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ACS Omega
ACS Omega
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ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c07009
Article
The Influencing Factors and Mechanism of Anionic and Zwitterionic Surfactant on Viscosity Reduction in Heavy O/W Emulsions
https://orcid.org/0000-0003-4219-0829
Jing Jiaqiang †‡
https://orcid.org/0000-0002-3895-5908
Shan Yuting *†
Wang Ning †
Sun Jie †‡
Jiang Cancan †
Cao Lei †
Song Xiyuan §
† School of Oil & Natural Gas Engineering, Southwest Petroleum University, Chengdu 610500, China
‡ Oil & Gas Fire Protection Key Laboratory of Sichuan Province, Chengdu 610500, China
§ School of New Energy and Materials, Southwest Petroleum University, Chengdu 610500, China
* E-mail: shanyt12138@163.com.
06 09 2024
17 09 2024
9 37 3925939276
31 07 2024
30 08 2024
22 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The high viscosity of heavy crude oil has been an obstacle to its safe production and economic transportation. In this work, a screened emulsified viscosity reducer system is conducted. Experimental results demonstrate that the most effective viscosity reducing agent comprises sodium oleate (NaOl) and cocamidopropyl betaine (CAB-35) in a ratio of 1:2, achieving a viscosity reduction rate of 94.65%. Additionally, the interfacial tension between oil and water decreases from 27 to 4 mN/m with 0.1 mass % TEOA and NaOH in a 1:1 ratio. The oil droplet size is uniformly distributed with Dmean is 14 μm and D50 is 11 μm. Droplets flocculate as the salinity increases to 0.2 mol/L, which corresponds to the apparent increase of viscosity. The adsorption of long alkyl chain lipophilic groups on surfactant molecules at the oil–water interface and the water film alters the wettability of pipe steel to water-wet, further enhancing the application of emulsification and viscosity reduction effects. The primary mechanism behind the viscosity reduction in emulsification is attributed to strong electrostatic interactions stemming from molecular electrostatic potential distributions.

National Natural Science Foundation of China 10.13039/501100001809 52106208 Natural Science Foundation of Sichuan Province 10.13039/501100018542 2023NSFSC0924 document-id-old-9ao4c07009
document-id-new-14ao4c07009
ccc-price
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pmc1 Introduction

Currently, the majority of recoverable oil resources in many countries are heavy crude with API degrees between 10.1 and 22.3.1 Large-scale heavy crude oil reservoirs have been discovered in multiple blocks and layers in the South China Sea with continuous exploration (Figure 1). However, the poor fluidity of heavy crude oil in pipelines makes it difficult to deliver economically and efficiently. Various viscosity reduction technologies have been investigated to tackle these challenges, such as diluting light oil, pipe heating, microwave heating, modification, using viscosity reducers, and microbial viscosity reduction. Offshore platforms encounter space constraints and demand efficient transportation and construction of raw materials. Consequently, surfactant emulsification viscosity reduction has emerged as a viable option due to the availability of materials and the simplicity of the process.

Figure 1 History of offshore oil and gas production in China.

The mechanical stirring disperses the heavy crude oil into small droplets, with the surfactant aqueous solution acting as the continuous phase. This emulsification process effectively reduces the apparent viscosity by converting friction between oil droplets into water films. Emulsifying viscosity reducers can be classified into various types based on their groups and compositions, including anionic, cationic, zwitterionic, and nonionic. Each type has its own set of advantages and disadvantages.2 Screening and application of emulsion viscosity reducers have been conducted in recent decades. The applications and mechanisms of surfactants in different oil fields are summarized in Table.1.

Table 1 Application and Characteristics of Oilfields with Different Viscosity Reduction Methods

Type	Characteristics	Applications	Surfactant	Viscosity reduction effect	Viscosity reduction mechanism	
Anionic	High output	PT. Pertamina EP Asset3	SLS	53.9%(DR)	Molecules attached to the interface film and form the electric double layer to enhance the droplet electrostatic repulsion4	
Low prices	
Material availability	Western Indian oilfield5	SDS	>99%	
Renewable and biodegradable	Xinjiang oilfield6	SDS	>90%	
Bohai oilfield7	AAGASs	96.8%	
Cationic	Little effect on heavy oil	Egyptian waxy crude oil8	Bola form	81.84%	Salting-out effect to change the driving force and mostly used in recovery enhancing9	
Good compatibility	Iranian oilfield10	CTAB	/	
Easily react with alkali	
Limited adaptability	
Zwitterionic	High surface activity	Indonesia T-KS11	SAE	/	The ionic groups regulate electrostatic interaction o realize π–π stacking12	
Low toxicity	Tahe oilfield13	RCOO–DMEAH+	98.4%	
Low irritation	
Good salt tolerance	
Nonionic	High output	Liaohe oilfield14	AOS	98.89%	Molecules attached to the interface film and form micelles15	
High stability	Indian Mehsana oilfield16	Madhuca longifolia	70.84% (DR)	
Good compatibility	
Has “cloud point”	

Traditional surfactants have undergone extensive research and have shown a beneficial impact on reducing viscosity. However, certain limitations have been observed such as poor salt tolerance for anionic surfactants, high prices for cationic surfactants, and cloud points for nonionic surfactants. Therefore, it becomes crucial to employ a compounding method to overcome the drawbacks associated with individual agents.17 Ma18 evaluated the viscosity reduction effect of zwitterionic cocamidopropyl sulfobetaine (CSB) and anionic sodium dodecylbenzenesulfonate (SDBS) on Shengli heavy crude oil. The combination of CSB and SDBS can be closely arranged at the heavy crude oil–water interface, which enhances the strength of the interface film. Si19 synthesized the anionic-nonionic surfactant SYW using 1,3 propylene glycol polyether (PPG), boric acid, maleic anhydride (MA), and sodium metabisulfite. SYW demonstrated a significant viscosity reduction of 98.6% in Xinjiang crude oil. Some references report different structural surfactants for viscosity reduction and emulsification of heavy oil such as different chain lengths20 and electrical properties.21

