Preparation and Property Study of Highly Efficient Demulsifier for Oil-in-Water Emulsion at Low Temperature
Introduction
Against the backdrop of tightening domestic oil and gas resource supply, ensuring stable production from mature oilfields has become as important as exploring new fields. As mature oilfields are exploited more intensively, recovery has entered secondary or tertiary stages, with many fields reaching ultra-high water cut phases. To stabilize the recovery rate of high-water-cut oilfields, targeted high-performance oil-displacing agents with ultrahigh interfacial activity and excellent emulsifying capacity have been widely adopted [1~4]. The presence of surface-active substances in injected water leads to complex oil-water emulsification properties in produced fluids, making demulsification challenging. Meanwhile, the high oil content in produced water fails to meet environmental discharge standards. To ensure the smooth implementation of surfactant flooding, rapid and efficient demulsification of produced water is required, underscoring the importance of research on oil-in-water demulsifiers.
Currently, common demulsifiers for oil-in-water emulsions mainly include cationic types, sulfonated polystyrene, and polyacrylic acid [9~11], most of which require heating and combined use with flocculants [12]. This adds pressure related to equipment corrosion, aging, and energy consumption reduction in mature oilfields, making the development of novel low-temperature, high-efficiency oil-in-water demulsifiers essential for supporting stable production in high-water-cut mature oilfields.
Targeting oil-displacing agent-stabilized oil-in-water emulsions, this study prepared a cationic polymeric demulsifier (SHP), investigated its laboratory-scale performance and demulsification mechanism, and provided an efficient technical solution for treating oil-in-water produced water after tertiary oil recovery.
1. Experimental Section
1.1 Raw Materials and Instruments
Epichlorohydrin, 40% aqueous dimethylamine solution, and petroleum ether (boiling range 90~120 °C) were chemically pure reagents purchased from Sinopharm Chemical Reagent Co., Ltd.; absolute ethanol was analytically pure, also sourced from Sinopharm Chemical Reagent Co., Ltd.; SH-HK-I oil-displacing agent (50% active content, industrial grade) was supplied by Sinopec Shanghai Research Institute of Petrochemical Technology Co., Ltd.; Hekou produced water (total salinity 15 000 mg/L) and Hekou crude oil (underground viscosity 44 mPa·s, surface viscosity 315 mPa·s) were collected from field sites.
Instruments used included an Evolution 201 UV-Vis spectrophotometer (Thermo Scientific) and a TX-500C spinning drop interfacial tensiometer (Temco).
1.2 Synthesis of Cationic Polymeric Demulsifier SHP
SHP was synthesized via polymerization of epichlorohydrin and dimethylamine, with the synthetic route shown in Figure 1.
The detailed procedure was as follows: Aqueous 40% dimethylamine solution and ethanol were added to a reactor and stirred until fully dissolved. The system temperature was maintained at 5 °C, followed by dropwise addition of epichlorohydrin. After the addition was completed, the reaction was allowed to proceed at a temperature below 50 °C for 4 h, then heated to 80 °C for an additional 5 h. The reaction was terminated, and the reaction mixture was collected and subjected to vacuum distillation to remove low-boiling solvents and volatiles, yielding a colorless transparent solid product, SHP.
1.3 Evaluation of Demulsification Performance of SHP
1.3.1 Preparation of Crude Oil Emulsion
A 0.3 wt% SH-HK-I oil-displacing agent solution was prepared using Hekou produced water. Then 15 g of Hekou crude oil was added to 1 L of the above solution, heated to 65 °C, shaken repeatedly to ensure homogeneity, cooled to room temperature, and left to stand for 1 week to obtain a stable crude oil emulsion.
1.3.2 Demulsification Test
SHP was first dissolved in Hekou produced water to prepare a 1 wt% active stock solution, which was further diluted with produced water to a 0.1 wt% working solution for use. For each test, 10 g of crude oil emulsion was placed in a test bottle, and SHP working solution was added at effective dosages of 50, 100, and 200 mg/L respectively under room temperature. The bottle was shaken 10 times, then left to stand at room temperature. Demulsification performance was observed every 10 min, and the state of oil-water interface was recorded photographically.
1.4 Determination of Oil Content in Water After Demulsification
1.4.1 Establishment of UV Absorbance Standard Curve for Crude Oil
A series of Hekou crude oil petroleum ether solutions with concentrations ranging from 10 to 150 mg/L were prepared, shaken to ensure homogeneity, and their absorbance in the wavelength range of 250~750 nm was measured at room temperature. The absorbance at the absorption peak of 260 nm was selected as the ordinate, and concentration as the abscissa, to plot the UV absorbance standard curve for Hekou crude oil. Linear regression was performed to obtain the relationship A = B*c + C, where A is absorbance, B is the absorptivity coefficient, c is the concentration of crude oil solution, and C is the correction coefficient.
