A wide variety of polymer types can be synthesized, and special properties can be imparted via different routes and suitable end capping reactions. In this article, the hyperbranched macromolecules were synthesized by the polymerization of an intermediate (AB_x or latent AB_x monomers). First, a diethanolamine methanol solution was prepared by dissolving diethanolamine in a methanol solution. Then, succinic anhydride was added into anhydrous methanol and stirred until it was completely dissolved. Then, the diethanolamine solution was added to the succinic anhydride solution, and the final molar ratio of diethanolamine to succinic anhydride was found to be 1:1. The solution was placed in a cold water bath and maintained for 6 h, and then the methanol solution was removed using a rotary evaporator to obtain the AB₂-type monomer intermediate. Its molecular structure is shown in Figure 1.

TMP (trimethylolpropane) is esterified with the AB₂ monomer at 120°C in the presence of p-toluenesulfonamide (PTSA) as the catalyst. The three terminal hydroxyl groups of TMP are equally replaced by the AB₂ monomer in the reactions, and then the Gen. 1 intermediate is obtained. When each hydroxyl group of TMP is substituted by esterification, two hydroxyl groups are introduced into the end position, and the esterification reactions continue to proceed with the monomer. In order to minimize the consumption of the Gen.1 intermediate by the AB₂ monomer and assure the equal substitution of terminal hydroxyl groups in TMP, a molar ratio of 1:3 (TMP to the intermediate) is used, and the yield is about 75%. The reaction scheme is shown in Figure 1.

The Gen.1 intermediate is esterified with the AB₂ monomer at 120°C in the presence of PTSA as the catalyst. The six terminal hydroxyl groups of the Gen.1 intermediate are esterified with the AB₂ intermediate, and then the second generation hyperbranched polyurethane (Gen.2 HPAE) is obtained. In the experiments, the molar ratio of 1:6 (Gen.1 intermediate to AB₂ monomer) is used to minimize the side reactions, and the yield is about 65%. The reaction scheme is shown in Figure 2.

Modified hyperbranched polyurethane (m-HPAE) and NaOH were put into a four-port bottle and heated to 80–100°C under magnetic stirring. A 3-chloropropionic acid solution (80 wt%) was vacuumed for 30–50 min and then poured into the bottle, leaving the reaction to proceed for 4–6 h. The solvent in the system was removed, and the product was washed using anhydrous ethanol and then filtered. The residues with low boiling point were removed using a rotary evaporator and sodium carboxylate–terminated HPAE (SCHP) was obtained. The reaction scheme is shown in Figure 3. In the process of synthesis, the molar ratio (hyperbranched polyurethane to 3-chloropropionic acid and NaOH) was determined to be 1:12:12 to replace 12 terminal hydroxyl groups in the synthesized HPAE.

Surface tension was determined using the ring method where a ring attached to a torsion meter was dipped into the hyperbranched carboxylate surfactant solution. The liquid level was lowered after slow pull-up, and the force on the ring was measured before the liquid film tore off. The surface tension was calculated based on the diameter of the ring and the tear-off force.

The CMC was determined by a conductivity method where a conductimeter was used to measure the conductivity of the solution. The Krafft point is the minimum temperature at which surfactants form micelles, that is, micelles cannot be formed below this critical micelle temperature. At the Krafft point, the solubility is equal to the CMC. The Krafft point was measured in the experiment. The solution of m-HPAE with a concentration of 1% was prepared and poured into a test tube which was heated in a water bath and stirred until the solution became clear and transparent. Then, it was placed in a cold water bath to allow the temperature to drop under continuous stirring until the precipitation was observed. The measurements were repeated, and the temperature was recorded.