Alkali is also used as a synergistic agent to reduce heavy crude oil viscosity by reacting with the acidic components22 and reducing the interfacial tension.23 Additionally, surfactant and alkali can also improve the wettability of rock and pipeline surface,24 leading to enhanced transportation efficiency and cost reduction.25 Gao26 compounded CAO-15, sodium oleate and NaOH. It resulted in a significant reduction in viscosity rate by 97.46% and a decrease in pour point by 25 °C. Liu27 optimized a compound system of fatty alcohol polyoxyethylene (AEO-12), sodium dodecyl sulfate (SDS) and NaOH for Liaohe heavy crude oil. This system significantly reduced the size of oil droplets to 0.2 μm and the interfacial tension to 0.008 mN/m. The addition of alkali enhanced the stability of the emulsion by tightly arranging the surfactant ion diffusion layer. Further research is still needed to investigate the varying effects on viscosity as there is currently no consensus.28 For instance, Abdurahman29 observed a slight increase in viscosity as the pH went from 5 to 8. While, Kumar30 found a reduction in viscosity as the pH increased from 7 to 11. Pu31 suggested that the viscosity of the emulsion would decrease under acidic and alkaline conditions.

Current research predominantly focuses on the mechanism of viscosity reduction through the single surfactant agent or binary surfactants interaction. Zhang32 conducted molecular dynamics simulations to reveal that anionic surfactants interact with polar macromolecules through weak dipole–dipole force, CH−π interaction and N–O interaction. Cai33 observed challenges in achieving lower interfacial tension on the decane-water surface with a single amphoteric betaine. The competitive adsorption occurs between betaine and anionic surfactant, leading to an increased density of interface surfactant molecules. Zhang34 synthesized an amphiphilic copolymer that weakly interacts with asphaltene disrupting asphaltene association structures and extending distances between asphaltene groups to achieve viscosity reduction in O/W emulsions. Wang35 demonstrated that the nonionic viscosity reducer TPVR7 and anionic disuccinimide suberate (DSS) can enhance the anticoalescence performance of oil droplets in emulsions and strengthen interfacial films. The hydrophilicity of the viscosity-reducing agent group promotes its adsorption on the interface film. However, interactions between the two agents are not considered.

Current research on O/W emulsions primarily focuses on Newtonian fluids with a water content exceeding the inversion point. However, in practical applications, the water content for reducing the viscosity in heavy crude oil emulsions is typically around 30%. Additionally, the diverse components of oil pose challenges in using existing surfactant systems as a reference. While there is extensive research exploring the detailed mechanisms of viscosity reduction, there is a lack of substantial evidence at the molecular scale and standardized evaluation criteria. Therefore, it is essential to explore different surfactants and viscosity reduction mechanisms to streamline the screening process of viscosity reducers and reduce the costs associated with industrial applications.

This study focuses on the alkali-sensitive characteristics of anionic and zwitterionic surfactants to achieve stable emulsification and viscosity reduction of heavy crude oil. It is determined that the optimal anionic-zwitterionic-alkali surfactant agent is composed of NaOl, CAB-35, TEOA, and NaOH by evaluating the viscosity reduction rate. Microscopic/scan electron microscope observation (SEM), interfacial tension test, and molecular simulation are conducted to identify the morphological transformation of emulsion droplets and the mechanism of surfactant synergism. The insights obtained from this investigation provide valuable knowledge for engineers in comprehending the impact of emulsification factors and mechanisms on alleviating crude oil pipeline transportation consumption.

2 Experimental Section

2.1 Materials and Test Instruments

Degassed and dehydrated heavy crude oil is from the South China Sea. The surfactant reagents obtained as follows, anionic surfactants sodium dodecyl sulfate (SDS), sodium dodecyl benzenesulfonate (SDBS), sodium alpha-olefin sulfonate (AOS), sodium oleate (NaOl), fatty alcohol polyoxyethylene ether sulfate Sodium (AES), sodium fatty alcohol polyoxyethylene ether carboxylate (AEC), zwitterionic surfactant cocamidopropyl betaine (CAB-35), dodecyldimethyl betaine (BS-12), nonionic surfactants sorbitan monolaurate (Span-20), octylphenol polyethylene ether (Triton X-100), polysorbate (Tween-80) and octylphenol polyoxyethylene ether 10 (Op-10). The surfactant reagents are purchased from Shandong Yousuo Chemical Co., ltd.

2.1.1 Density

The heavy crude oil density is measured using a 10 mL pycnometer, an electronic balance, and a thermostatic water bath. Sample density at different temperatures is operated steps are as eq 1. The heavy oil density at 20 °C is 939.75 kg/m3.1

where ρo is the oil sample density, kg/m3; m0 is the quality of pycnometer and oil sample, kg; mp is the quality of pycnometer, kg; and Vp is the volume of pycnometer, 10 mL.

2.1.2 Viscosity

The coaxial cylinder rotor, coaxial cylinder, and HAAKE intelligent rheometer are used to assess the viscosity of heavy crude oil. The shear rate is set as 0–1000 s–1 and the temperature is set as 20–90 °C driven by the heating module and the rotating module of the rheometer. The heavy oil viscosity is 5047.52 mPa s at 20 °C.