1.4.2 Measurement of Oil Content in Post-Demulsification Samples
2 mL of water sample (lower layer) was transferred to a 5 mL PE tube, mixed with 2 mL of petroleum ether, sealed tightly, and shaken thoroughly for extraction. After standing to allow phase separation, 1 mL of the upper petroleum ether layer was collected, diluted uniformly to 5 mL, and further diluted to a concentration within the range of the standard curve. The absorbance was measured using a UV spectrophotometer, and the oil content in water was calculated based on the linear relationship and dilution factor.
1.5 Measurement of Oil-Water Interfacial Tension
Hekou crude oil was used as the oil phase, and the 0.3% SH-HK-I oil-displacing agent solution was used as the water phase. The rotational speed of the spinning drop interfacial tensiometer was set to 4500 r/min, the temperature was maintained at 65 °C, and the corresponding oil-water interfacial tension was measured.
2 Results and Discussion
2.1 Characterization of Cationic Polymeric Demulsifier SHP
SHP exhibited good water solubility, so deuterium oxide was used as the solvent to characterize its NMR spectrum, as shown in Figure 2.
As shown in Figure 2, the peaks at δ 3.28~3.41 ppm corresponded to hydrogen signals of methyl groups attached to nitrogen atoms; peaks at δ 3.56~3.80 ppm were assigned to hydrogen signals of methylene groups adjacent to nitrogen atoms; peaks at δ 4.58~4.62 ppm and δ 5.04~5.06 ppm corresponded to hydrogen signals on carbons bearing hydroxyl groups. A singlet at δ 3.18 ppm was attributed to hydrogen signals of methyl groups in free unreacted dimethylamine. The integration ratio of the NMR spectra indicated that over 97% of the dimethylamine raw material participated in the polymerization reaction, demonstrating high polymerization efficiency.
2.2 Demulsification Process
The demulsification performance of SHP is presented in Figure 3, where (a) shows the initial oil-water interface after adding 50, 100, and 200 mg/L SHP (left, middle, right respectively) to 10 g of 0.3% SH-HK-I crude oil emulsion; (b)–(e) show the oil-water interface at 0.5, 10, 20, and 30 min after shaking, respectively.
As shown in Figure 3, SHP exhibited good demulsification performance for the crude oil emulsion at all tested dosages. Within 30 min, the aqueous phase became clearer compared to the initial state. Particularly when the SHP dosage reached 200 mg/L (Figure 3(e), right), the lower aqueous phase was almost completely clear and transparent, with demulsified crude oil floating on the surface or adhering to the bottle wall, without any flocculation. This indicates that the demulsifier can efficiently break oil-in-water crude oil emulsions at low dosages without requiring additional harsh conditions. At relatively low dosages (50 and 100 mg/L effective dosage), although the aqueous phase remained yellowish after 30 min of demulsification, a large amount of demulsified crude oil was still produced, indicating that SHP retains certain demulsification effects even at low concentrations. Overall, higher SHP dosages resulted in faster demulsification rates and better demulsification performance.
2.3 Oil Content in Water After Demulsification
To further evaluate the demulsification performance of SHP, the oil content in water after demulsification was measured. The UV-Vis spectra of Hekou crude oil petroleum ether solutions at different concentrations are shown in Figure 4.
As shown in Figure 4, all Hekou crude oil petroleum ether solutions exhibited an absorption peak at 260 nm, which originated from aromatic components in the crude oil. The relationship between absorbance A and concentration c at 260 nm was determined as A = 0.01184c – 0.0344, with a coefficient of determination (R²) of 0.9933, indicating a strong linear correlation within the concentration range of 10–130 mg/L, which is suitable for quantitative determination of samples.
The oil content in the untreated crude oil microemulsion was measured to be as high as 11534 mg/L, confirming the ultrahigh emulsification and solubilization capacity of the oil-displacing agent for crude oil. Samples from demulsification tests with effective SHP dosages of 50, 100, and 200 mg/L were collected after 30 min of room-temperature demulsification, and the oil content in the lower water layer was measured. The results are summarized in Table 1.
| Sample | Oil content / (mg/L) | Oil removal rate / % |
|---|---|---|
| Crude oil microemulsion | 11534 | / |
| Crude oil microemulsion + 50 mg/L SHP | 554 | 95.2 |
| Crude oil microemulsion + 100 mg/L SHP | 313 | 97.3 |
| Crude oil microemulsion + 200 mg/L SHP | 105 | 99.1 |
As shown in Table 1, when the SHP dosage was 200 mg/L, the oil content in the post-demulsification water was only 105 mg/L, with an oil removal rate exceeding 99%, meeting relevant water treatment requirements.