The adsorption property of m-HPAE is a significant parameter to determine its suitability for enhanced oil recovery, relevant to its ability and effectiveness to reduce surface tension at the oil–water interface and strip off oil drops from the rock surface. m-HPAE varying from 100 mg/L to 900 mg/L was prepared and mixed with 2 g oil sands (from Daqing Oilfield in China) at the liquid/solid ratio of 60:1. The container was placed at 45°C and was continuously shaken for 8 h. The clear solution was obtained after separation and still stratification, and the concentration of the hyperbranched surfactants was determined by phase titration using Bromocresol green based on the following equation. M = ( C 1 − C 2 ) ∗ V m , where m is the weight of the oil sands, g; C₁ is the concentration before adsorption, mg/L; C₂ is the concentration after adsorption, mg/L; V is the volume of m-HPAE, mL; and M is the adsorption capacity, mg/L.

Micellar solubilization is the process of taking the solubilizate onto or into micelles as a part of the whole system. In the experiment, the hyperbranched surfactant solution was prepared with a concentration greater than the CMC. 10 ml distilled water was added into seven 50-ml bottles containing benzene with various volumes (10, 20, 30, 40, 50, 60, and 70 μl). The bottles were placed in the water bath at 40°C for 15 min, then taken out, and cooled down to atmospheric temperature. The absorbance value of each solution was measured using a spectrophotometer, and the solubilization value was calculated based on the following equation. Z = 1000 A c V , where Z is the solubilizing ability, mL/mol; c is the hyperbranched molecule solution concentration, mol/L; V is the hyperbranched molecule solution volume, mL; and A is the benzene content when reaching the solubilization limit, mL.

Liquid droplets on the solid surface will form droplets under the action of the interfacial tension at the gas–liquid–solid three-phase interface, and the droplets will form a contact angle θ at the gas–liquid–solid three-phase interface. When equilibrium is reached, the wetting formula can be obtained. σ g − s = σ l − s + σ g − l c o s θ .

The contact angle θ can judge the wetting of the surface. The smaller the contact angle, the better the hydrophilic performance. The m-HPAE solution was prepared with a concentration of 400 mg/L to fully soak the rock sample and then dried to obtain the processed rock sample, and a contact angle tester was used to measure the contact angle of the rock sample surface before and after the treatment.

In the process of water flooding, the migration of oil and water in the rock pores will be affected by the wettability of the rock. The wettability of the pores directly affects the efficiency of water flooding. The capillary resistance formed by the lipophilic pores will reduce the recovery. The capillary force formed by hydrophilic pores can be used as a driving force to improve oil recovery. When the oil-in-water emulsion system flows through the bottom layer, the more hydrophilic the surface of the rock, the lower will be the flow resistance. If the surfactant can change the originally hydrophobic rock surface into a hydrophilic rock surface, it will convert the capillary force from resistance to motive force and increase the oil recovery.

There are polymer chains in the m-HPAE structure, which will affect the extension structure in a high-concentration salt solution, and the formation environment is often accompanied by high salinity. This experiment explores the influence of salinity on the rheology of m-HPAE. An m-HPAE solution was prepared with a concentration of 400 mg/L, and NaCl was added to make its concentration reach 1000 mg/L and 10000 mg/L. The viscosity at different shear rates was measured by using a Brookfield rotary viscometer.

Two binary surfactant systems, namely, m-HPAE/sodium dodecylsulfate (LAS) and m-HPAE/NaOH, were prepared, and the interfacial tension was measured prior to the core flooding experiments. The viscosity of the crude oil (obtained from Daqing Oilfield) is 10cP at 45°C, and the salinity of the formation water is 3 × 10³ mg/L. The interfacial tension was measured using a TX-500C interfacial tensiometer at 500 rpm, and the data were recorded every 30 mins until the difference between the two consecutive readings was less than 0.001.

The core flooding experimental setup is composed of pressure sensors, the core holder and the sample collecting container, and the hand pump and the constant rate pump (Figure 4).

All components were placed in an oven with the temperature set at 65°C, except the hand pump and the constant rate pump. The core was vacuumed and saturated with formation water at the atmospheric temperature and then with the crude oil at 65°C. The core was flooded by water at the same temperature until 98% water cut and the oil recovery factor by water flooding were calculated. Then, 0.3 pore volume of the surfactant solution was injected followed by water flooding until 98% water cut and the oil recovery factor by surfactant flooding were calculated. The injection rate was constant at 0.3 ml/min, and pressure data were recorded every 30 min.