2.1.3 Saturates, Aromatics, Resins, and Asphaltene (SARA)

The heavy crude oil is divided into four components according to the “NB/SH/T0509–2010 Test method for separation of asphalt into four fractions”. The N-heptane, toluene, and ethanol are used to flush through the adsorption column which is filled by activated alumina. The mass fraction of asphaltene, saturates, aromatics, and resins are 22.07%, 48.07%, 18.17%, and 11.69%, respectively. High levels of colloidal asphaltenes can affect the crude oil viscosity.

2.1.4 Hydrocarbon Component

Oil hydrocarbon components are tested by Agilent 7890A chromatograph. The heavy crude oil is poured into the beaker, sampled with a 10 μL microinjector, and injected into the chromatograph. As shown in Figure 2, the carbon number distribution of heavy oil exhibits a broad range with the peak content observed at C12, accounting for 13.06 wt %. Oil density, viscosity, SARA and component properties are summarized in Table 2.

Figure 2 Oil hydrocarbon components distribution.

Table 2 Properties of Heavy Crude Oil

Test	Value	Compound	Fraction (wt %)	
Density (kg/m3)	939.75 (20 °C)	C6–C10	6.76	
Viscosity (mPa s)	5047.52 (20 °C)	C11–C15	50.62	
Saturates (wt %)	48.07	C16–C20	9.47	
Aromatics (wt %)	18.17	C21–C25	10.20	
Resins (wt %)	11.69	C26–C35+	22.95	
Asphaltene (wt %)	22.07	∑Cn	100.00	

2.2 Preparation of Samples

O/W emulsions are prepared by the HH-4 digital display constant temperature water bath (Changzhou Guohua Electrical Appliance Co., Ltd.) and GJ-3S digital display electronic stirrer (Qingdao Xinruide Petroleum Instrument Co., Ltd.). Initially, the crude oil and surfactant solution are preheated in the water bath to 30 °C for one hour. Subsequently, 70 mL of oil is uniformly injected into 30 mL of surfactant solution at a stirring rate of 1000 r/min and a stirring time of 90s. After addition of all the crude oil, the mixture is stirred for an additional 90s. The oil samples, solution samples, and prepared emulsions are tested for rheology, surface tension, and microscopic properties. The experimental process is summarized in Figure 3.

Figure 3 Experimental process for sample preparation and characterization test.

2.3 Experimental Device and Processes

2.3.1 Rheological and Water Separation Evaluation

Different types of rotors and cylinders are used for emulsions with varying viscosities, as shown in Figure 4. The coaxial cylinder rotor and coaxial cylinder are employed for heavier emulsions (Figure 4a and Figure 4c). The double slit rotor and double slit cylinder are used for low viscosity O/W emulsions (Figure 4b, Figure 4d). The emulsion samples prepared in section 2.2 are immediately transferred to the rheometer. The shear rate is set as 0–1000 s–1 and the temperature is set as 20–90 °C. The viscosity reduction rate is calculated according to the following eq 2.2

where δ is viscosity reduction rate, %; ηo is the viscosity of crude oil, mPa s; ηe is the viscosity of emulsion, mPa s.

Figure 4 rheometer components and test tubes: (a) coaxial cylinder rotor; (b) double slit rotor; (c) coaxial cylinder; (d) double slit cylinder; (e) rheometer; (f) test tube.

The emulsion prepared in Section 2.2 is transferred to a 100 mL test tube (Figure 4f). Initially, the total height of the emulsion is measured, followed by hourly measurements of the height of separated water. The water separation rate of the emulsion is determined using eq 3.3

where f is the water separation rate, %; v1 is the volume of precipitated water, mL; v2 is the total volume of emulsion, mL; h1 is the height of precipitated water, cm; and h2 is the total water height, cm.

2.3.2 Interfacial Tension Test

As shown in Figure 5, the JC2000D2 contact angle measuring instrument (Shanghai Zhongchen Digital Equipment Co., Ltd.) is utilized to measure the surface tension and interfacial tension at the temperature of 30 °C. The microsyringe is installed on the fixed frame to control the oil output being 3 μL each time (Figure 5a). The data are recorded once the shape of oil droplet stabilized and measured by the pendant-drop method (Figure 5b).

Figure 5 Interfacial tension test components: (a) testing device; (b) captured image.

2.3.3 Droplet Size and Distribution Analysis

As depicted in Figure 6a, the transmission and reflection polarizing microscope (Shanghai Cai Kang Optical Instrument Co., Ltd.) is used to observe the O/W emulsion. The objective lens had magnifications of 10, 25, and 60 times, corresponding to the droplet size (Figure 6b). The mean droplet diameter (Dmean) and median droplet diameter (D50) are evaluated by ImageJ software.

Figure 6 Microscopic observation. (a) Microscope; (b) eyepiece and emulsion slide.

2.3.4 Morphology Analysis at Micrometer-Scale

Scanning electron microscopy (SEM) is utilized to examine the microscopic morphology of crude oil and emulsion on the pipe surface (Figure 7). The materials undergo pretreatment with drying and gold spraying. Subsequently, the sample is chemically characterized and elementally analyzed using Energy Dispersive Spectroscopy (EDS). Elements are identified based on the peak positions in the spectrum, with signal intensity reflecting element concentration.

Figure 7 SEM analysis. (a) EVO MA 15 SEM; (b) SEM structure and samples.

