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Influence of Aggregation State of Asphaltenes on Structural Properties of Model Oil/Brine Interface Jia You, Chuanxian Li, Daiwei Liu, Fei Yang, and Guangyu Sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04439 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019
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Energy & Fuels
Influence of Aggregation State of Asphaltenes on Structural Properties of Model Oil/Brine Interface Jia You,1 Chuanxian Li,1,2 Daiwei Liu,1 Fei Yang,1,2 Guangyu Sun1,2,* 1College
of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong
266580, People’s Republic of China 2Shandong
Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao,
Shandong 266580, People’s Republic of China *Corresponding author: Guangyu Sun, Email:
[email protected] ABSTRACT: With the continuous exploitation of heavy oil resources, the stability of heavy oil emulsions and further its demulsification technology are receiving more and more attention. One of the most important elements that affect the stability of heavy oil emulsion is asphaltenes. Under this background, the effect of the aggregation state of asphaltenes on the structure-related properties of the model oil/brine water interface is investigated in this study, and further the relation between the structural properties of the interface and the macroscopic stability of the emulsion is studied. It is observed through the dynamic light scattering (DLS) experiment that there is an abrupt increase in the particle size of the asphaltene aggregates at the concentration of 100 ppm, indicating the enhancement of the aggregation degree above this concentration. The higher level of aggregation changes the adsorption kinetics of the asphaltenes at the interface, causing a slower descending rate of the diffusion coefficient. Meanwhile, although the interfacial viscoelastic experiment demonstrates that the interfacial dilational modulus is increased with the addition of asphaltenes, the rising trend becomes gentle and the loss angle hardly changes at the concentrations above 100 ppm, indicating that the cross-linked structure in the interfacial film
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changes slightly with the addition of asphaltenes. In addition, it is found through the interface contraction experiment that no crumpling appears at the interfacial film below 100 ppm. At the concentrations above 100 ppm, the crumpling of the interfacial film is observed. However, the contraction and expansion processes are reversible. At last, it is proven with the bottle test method that there is a positive correlation between the macroscopic stability of the emulsion and the interfacial dilational modulus.
1. Introduction As one of the non-renewable energy sources, petroleum has limited reserves on the earth. After the massive exploitation for more than a century, the light crude oil resources with high quality are gradually depleting with each passing day. As a result, the development of heavy oil is getting more and more attention. However, the recovery of heavy oil is a more complicated technology due to its higher content of resins and asphaltenes. One issue is that the water-in-oil (W/O) emulsion formed by heavy oil and formation water is more stable.1-3 During the multiphase transportation of produced fluid, the existence of stable W/O emulsion will increase the frictional resistance in pipeline, thus raising the transportation cost and causing the waste of energy and money.4,5 If the emulsion cannot be completely broken in the treating station, the discharged treatment water may contain crude oil, which is harmful to the environment. Moreover, even a small quantity of stable emulsion could bring great damage to the refining devices.6 Therefore, the stability of heavy oil emulsion and its mechanism need to be intensively studied. However, crude oil emulsion is a rather complex disperse system. As we all know, many factors could affect the stability and rheological properties of crude oil emulsion, such as its producing area, the recovery method, temperature, pressure, the constituents in aqueous phase,
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etc. Among these factors, the stability of emulsion is mostly determined by the natural emulsifiers in crude oil, such as asphaltenes and resins.7,8 Asphaltenes and resins are the major polar components in crude oil. Asphaltenes have stronger polarity and larger molecular weight than resins. These polar components could form a thin film at the oil-water interface. The interfacial film has two notable features. One feature is that the interfacial tension is relatively high, i.e., the interfacial activity of the film-forming matters is not strong. Another study developed by our lab shows that asphaltenes have stronger influences than resins at the interface.9 The other is that the film has a high structural strength. The latter feature makes the dispersed droplets hard to rupture when they collide with each other, consequently stabilizing the emulsion.10-12 From the perspective of interfacial rheology, the structural strength of an interfacial film can be characterized by