The processes where the intermediate and the final product are produced belong to the esterification and carboxymethylation reactions, respectively. The conversion rate in the esterification reaction is evaluated by the acid number which is the mass of potassium hydroxide (mg) required to neutralize the chemical (1 g). The conversion rate in the carboxymethylation reaction is calculated based on the hydroxyl value, which is potassium hydroxide (mg) required to neutralize the acetic acid taken up on acetylation of the chemical substance (1 g) that contains free hydroxyl groups.

As esterification reaction proceeds slowly at low temperature, and we recorded the data from the reaction conducted at 100°C. More intermediates are produced as the temperature increases. When the temperature was elevated to 115°C, the color of the product is light yellow, close to the color of the intermediate. The color of the product turns dark at a temperature above 130°C, indicating that the structure of the intermediate is broken, and an unexpected substance is produced.

There were no catalysts required in the carboxymethylation reaction, while various concentrations of PTSA (1, 2, 3, 4, and 5%) were used to accelerate the esterification reaction, since it is a reversible reaction. The reaction temperature was set at 120°C, reaction time was 8 h, and molar ratio of the intermediate and TMP was 9:1. The relationship between the acid number and concentration of PTSA is demonstrated in Figure 5A. The acid number decreases when PTSA concentration is increased till 3%. The acid number is determined to be 26.62 mg/g at 3% concentration, and in the meanwhile, the product exhibits light yellow color and viscous property. The reaction proceeds toward the AB₂ intermediate, and the carboxyl groups are continuously consumed. The effects of PTSA itself on the acid number are neglected, considering its relatively small usage. The acid number of the system is abnormal when PTSA concentration is more than 4%. It indicates that some unexpected byproducts are produced by increasing the concentration of the catalyst. PTSA concentration is determined to be 3% in the esterification reaction to avoid byproducts during the syntheses of m-HPAE.

Figure 5B shows the influence of temperature and reaction time on the acid number in the esterification reaction. Acid number can be used to indirectly demonstrate the extent of the reaction. It decreases with the increasing temperature due to the fact that the hydroxyl groups are reacting with the carboxylic acid in the previously formed monomer. The stabilized acid number indicates reaction equilibrium at 120°C, which is also the temperature limit of the light-yellow intermediate being produced. The influence of various reaction times was also evaluated at 120°C, where 3% PTSA as the catalyst and 9:1 M ratio of the intermediate and TMP were used. The acid number presents a stabilized trend after the 8-h reaction, which indicates that the esterification reaction achieves equilibrium at 8 h. Therefore, in the esterification reaction, the temperature is determined to be 120°C, and the reaction time is 8 h.

Figure 6A depicts the influence of temperature and reaction time on the hydroxyl value in the carboxymethylation reaction. The hydroxyl value decreases with the increasing temperature and becomes stable at temperature above 80°C. With extended reaction time, the hydroxyl value also decreases and becomes stable when the reaction time is more than 4 h. Therefore, in the carboxymethylation reaction, the temperature is determined to be 80°C, and the reaction time is 4 h.

Figure 6B shows the influence of temperature and reaction time on conversion in the carboxymethylation reaction. When the temperature is above 80°C and reaction time is over 4 h, the conversion trend in the carboxymethylation reaction does not present obvious change, which is in line with the conclusion given in Figure 7. The conversion at 80°C reaction for 4 h is 80.77%.

The spectral data of HPAE and the modified HPAE (m-HPAE) were recorded by using a FT-IR spectrometer in a wide spectral range, as is shown in Figure 7.

The strong absorbance bands at 3,420 cm⁻¹ (m-HPAE)and 3,386 cm⁻¹ (HPAE) indicate that the -OH group is present in the molecular structure. In the process of the hydroxylation reaction of HPAE, -COONa is formed, and the terminal–OH is consumed, resulting in a less-intense absorption band. Meanwhile, it is normal that the amide–hydroxyl bonding peak appears in this region. The saturated carbon chain is elongated as a result of the hydroxylation of HPAE. This largely weakens the electron-withdrawing effect of adjacent–OH on amide groups, resulting in the enhanced inductive effect of adjacent carbon carbonyl C=O oxygen atoms on amide groups. That explains why the absorption peak has shifted to a higher wavenumber (3,420 cm⁻¹).