2.3.5 MD Simulation Methodology

Asphaltene molecules are distinguished by the presence of hydroxyl groups, ketone groups, and heteroatoms, all of which significantly have a notable influence on the physical characteristics of crude oil. Researchers commonly rely on the average molecular structure to study asphaltene mixtures. The classic molecular structure model of asphaltene is presented by Jia.36 The molecular structures of surfactants and asphaltene are shown in Figure 8. The density functional theory (DFT) DMol3 program in Materials Studio is utilized to investigate the electrostatic potential of surfactants and asphaltene molecules. The interactions between molecules are obtained by using the Forcite module. For DMol3 settings, surfactant and asphaltene molecules are optimized using the Generalized Gradient Approximation (GGA) with a fine quality energy of 10–5 Ha, maximum of 500 iterations, and a maximum step size of 0.3 Å. For Forcite settings, the smart algorithm is employed, utilizing the COMPASS III force field with an energy of 0.001 kcal/mol and a force of 0.5 kcal/mol/Å. The geometry-optimized asphaltene molecules are annealed using the Anneal module through 10 cycles in the COMPASS III force field of the NVT ensemble at temperatures ranging from 300 to 500 K. The electrostatic and van der Waals force are calculated using the Atom-based algorithm, with a time step of 1 fs, Nose thermostat at 298 K, and initial random speed.

Figure 8 Molecular structures of asphaltene and surfactants. (atom color: gray, C; white, H; red, O; yellow, S; blue, N; purple, Na).

3 Results and Discussion

3.1 Effects of Surfactant Type and Compounding on Viscosity

The O/W emulsion preparation is carried out under the following conditions: deionized water with a mass fraction of 1% surfactants, emulsification temperature of 30 °C, and stirring rate of 1000r/min. Different surfactants exhibit varying effects as shown in Figure 9a. These emulsions are considered to be shear dilution fluids. Notably, the anionic surfactant NaOl, the zwitterionic surfactant CAB-35, and the nonionic surfactants Span-20, demonstrate significant viscosity reduction effects of over 90%. Figure 9b illustrates the water separation rates of emulsions over a 6 h period. The water separation rates for individual agent emulsions of NaOl and CAB-35 are 47.01% and 49.56% respectively, while Span-20 and OP-10 exhibit rates of 59.21% and 60.32%. To further investigate viscosity changes in the emulsion, NaOl and CAB-35 are used as primary agents, supplemented by Span-20 and OP-10.

Figure 9 Impact of surfactant single agent type on viscosity and water separation. (a) Emulsion viscosity; (b) water separation rate.

The effect of compounding two surfactants is shown in Figure 10. The combination of NaOl and CAB-35 at a mass ratio of 1:2 has the best viscosity reduction effect, with the viscosity reduction rate is 94.65%. NaOl aqueous solution is alkaline and can be partially hydrolyzed into NaOH to react with the petroleum acid in heavy oil. The carboxylate and the double bond can increase the curvature of the oil–water interface film.37 It is more conducive to emulsification and viscosity reduction. Additionally, the molecular structure of CAB-35 contains both anionic and cationic polar functional groups, which enhances its water hardening ability.38 CAB-35 also exhibits a significant viscosity reduction effect both as a standalone agent and in combination with anionic NaOl. The compounds of NaOl and CAB-35 are selected for subsequent experiments due to the compatibility.

Figure 10 Impact of the compound surfactant agent type on viscosity.

3.2 Effects of Emulsification Conditions on Viscosity

3.2.1 Surfactant Content

According to the results presented in Figure 11, the emulsion viscosity initially decreases and then increases as the surfactant concentration increases. The most significant reduction in viscosity occurs at a surfactant concentration of 2%. This can be attributed to the gradual increase in the number of surfactant molecules adsorbed on the oil–water interface prior to the concentration of 2%. The interface film strength gradually improves, leading to a reduction in viscosity.39 However, at higher surfactant concentrations, the effect of adsorption on the heavy components in the crude oil becomes saturated. Consequently, additional surfactant molecules will contribute to an increase in the viscosity of the external phase, leading to an overall increase in the emulsion viscosity.40 Given that a surfactant content of 1% effectively reduces viscosity, we subsequently use 1% as the baseline concentration to minimize costs in a follow-up experiment.

Figure 11 Impact of compound mass fraction of the surfactant agent on viscosity.

3.2.2 Alkali Type and Content

The inorganic bases NaOH, Na2CO3 and organic base TEOA are selected to carry out viscosity reduction experiments with the mass fraction of 0.1–0.5%. Figure 12 illustrates the effect of these substances on viscosity reduction. The optimal viscosity reduction rates are observed at 94.95% (0.1% NaOH), 93.53% (0.3% Na2CO3), and 94.13% (0.2% TEOA), respectively. Polar functional groups present in heavy oil, such as colloids, asphaltenes, naphthenic acids and fatty acids have the ability to ionize easily with alkali.41 As a result, active substances are formed that can work in synergy with the existing surfactant compounding system. It stabilizes a greater number of oil–water interfaces, leading to a more even dispersion of oil droplets with smaller particle sizes. The hydrolysis ionization equilibrium equations of the three bases are shown as eqs 4 to 7.4

5

6

7

In the case of strong alkali (NaOH), the viscosity of the emulsion increases with rising alkali content. Conversely, for weak alkalis (TEOA and Na2CO3), the emulsion viscosity initially decreases, before subsequently increasing. Among the three alkalis, NaOH is a fully ionized strong base that generates the highest number of surface-active substances and exhibits the strongest stabilizing effect. Weak bases have a limited ability to react with macromolecular acidic substances. And low concentrations of alkali cannot fully ionize the petroleum acid in heavy oil. Consequently, as the alkali concentration increases, the dissociation effect gradually strengthens. It leads to the decrease in emulsion viscosity once the concentration reaches the optimal viscosity reduction rate.42 However, as the alkali concentration continues to increase, the balance between dissociated and undissociated acids is disrupted, resulting in the decrease in the effectiveness of viscosity reduction.