its viscoelasticity. According to existing literature, the dilation/contraction deformation of the interface is more close to the real coalescence process of droplets.13,14 Therefore, the in-depth study of interfacial dilational viscoelasticity could offer an insight into the structural properties of the interfacial film and further the stability mechanism of crude oil emulsion. Up to now, many studies have demonstrated that there exists a positive correlation between the emulsion stability and the interfacial dilational modulus.15-20 The increase of the interfacial dilational modulus indicates that the asphaltene aggregates have formed physical cross-links inside the interfacial film.21-23 It is also important to note that the dilational property of the interface is not the only factor determining the stability of emulsions. Many other factors also function, such as the viscosity of the continuous phase, the size and distribution of the dispersed droplets, the interfacial shear viscosity and modulus, etc. In particular, the relationship between the stability of emulsions and the shear rheological properties (shear viscosity and viscoelasticity) of the interface has been investigated by researchers.24-27
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Existing research has verified that asphaltenes tend to adsorb irreversibly at the water/oil interface.28,29 Owing to the intermolecular action of π-π bonds between polyaromatic cores, asphaltene molecules can stack into nano-aggregations at a very low concentration. This concentration is called the critical nano-aggregation concentration (CNAC).30 Asphaltene nanoaggregates can further form clusters when the asphaltene concentration is higher than the critical clustering concentration (CCC).31 It is proved that resins can improve the dispersed state of asphaltenes in crude oil through a solvation effect.32-35 The small angle neutron scattering (SANS) study of Jestin et al. manifests that the asphaltene molecules are structured as a monolayer at the interfacial film, and the thickness of the film layer is in connection with the size of asphaltene aggregates. The structure of the interfacial layer resembles the structure of the nanoaggragates in the bulk, indicating that the asphaltenes adsorb at the interface in the form of nanoaggragates instead of monomers.36 Likewise, the FTIR spectroscopy experiment also shows that the alkyl chains in asphaltene molecules at the interface exhibit crystallization behavior, reflecting the formation of nanoaggragates among the asphaltene molecules.37 There is still controversy on the size of asphaltene aggregates. Many studies demonstrate that the typical size of asphaltene aggregates is a few nanometers,30,36,38-40 for instance, 5.6 nm in the study of Zhang et al.30 and 8.4 nm in the article of Jestin et al.36 However, the result of DLS experiment carried out by Rane et al. manifests that the average particle size of the asphaltene aggregates is about 75 nm immediately after the asphaltenes are diluted in the stock solution of toluene and Nexbase oil 2000 series, regardless of concentration. Moreover, the particle size increases with quiescence time for concentrations above 80 ppm while remains constant below 80 ppm.41 The results of another two studies are even larger (asphaltenes diluted in alkanes).42,43
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This size scale of particle sizes indicates that part of the asphaltenes stay in aggregation state even at very low concentrations. In this study, the influence of the aggregation state of asphaltenes on the structural properties of the oil/water interface is characterized by testing the response of a pendant drop under dilation and contraction conditions. Since the composition of crude oil is very complex, the model oil prepared with specified components (n-decane and 1-methylnaphthalene) is used. n-Decane is chosen to simulate the saturates in crude oil, while 1-methylnaphthalene is chosen to simulate the aromatics. In order to obtain different aggregation states, the concentration of asphaltenes added into the model oil is altered. The particle size distribution of the asphaltene aggregates is determined with the DLS method. Then, the adsorption process of the asphaltenes with different aggregation states and the structure of the formed interface are explored by different dynamic behaviors of the interfacial film. At last, the structural properties of the interface are correlated to the macroscopic stability of the emulsion. We hope this study could deepen the comprehension about effects of asphaltenes on the stability of crude oil emulsions. 2. Experimental Section 2.1. Materials The asphaltenes used in this study were extracted from the Tahe heavy oil produced in China. The basic physical properties of the heavy oil are listed in Table 1. The contents of SARA (saturates, aromatics, resins, and asphaltenes) were determined according to the ASTM D412409 Standard Test Method for Separation of Asphalt into Four Fractions.
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Table 1. Basic Physical Properties of the Tahe Heavy Oil Parameter
Value
Density at 20 oC (kg/m³)
972
Wax content (wt.%)