The absorption bands at 2,966 cm⁻¹ and 2,927 cm⁻¹ correspond to CH^3- and CH₂- in the molecule, respectively. The C–H absorption band at 2,890 cm⁻¹ with low intensity is associated with the saturated carbon chain structure in the molecule.

Both HPAE and m-HPAE molecules show strong -COO- antisymmetric stretching vibration at 1,599 cm⁻¹. This absorption band is not associated to the amino acid (-CONH-) -COO- structure because the intensity is not highest. The presence of the -COO- structure in HPAE molecules throughout the modification processes is a strong evidence that the branched structure is built from the esterification reaction of HPAE and m-HPAE.

There are multiple strong absorption bands in the m-HPAE spectrum. 1722 cm⁻¹ and 1,632 cm⁻¹ correspond to ester carbonyl and tertiary amide carbonyl groups, respectively, which proves that the molecular structure at the position of the end group is modified to be –CO–N–COO-. Meanwhile, the carboxylic acid radical is present as indicated by the weak but characteristic absorption band at 550 cm⁻¹ (-COO- vibration).

Multiple weak absorption bands are detected in the range of 1,500–1,000 cm⁻¹ for both HPAE and m-HPAE, respectively. 1,380 cm⁻¹ and 1,324 cm⁻¹ correspond to amide C–N, while the broad absorption band at 1,094 cm⁻¹ is connected to the aliphatic amide and C–O–C structure. The broad shape of the absorption band results from the increased contents of C–O–C by m-HPAE modification.

In theory, the products synthesized in this experiment can only be hyperbranched products or monomeric self-polymerized products. HPAE and m-HPAE are successfully obtained, and the expectation of the product design is met, as verified by infrared spectroscopy. The presence of hyperbranched and carboxymethylated products indicates the success of the branching and the carboxymethylation reactions. This is also confirmed by the increase in the value of the hydroxyl group.

Surface tension dependence on the concentration of m-HPAE at 25°C is given in Figure 8 m-HPAE is able to lower surface tension to around 28 mN/m, exhibiting excellence compared to the conventional surfactants such as OP-10 (around 31 mN/m) and LAS (around 32 mN/m).

Conductivity dependence on the concentration of m-HPAE is shown in Figure 9. The conductivity was increasing with the increasing concentration of m-HPAE; however, the rate of the increase was different presenting a turning point (intersection of both lines) in the trend. This turning point, namely, CMC, is calculated to be 433.63 mg/L. The Krafft point is determined to be 5°C, which means that it can be used under room temperature conditions.

Adsorption dependence on the concentration of m-HPAE is given in Figure 10. The adsorption was increasing with increasing concentration of m-HPAE. In the range of 400–500 mg/L concentration, the adsorption onto the oil sand surface achieved a stable value. The CMC was also within this range, indicating that the adsorption equilibrium was achieved at the CMC. When the concentration was above 500 mg/L, the adsorption started to increase, breaking the stabilization. It is well-known that the dual layers of electrodiffusion were formed at the surface of the oil sands when contacting an aqueous solution. The negative ions in the surfactant solution were opposite to the electric charge on the surface of the oil sands, resulting in reduction in electrostatic action and weakening in the dual layers of electrodiffusion. More surfactant monomers tend to seek adsorption vacancy at higher concentration.

Absorbance dependence on benzene content at a wave number of 560 nm is shown in Figure 11. When benzene content was more than 40 μl, the absorbance of the solution was increasing dramatically. Micellar solubilization was determined to be 600 ml/mol at 40 μl of benzene content.