Figure 12 Impact of the alkali mass fraction on viscosity.

Surfactant solutions are prepared consisting of 1% NaOl and CAB-35 at a mass ratio of 1:2, combined with various alkalis. The crude oil is emulsified with the solution at a ratio of 7:3 at 30 °C and 1000 rpm. The emulsion is then transferred into a 50 mL colorimetric tube. Subsequently, the heights of the oil and solution phases are measured to calculate the water separating effect using the formula eq 8 provided below. The experimental results are presented in Table 3.8

where f is the water separating rate,%; v1 is the volume of precipitated solution, mL; v2 is the volume of emulsion, mL; h1 is the height of precipitated solution, cm; and h2 is the height of emulsion, cm.

Table 3 Effect of Alkali Compound on the Water Separation Rate of Emulsions

System	TEOA:NaOH	TEOA:Na2CO3	
Ratio of alkali agents	2:1	1:1	1:2	2:1	1:1	1:2	
24 h water separation rate (%)	7.24	2.50	5.02	16.32	17.15	21.60	

As shown in Table 3, a low-corrosive organic base is combined with inorganic bases at a concentration of 0.1%. The emulsion containing TEOA and NaOH, mixed in a 1:1 ratio, exhibited the lowest water separation rate after 24 h, measuring only 2.50%. Consequently, this formulation has been selected as the alkaline agent for this study to ensure the long-term stability and safe transportation of the emulsion on the offshore platform.

3.2.3 Oil–Water Ratio

Emulsions at oil–water ratios of 9:1, 8:2, 7:3, 6:4, and 5:5 are selected for viscosity reduction experiments. As shown in Figure 13, the emulsion quickly inverts into the water-in-oil (W/O) type when the oil–water ratio is 9:1. At this point, the surfactant has no viscosity-reducing effect. As the oil/water ratio decreases, the influence of water on the viscosity gradually increases. The emulsion viscosity also decreases with the shear weakened dilution effect. Sufficient surfactant molecules can generate and stabilize a larger oil–water interface area, resulting in smaller oil droplets and increased droplet separation distance.43 It leads to a reduction in collision frequency and frictional resistance, ultimately reducing the apparent viscosity of the emulsion. From safety and economic standpoint, it is preferable to maintain an oil/water ratio of 7:3. Increasing the content beyond this ratio will not significantly reduce the emulsion viscosity.

Figure 13 Impact of the oil–water ratio on viscosity.

3.2.4 Emulsifying Temperature

The viscosity reduction effects at emulsification temperatures of 30 °C, 50 and 70 °C are shown in Figure 14. As the temperature increases, the emulsion viscosity initially increases and then decreases. The temperature-dependent effect on viscosity reduction can be attributed to the impact on the internal phase viscosity, the dissolution and adsorption effect of surfactant molecules at the interface, and the Brownian motion. Specifically, the increase in temperature enhances the solubility of surfactant molecules at the interface within the range of 30 to 50 °C. It strengthens the oil–water interface film and increases the shear resistance during rheological analysis. The macroscopic performance is characterized by an increase in the viscosity. The continuous increase in temperature causes polar substances to become more soluble and weakens the associations adsorbed on the oil–water interface. As a result, the surface charge density of the oil droplets decreases, and the repulsion weakens. The distribution of surfactant molecules on the oil–water interface film is looser. Besides, the enhanced Brownian motion of oil droplets leads to faster aggregation and sedimentation.44 The water phase wrapped in the oil droplets also accelerates to precipitate. Therefore, 30 °C is identified as the most suitable emulsification temperature.

Figure 14 Impact of emulsifying temperature on viscosity.

3.2.5 Stirring Rate

The effects of stirring rate at 500, 1000, and 1500 rpm on viscosity reduction are shown as Figure 15. The results indicate that the emulsion viscosity initially decreases and then increases significantly as the stirring rate rises. This phenomenon can be attributed to the fact that an optimal stirring rate (1000 rpm) promotes the uniform distribution of oil droplets.45 However, an excessive stirring rate leads to a smaller particle size and a larger number of dispersed droplets. This excessive dispersion overconsumes the limited surfactant molecules,46 resulting in a loose arrangement of the interface film and a reduction in strength. Consequently, the O/W emulsion becomes unstable and undergoes phase reversal, leading to an increase in the viscosity.

Figure 15 Impact of stirring rate on viscosity.

3.2.6 Inorganic Salt Type and Content

Formation water contains a significant amount of inorganic salt ions. The type and content of these ions play a crucial role in the hydrophobic effect, alkali loss, and double electric layer. The inorganic salts (NaCl, KCl, CaCl2, MgCl2, Na2SO4, NaHCO3, and Na2CO3) are selected as the research objects. The changes in the emulsion viscosity with different types and concentrations of ions are presented in Figure 16a. The viscosity of emulsions containing ions is higher compared to emulsions without ions. The data indicate an initial increase in emulsion viscosity, which is subsequently followed by a decrease as ion concentration rises. Turning points are observed at approximately 0.15 mol/L for bivalent cations and 0.2 mol/L for monovalent cations and anions. Ions increase the polarity of the continuous phase due to salting-out effect at low concentrations. It prompts the migration of surfactant molecules to the oil–water interface and strengthened the film. However, this effect hinders rheological shear, resulting in a slightly higher viscosity. The gradually increasing ion concentration also screens the electrostatic repulsions between quaternary ammonium cations, enhances the lipophilicity of surfactant molecules.47 Counter ions neutralize excessive surface charges, leading to a weakening of electrostatic repulsion between the droplets.

Figure 16 Impact of ion type and content on viscosity.