Combined with Figure 3, although the aqueous phase appeared darker yellow at lower SHP dosages, the oil contents in water after 30 min of demulsification were 554 mg/L and 313 mg/L at 50 mg/L and 100 mg/L dosages respectively, with corresponding oil removal rates of 95.2% and 97.3%, confirming that SHP still delivers favorable demulsification performance at low concentrations. However, after the oil content drops significantly, the remaining oil-in-water emulsion requires a higher demulsifier dosage to achieve complete oil-water separation.
2.4 Effect of Demulsifier on Oil-Water Interfacial Tension
To investigate the action mode of the demulsifier, its impact on the oil-water interface in the aqueous phase was studied by measuring the oil-water interfacial tension at formation temperature (65 °C).
Hekou produced water was used to prepare 0.3% SH-HK-I oil-displacing agent mixed solutions with SHP effective dosages of 50, 100, and 200 mg/L respectively. Taking Hekou crude oil as the oil phase, the oil-water interfacial tension between these mixed solutions and the 0.3% SH-HK-I oil-displacing agent solution was measured sequentially using a spinning drop interfacial tensiometer, and the results are shown in Figure 5.
As shown in Figure 5, the interfacial tension between the 0.3% oil-displacing agent solution and crude oil could be maintained at 10-3 mN/m at 65 °C, and even reached 10-4 mN/m, indicating the ultrahigh interfacial activity of the oil-displacing agent.
When 50 mg/L SHP was added to the 0.3% oil-displacing agent solution (Figure 5a), the oil-water interfacial tension could still reach 10-3 mN/m and remain stable for 1 h. This indicates that low-concentration SHP does not affect the interfacial activity of the oil-displacing agent, meaning SHP does not enter the oil-water interface to alter the composition of the interfacial film. Under this dosage condition, 95% of the emulsified crude oil was broken, which is primarily attributed to the positively charged demulsifier compressing the electric double layer of the emulsion through electrostatic interaction, promoting the coalescence of emulsion droplets and thus driving the demulsification process [13].
As the SHP dosage increased to 100 mg/L (Figure 5b), the oil-water interfacial tension rose, and the overall interfacial activity weakened. Although the interfacial tension reached 10-3 mN/m at 55 min, the inhibitory effect of SHP on interfacial activity had already emerged, indicating that demulsifier molecules began to partially enter the oil-water interface, altering the composition of the interfacial film, further reducing emulsion stability, and enhancing the degree of demulsification.
When the SHP dosage reached 200 mg/L (Figure 5c), the oil-water interfacial tension could only be maintained at 0.1 mN/m. Compared with the ultralow interfacial tension of the original oil-displacing agent solution, the system essentially lost interfacial activity. This indicates that under this dosage condition, the demulsifier exerts a deeper demulsification effect via the displacement mechanism [14]: demulsifier molecules enter the interfacial film and almost completely destroy it, achieving thorough demulsification.
Therefore, the demulsification process of SHP for crude oil emulsions involves two synergistic mechanisms: electric double layer compression and interfacial film displacement. The electric double layer compression mechanism contributes more to the overall oil removal rate, accounting for the majority of demulsified crude oil when the oil content is high. The displacement mechanism plays a more prominent role in breaking residual emulsions at low oil content, enabling thorough demulsification with low residual oil in water.
3 Conclusions
A cationic polymeric demulsifier (SHP) was synthesized using epichlorohydrin and dimethylamine as raw materials. The synthesis method features simplicity, high efficiency, easy post-treatment, and no generation of polluting waste. At room temperature and a dosage of 200 mg/L, SHP reduced the oil content in the water phase of the corresponding oil-displacing agent-crude oil emulsion to 105 mg/L within 30 min, achieving an oil removal rate of 99.1%. The post-demulsification aqueous phase was clear and transparent, with no discoloration.
The demulsification process of SHP for crude oil emulsions involves two synergistic mechanisms: electric double layer compression and interfacial film displacement, which together realize high oil removal rate and low residual oil content in water. SHP provides a feasible treatment solution for oily wastewater generated after tertiary oil recovery, and follow-up research will focus on field application verification and promotion.