Figure 12 shows that the contact angle (a) of the untreated rock sample is 116.2°, and the contact angle (b) after treatment is reduced to 61.1°, indicating that m-HPAE adsorbed on the rock surface can effectively improve the hydrophilicity of the rock surface, reduce the flow resistance of oil in water fluid in the water flooding system, reduce the possibility of blockage during the migration of formation of lipophilic particles, and achieve the purpose of improving formation permeability and enhancing oil recovery.

Figure 13 shows that the apparent viscosity of the m-HPAE solution decreases with the increase in salinity. After the -COONa group is ionized in the solution, the -COO^- group is found to be electronegative. The mutual exclusion between groups results in the molecular chain being more stretched, the flow resistance increases, and the viscosity increases. When the high-concentration salt solution is added to the system, the cations gather around the electronegative groups, shielding the effective charge, reducing the anion mutual exclusion, shrinking the molecular chain structure, and reducing the flow resistance and viscosity. The m-HPAE solution maintains the shear thinning characteristic when adding the low-concentration salt solution (1000 mg/L), while after adding the high concentration salt solution (10000 mg/L), the shear thinning effect decreases with the increase of the shear rate, which is similar to Newtonian fluid, and the viscosity is maintained near 31 MPa s.

The formulation of m-HPAE/LAS and m-HPAE/NaOH with various ratios was prepared, and the interfacial tension was measured using a TX-500C interfacial tensiometer, as shown in Figure 14. Figure 14A demonstrates the interfacial tension trend with various mass ratios of m-HPAE/LAS. The interfacial tension can be lowered to a level of 10^–2 by individual m-HPAE or LAS; however, this value was further lowered to a level of 10^–3 using the formulation of m-HPAE/LAS due to the synergistic effect exerted by petroleum sulfonate and the HB carboxylate surfactant. The negatively charged oxygen atoms in sulfonic groups enhanced the interaction with crude oil by making full use of anionic surfactants’ advantage of maximum interfacial adsorption, leading to a lower interfacial tension at a relatively low usage. Being formulated with m-HPAE, this advantage was further exhibited to achieve the optimum effect with least usage. The interfacial tension was lowered to 3.5 × 10^–3 mN/m at 1:3 mass ratio of m-HPAE/LAS.

Figure 14B depicts the interfacial tension change with different mass fraction of NaOH in 0.04 wt% m-HPAE. The interfacial tension was also lowered to a level of 10^–3 using the formulation of m-HPAE/NaOH, and the three mass fractions of NaOH did not present obvious difference. The reduction in interfacial tension was caused by competitive adsorption of m-HPAE and the alkali. The active materials were also produced by the reaction between the alkali and the acid in the crude oil, resulting in further reduction in interfacial tension.

Due to the well-known reservoir damage caused by alkali, it is not recommended to be used for flooding. The LAS was proven to be able to lower interfacial tension to the same level as NaOH when being formulated with m-HPAE; therefore, only the binary surfactant system of m-HPAE/LAS was used in core flooding experiments. The m-HPAE flooding and water flooding experiments were also conducted for comparison. The permeability of heterogeneous cores ranged from 100 mD to 1000 mD. 0.04 wt% m-HPAE and 1:3 mass ratio of m-HPAE/LAS were used in the flooding experiments. The results of m-HPAE flooding and m-HPAE/LAS flooding are listed in Table 1 and Table 2, respectively.

As shown in Table 1, the oil recovery was further enhanced by m-HPAE flooding after 98% water cut was observed in water flooding. The increment was ranging from 10.4 to 25.15% in the group experiments. Two aspects of the HPB contribute to enhanced oil recovery. On the one hand, m-HPAE, as described above being a surfactant, is able to lower interfacial tension to a great extent and on the other hand, the m-HPAE structure with an inward lipophilic group and many outward hydrophilic groups creates the perfect space for the migration of the oil drops.

As is given in Table 2, oil recovery was further enhanced by m-HPAE/LAS following water flooding, ranging from 19.36 to 32.21%. The formulation made full use of the combined chemicals, namely, polymer, for water inflow profile modification to enhance sweep efficiency and surfactants for lowering interfacial tension. The synergistic effect resulted in the formulation being more efficient, especially in low permeability cores, than the single use of m-HPAE.