Ca2+ and Mg2+ ions can react with OH– to form slightly soluble and insoluble substances, which reduces the effectiveness of active substances that stabilize the oil–water interface film.48 Organic bases and high-valent cations can form complexes, reducing the consumption of OH– in the surfactant solution. These combined effects result in a slightly higher emulsion viscosity with added Mg2+ and Ca2+ ions compared to the Na+ and K+ ions. As shown in Figure 16b, divalent anions have a more significant impact on the compression double electric layer and exhibit a stronger salting out effect compared to monovalent anions.49,50 NaHCO3 can undergo incomplete ionization and hydrolysis to generate OH–. It promotes the reaction of acidic substances in heavy oil, leading to a lower apparent viscosity of the system.

3.3 Mechanism of Viscosity Reduction

3.3.1 Interfacial Tension

The interfacial tension between heavy oil and various solutions is presented in Figure 17 and Table 4. Initially, the interfacial tension between heavy oil and deionized water is measured to be 27 mN/m (Figure 17a), which significantly decreases to 4 mN/m after adding the surfactant solution (Figure 17b).

Figure 17 Interfacial tension between oil-solution and double electric layer. (a) oil-deionized water; (b) oil and 1% NaOl and CAB-35 (1:2); (c) oil-1% compounding system and 0.1% alkalis (TEOA:NaOH = 1:1); (d) alkali-free solution with 2000 mg/L salinity; (e) 10 000 mg/L salinity; (f) 20 000 mg/L salinity; (g) alkaliferous solution with 2000 mg/L salinity; (h) 10 000 mg/L salinity; (i) 20 000 mg/L salinity; (j) droplet double electric layer.

Table 4 Interfacial Tension of Heavy Oil and Surfactant Solutions

Solution	Interfacial tension (mN/m)	Solution	Interfacial tension (mN/m)	
Deionized water	27	0.1% Na2CO3	 	4.13	
1% Compounding system (NaOl:CAB-35 = 1:2)	3.68	0.1% TEOA	 	5.07	
0.1% Alkaliferous (TEOA:NaOH = 1:1)	3.74	0.1% NaOH	 	3.64	
Alkali-free	2000 mg/L	3.38	Alkaliferous	2000 mg/L	3.30	
10000 mg/L	1.93	10 000 mg/L	3.19	
20000 mg/L	2.17	20 000 mg/L	2.58	

The introduction of 0.1% alkali substance results in a slight increase in interfacial tension due to changes in the molecular interaction between anionic and zwitterionic surfactants (Figure 17c). For alkaline conditions, the negative electrons of the carboxyl group of CAB-35 and the negative ions of the fatty acid group of NaOl associate through Na+. Conversely, the quaternary ammonium salt cation of CAB-35 interacts with the fatty acid anion of NaOl via ionic bonds. The interfacial tension between heavy oil and the TEOA solution is slightly larger due to the weaker ability to stabilize the interface.

The increase in mineralization contributes to a reduction in the interfacial tension (Figure 17d–i). Solutions containing 20 000 mg/L inorganic salts without alkali shows a decrease in IFT and increases to 2 mN/m. The viscoelasticity of surfactant solutions can be affected by the salinity, which in turn affects the mass transfer dynamics of surfactant and oil molecules at the oil–water interface.51,52 This can be attributed to the formation of a double electric layer by surface active molecules adsorbed on the droplet surface and the salting-out effect (Figure 17j).53 The thickness of the double electric layer is inversely proportional to the square root of the ion valence and concentration, as indicated in eq 9.9

where κ–1 is the equivalent thickness of electric double layer, m; e is the electronic charge, 1.6 × 10–19 C; ni is the number of ions; Zi is the ionic valence number; ε is the dielectric constant of the medium, F m–1; K is Boltzmann constant; σ is the surface charge density, C m–2.

The positively charged inorganic salt ions diffuse on the droplet surface, thereby compressing the thickness of the electric double layer. Consequently, the arrangement of surfactant molecules at the interface becomes more compact,54 leading to alterations in the electrical repulsion between the oil droplets.55,56

3.3.2 Droplet Size Distribution

Microscopic images are taken of emulsion samples made with heavy oil and various solutions, as shown in Figure 18. The particle size distributions of oil droplets are illustrated in Figure 19, which can be summarized as reversion W/O emulsion, O/W emulsions, and flocculation. Taylor57 described the deformation of the internal phase. The interface film of droplets is deformed by the external force, while the interfacial tension maintains the sphericity. The competition between these two factors determines the shape of the droplets. Two methods are employed to quantitatively study the influence of the oil droplet size and structures. The first method involves analyzing the mean droplet diameter (Dmean), while the second method focused on the droplet median diameter (D50).

Figure 18 Measurement of particle size. (a) Micrograph; (b) Vampix; (c) auto threshold; (d) particle analyze.

Figure 19 Morphology of emulsions. (a) Water droplet at oil–water ratio 9:1; (b) oil droplet at 1% compounding system (NaOl: CAB-35 = 1:2); (c) oil droplet at 1% compounding system and 0.1% alkalis (TEOA:NaOH = 1:1); (d) oil droplet 1% compounding system, 0.1% alkalis, and 0.2 mol/L inorganic salt.

