accelerator concentrations. Specifically, TEA-22 significantly changed when the concentration reached 0.2%; the foam composite index reached the maximum value, the foam composite index was 5,916 ml·min, which was 2.5 times higher than that without TEA-22, and the foam performance was excellent. Therefore, compared with TEA-20 and TEA-18 and since TEA-22 has greater advantages, it was selected as the accelerator for system 1#.

Figure 6A shows that within the investigated concentration range, the foaming volume did not significantly change with the concentration, most of which were concentrated from 110–140 ml. However, while the foaming volume of CQS-1, CST-1, OS, NP-7, DTAC, and CAD-40 increased with an increase in concentration, SDS, CST-2, and FH-1 showed an obvious downward trend. Thus, we judged that the best concentration range was 0.3%–0.5% to ensure sufficient foaming volume. In addition, since CQS-1 showed obvious increasing changes, we determined that a higher concentration could be used to achieve better foaming volume when the cost allows. Therefore, CQS-1 was selected as the main foaming agent. This conclusion is consistent with Section 2.2.5.

As observed in Figure 6B, the foam stability of CQS-1 and LH-1 with excellent foaming properties decreased with increasing concentration. We also observed that while the foaming properties of the nonionic surfactant FH-1 and the four amphoteric surfactants were rich and fine, with good viscoelasticity, the foam was the most stable. However, with an increase in the concentration, the foam decreased first and then increased, and the 0.7% FH-1 composite index was the highest, reaching 2071 ml·min. Therefore, combining the 0.5% aqueous solution foam performance of the surfactant series shown in Figure 7 with the foaming performance, foam stabilizing performance, and cost factors, the optimal foam stabilizing compound was FH-1.

Since the synergism between surfactants can enhance the effect of foaming and stabilizing bubbles, we chose CQS-1 and FH-1 to make the foaming-agent mixture. We investigated the effect of the mass ratio of CQS-1 and compound additives on the foam properties of the composite foam system. Experiments were conducted according to the 0.5% foaming agent/0.15%DQL/0.15%NS-2 solution using the DFA method and the ratio of gas to liquid of 1:1. Figure 8 shows the experimental results.

It was evident from Figure 8 that when the mass ratio of CQS-1 to FH-1 was less than 1:1, it had good compound and positive synergistic effects and effectively stabilized the foam. Hence, considering the cost of use, we determined that while the suitable compounding ratio was CQS-1:FH-1 = 4:6, the composition of the compounding foam ternary composite reinforced foam system was 0.5% (CQS-1:FH-1 = 4:6)/0.15% DQL/0.15% NS-2 and was recorded as the 2# system.

With a DFA-100 dynamic foam meter and according to a gas–liquid ratio of 1:1, the temperature resistance performance of the 2# system (0.5% (CQS-1:FH-1 = 4:6)/0.15% DQL/0.15% NS-2) was measured at 60 °C, 70 °C, 80 °C, and 90 °C. Figure 9 shows the experimental results.

As shown in Figure 9, the 2# system had excellent temperature resistance with an increase in temperature related to higher foaming ability, but this also caused the foam stability to slightly fluctuate. Similarly, the foam index was not great at the 60–90 °C interval and fluctuated slightly at 500 ml·min; and therefore, the 2# system has excellent temperature resistance.

The 2# system was mineralized with a salinity of 1,000 mg/L, 3,000 mg/L, 5,000 mg/L, 8,000 mg/L, 12,000 mg/L, and 15,000 mg/L after which the 2# foam system with 0.15% TEA-22 was added. The influence of salinity on the foam performance of the 2# system was also investigated at 90 °C using the DFA-100 dynamic foam tester. Figure 10 shows the experimental results.

It was evident from Figure 10A that with an increase in the salinity of simulated water, the foaming ability of the 2# system and its TEA-22 strengthening system were hardly affected. However, in the range of 1,000–5,000 mg/L and as the salinity increased, the foam half-life of the 2# system slightly increased, the salinity continued to increase, and the half-life decreased. Finally, at 12,000 mg/L, the half-life dropped to a minimum of 136 s. This is because the addition of a strong electrolyte led to the diffusion of the electric double layer on the surface of the liquid film and decreased the zeta potential, which resulted in poor foam stability. In contrast, the half-life increased sharply to 355 s when the salinity was 15,000 mg/L and caused the foam composite index to be 980 ml·min.

Figure 10B shows that while the foaming volume changed little after adding TEA-22 to the 2# system, the half-life of the foam increased with an increase in salinity. As a result, while high foam performance was produced when the salinity was 12,000 mg/L (a half-life of 350 s and a comprehensive foam index of 910 ml·min), the high foam performance was maintained in the salinity range of 5,000–15,000 mg/L (comprehensive foam index >500 ml·min).

Within the mineralization degree of 1,000–15,000 mg/L and while the comprehensive foam index of the 2# system was higher than the 350 ml·min, the optimal formulation system showed better salt resistance after TEA-22 reinforcement, which indicates that the preferred 2# composite foam system had excellent salt resistance. However, TEA-22 was considered if improvements were required.

The core adsorption and retention conditions indicate the industrial application feasibility of an oil-repellent system. After separately weighing 2 g of oil sand and adding the appropriate amount of water to wet it, the sand-foaming liquid system with a solid-liquid ratio of 1:9, 1:4, and 3:7 was prepared and the adsorption was shaken at 90 °C for 24 h. Then, the adsorption capacity of the main foaming agent on the core was measured by two-phase titration. A comparison of the petroleum sulfonate PS with the main foam agent CQS-1 2# system and the TEA-22 enhanced 2# system and CQS-1 adsorption on oil sands is shown in Figure 11.

However, from Figure 11 we observed that within the investigated range, while the adsorption of the main foam CQS-1 on the oil sand increased with an increase in the solid-liquid ratio, the adsorption amount varied from 1.4 to 3.0 × 10⁻⁴ mmol/g. This finding shows that the preferred system strengthened by TEA-22 was slightly helpful in reducing the adsorption loss. However, the system’s viscosity further increased, which decreased the foaming agent’s diffusion speed to the oil sand’s surface.