Significant differences in droplet microstructure are detected in Figure 19. Figure 19a is prepared using an oil–water ratio of 9:1. In the W/O emulsions, the water droplet is small with the Dmean of 2.30 μm and the D50 of 1.88 μm (Figure 20a). The emulsion transformed into an oil-in-water type (Figure 19b) under the conditions of an oil–water ratio of 7:3, a mass fraction of 1% surfactant (NaOl: CAB-35 = 1:2). The oil droplets display an irregular spherical shape with a tail extending up to 300 μm. The Dmean is 57 μm, with the D50 value of 44 μm (Figure 20b). It can be observed that the size range of the oil droplets becomes narrower with the addition of alkalis in Figure 19c and Figure 20c. Additionally, there is less deformation on the outer surface, leading to an increase in roundness. A direct correlation among emulsion droplet size, emulsion stability and apparent viscosity has been revealed.58 Smaller oil droplets are indicative of a more stable emulsion system. The experimental results presented in Section 3.2.2 confirm that the stability of alkali-added emulsions is enhanced. In Figure 19d and Figure 20d, it is observed that a large number of loose chain flocs appeared in emulsions with adding alkalis and inorganic salts. The small oil droplets are found to be distributed in a three-dimensionally manner around the larger droplets, and these oil droplets did not coalesce upon collision. Microscopic measurements revealed that the particle size distribution of the system became narrower after flocculation, where Dmean is 3 μm and D50 is 2 μm.

Figure 20 Droplet size distribution of emulsions.

The degree of flocculation of oil droplets depends on the balance of attraction and repulsion, which mainly includes van de Waals, electrostatic, steric, depletion, and hydration.59 The surfactants used in this study are anionic and zwitterionic, leading to a combination of van der Waals forces and electrostatic repulsion as the main repulsive effects. In the absence of the surfactants, the repulsive force is strong enough to counteract the van der Waals force, avoiding oil droplet flocculation. The attraction between droplets increases and the droplets start to flocculate as the salinity increases to 0.2 mol/L. This corresponds to the point where the viscosity transition occurs as observed in Figure 16.

Previous studies have suggested that the adsorption amount of active agents on the oil–water interface increases with the increase in ion concentration.60 It results in a decrease in the effective radius of active substances. Therefore, at high ion concentrations (>0.2 mol/L) and low electrostatic repulsion, oil droplets tend to coalesce rather than flocculate,42,61 leading to a decrease in emulsion viscosity.62

3.3.3 Micrometer-Scale Appearance

SEM and EDS are two commonly used advanced microscopic analysis techniques. In Figure 21, the micromorphology and distribution of carbon atoms are presented for both crude and emulsion samples. Figure. 21a illustrates that crude exhibits a high wetting morphology on steel for filling the surface scratching traces. Upon the addition of surfactants, the wetting behavior of the emulsion on the metal surface diminishes, resulting in a clearer outline of the emulsion droplets in Figure 21b. This observation is further corroborated by the surface scanning of the carbon atomic distribution shown in Figure 21c and Figure 21d, indicating a shift in wettability due to the substitution of oil with the surfactant solution. The shift in wettability is caused by the adsorption of the long alkyl chain lipophilic groups on surfactant molecules at the oil–water interface. The polar hydrophilic groups extend to the interface, forming hydrogen bonds with water molecules and creating a water film that alters the wettability of the pipe.63 This alteration in wettability ultimately leads to a reduction in the flow resistance of emulsions during subsequent transportation.

Figure 21 SEM and EDS images of crude and emulsion: (a) crude SEM; (b) emulsion SEM; (c) carbon atom EDS of crude; (d) carbon atom EDS of emulsion.

3.3.4 Synergistic Mechanism of Anionic and Zwitterionic Surfactants

The electrostatic potentials of surfactants and asphaltene are visualized in red, white, and blue, as depicted in Figure 22. The red region indicates a positive charge where atoms have difficulty losing electrons, but atoms can easily receive electrons to form hydrogen bonds. Conversely, the blue region represents a negative charge where atoms are more likely to lose electrons. The white region has a near-zero potential and exhibits a relatively stable structure. Specifically, in the NaOl molecule, significant positive and negative charges gather near the carboxyl substituent and Na atom (Figure 22a). Similarly, in the CAB-35 molecule, a substantial accumulation of charges is observed around the N atom and carboxyl substituent (Figure 22b). In asphaltene, this phenomenon is prominent near N atoms and hydroxyl groups (Figure 22c).

Figure 22 Electrostatic potential of surfactants and asphaltene.

As illustrated in Figure 23, the amorphous cells (AC) for both the oil phase and the solution phase have been established. The oil phase contains 10 asphaltene molecules, while the solution phase comprises 5 NaOl molecules, 10 CAB-35 molecules, 2 TEOA molecules, 2 NaOH molecules, and 600 water molecules. Additionally, a single surfactant solution AC is also prepared, consisting of 600 water molecules and either 10 NaOl or 10 CAB-35 molecules. Following the optimization process detailed in Section 2.3.5, the phase interface is constructed using the Build Layer function. A vacuum layer of 20 Å is introduced between the interfaces, after which the Forcite module facilitates a 500 ps molecular dynamics simulation.

Figure 23 Amorphous cells and phase interface.

3.3.4.1 Interaction Energy

The interaction strength of each component in the emulsion can be characterized by the interaction energy (Eint). The lower the Eint indicates the stronger interaction of components and the stabler structure.64Eint and Nonbond energy (Enon) are calculated using the COMPASS III force field of Forcite module as eq 10 and eq 11. The comparison of system energy is shown in Table 5.10

11

where Eint, EA-B, EA, and EB are the energy of interaction, the system total energy and the energy of each component, kcal/mol; Enon, EdvW, and Eelec are the Nonbond energy, van der Waals interaction energy and the electrostatic interaction energy, kcal/mol.

Table 5 Comparison of Surfactant/Asphaltene System Energy

 	System energy (kcal/mol)	
System	Enon	EvdW	Eelec	Eint	EA-B	EA	EB	
CAB-35/NaOL	12.97	–0.58	13.55	12.97	–299.71	–120.27	–192.41	
CAB-35/NaOL (Alkali)	2.68	0.05	2.63	2.68	–269.80	–101.05	–171.42	
NaOL/Asphaltene	–4.25	–0.07	–4.18	–4.25	611.45	807.60	–191.89	
CAB-35/Asphaltene	–15.28	–0.02	–15.26	–15.28	736.33	805.51	–53.89	
CAB-35+NaOL/Asphaltene (Alkali)	–16.52	–0.60	–15.92	–16.52	515.04	812.61	–281.04	

As shown in Table 5, the results of Eint and Enon are equal, indicating the adsorption of surfactants and asphaltene in the heavy oil emulsification.65 Under alkaline conditions, the interaction between CAB-35 and NaOL is enhanced, resulting in a decrease in the Eint. It is observed that a single surfactant can achieve a negative Eint with asphaltene. Furthermore, the combination of surfactants with asphaltene results in a lower Eint compared with using either agent individually. Importantly, the proportion of Eelec in Enon consistently surpasses that of EvdW in all of the systems. Hence, the viscosity reduction in emulsification is primarily attributed to the strong electrostatic interaction arising from molecular electrostatic potential distributions.

3.3.4.2 Radial Distribution Function (RDF)

Figure 24a illustrates the radial distribution function (RDF) curve for oxygen atoms in asphaltene and hydrogen atoms in water molecules during the emulsification process. The curve reveals two sharp peaks within the range of 1–4 Å. The alignment of the peak positions suggests that the aggregation structures of asphaltene and the solution molecules are similar within this range. Notably, the peak sizes indicate that asphaltene exhibits stronger interactions with water molecules and Na+ ions in NaOl solution compared to those in CAB-35 solution (Figure 24b). Furthermore, the peaks associated with NaOl, CAB-35, and alkali molecules are higher, signifying an enhanced interaction.

Figure 24 RDF between atoms in different surfactant systems.

3.3.4.3 Hydrogen-Bond Interaction

Figure 25 illustrates the changes in hydrogen bonds within the emulsion system, as calculated by the script. The criteria for identifying hydrogen bonds in the script stipulate that the donor–acceptor distance must be less than 3 Å, and the angle between the hydrogen atom-donor–acceptor is less than 30°. Initially, there are no hydrogen bonds present between the two phases due to the lack of contact. Approximately 60 ps into the simulation, the oil and water phases make contact, allowing the polar headgroup of the surfactant to form hydrogen bonds with water molecules. It rapidly establishes a new oil–water interface. Subsequently, the number of hydrogen bonds quickly increases at the expanded interface. And the surfactant molecules continue to diffuse due to hydrophobic interactions, leading to a gradual stabilization of the hydrogen bond distribution.66

Figure 25 Internal and interphase hydrogen bonding.

3.3.4.4 Synergistic Mechanism

The synergistic effect of the emulsification viscosity reduction mechanism of anionic and zwitterionic surfactants is shown in Figure 26. The introduction of surfactants effectively reduced the oil–water interfacial tension. Concurrently, the long alkyl chain lipophilic groups of the surfactant molecules adsorb at the oil–water interface. The polar hydrophilic groups extend and interact with water molecules through hydrogen bonds to form a water film that facilitates emulsification and reduces the viscosity. Under alkaline conditions, when NaOl and CAB-35 diffuse to the oil–water interface, the – OH in asphaltene and resin connect NaOl through hydrogen bonds67 and water bridge.68 The asphaltenes in the heavy oil partially replace with the surfactant, resulting in the formation of an O/W emulsion69 and exhibiting shear thinning behavior.70

Figure 26 Synergistic mechanism of the anionic and zwitterionic surfactants.

4 Conclusions

This study investigates the effects of a compounding surfactant system consisting of anionic NaOl and zwitterionic CAB-35 on the South China Sea heavy oil. The aim is to understand the impact of emulsification conditions and the viscosity reduction mechanism.

The most effective viscosity reduction solution consists of a combination of 1% NaOl and CAB-35 in a mass ratio of 1:2, along with the addition of 0.1% NaOH and TEOA in a mass ratio of 1:1. This solution maintains an oil–water ratio of 7:3, with emulsification occurring at a temperature of 30 °C and a stirring rate of 1000 rpm. The surfactant system demonstrated a significant viscosity reduction effect, achieving a viscosity reduction rate of 94.95%. The interfacial tension significantly reduced from 27 to 4 mN/m and transformed the oil droplet being rounder and smaller. The increase in salinity promotes flocculation between the surfactant and asphaltenes/colloids. The emulsion viscosity increases slightly at the salinity of 0.2 mol/L and the Dmean size is measuring as 3 μm. Surfactant molecules with long alkyl chain lipophilic groups adsorb at the oil–water interface, while the polar hydrophilic groups extend to form hydrogen bonds with water molecules, creating a water film. This alteration in steel surface wettability shifts it from being oil-wet to being water-wet, ultimately reducing the drag. The viscosity reduction in the emulsification is primarily attributed to the strong electrostatic interaction arising from molecular electrostatic potential distributions. The −OH in asphaltene and resin molecules connects the surfactant molecule through hydrogen bonds and a water bridge, resulting in the formation of a shear dilution of the O/W emulsion.

In summary, the results of this study offer insights for viscosity control. Since the research is conducted under specific experimental conditions, it is essential to conduct additional oil field experiments focusing on long-term storage and transportation. The conditions for maximizing of the synergistic mechanism warrant further investigation.

The authors declare no competing financial interest.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant No. 52106208) and Natural Science Foundation of Sichuan Province (Grant No. 2023NSFSC0924).
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