Yaqi Ren, Shuqian Xia

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

Keywords:Heavy oil Viscosity reduction Asphaltene Non-Newtonian fluid

ABSTRACT Oil soluble viscosity reducers have gradually attracted the attention of petrochemical research due to their cleanliness and high efficiency.Considering the high viscosity and non-Newtonian fluid properties of Chenping heavy oil found in China,a series of new oil soluble viscosity reducers with different proportions and molecular weights were prepared by free radical polymerization using octadecyl acrylate, 2-allylphenol and N-methylolacrylamide as monomers.The viscosity reducer was applied to different types of heavy oil and found that it exhibited a better effect on heavy oils with high asphaltene content.The test of rheological behavior of heavy oil with additive was performed at wide range of shear rate (3-90 s-1)and temperature range (30-100 °C).The apparent viscosity reduction rate was up to 70.09%, which was better than the industrially relevant ethylene-vinyl acetate copolymer under the same test condition.In addition, the effect of viscosity reducers on the components of heavy oil and the energy change of the system simulated by molecular dynamics simultaneously was investigated.The consistency of the simulated and experimental results show that the effect of viscosity reduction closely related to the crystallization process of wax and the viscosity reducer can reconstruct the surface structure of asphaltene and diminish the connection of benzene ring.

1.Introduction

Crude oil being one of the main energy pillars of modern society, its consumption has increased from 60 million barrels per day to 84 million barrels per day [1].However, oil resources are currently facing a serious crisis of exhaustion.It has been proved that the global geological reserves of heavy oil account for more than 70%of the world’s petroleum reserves[2-4].It is of great significance to alleviate the energy crisis [5,6].Heavy oil possesses a complex colloidal dispersion system,mainly composed of aliphatic hydrocarbons, aromatic hydrocarbons, resins and asphaltenesetc[7-11].It is a challenge to transport and exploit heavy oil due to its high viscosity [12](usually in the 1000-10,000 mPa·s region under oil layer temperature conditions).In the field of petrochemical industry, the research have focused on improving the fluidity of heavy oil along with exploring clean, efficient and low-priced modes of transport [13].Traditional viscosity reduction technologies, such as heating and thin oil blending technologies, have several limits,including the cost and the environmental issues[14,15].So,the utilization of chemicals to reduce the viscosity of heavy oil has drew much attention.

The oil-soluble chemical viscosity reducer can improve the flow behavior of heavy oil, which is developed on the basis of pourpoint depressant [16-18].Two kinds of oil-soluble viscosity reducer used widely are condensation compounds and polymers of unsaturated monomer [19].Previous studies [20-22]have presented that some special structures in viscosity reducer play vital roles in reducing the viscosity of heavy oil.Some domestic and foreign researchers have researched on compounds with various alkyl chain lengths.Quanet al.[23]obtained a hyperbranched polyester with different lengths of carbon chain for using in Xinjiang heavy oil,China.And the polar groups are profitable for viscosity reducer to combine with asphaltene through hydrogen bonds,and improve the dispersity of asphaltene.Kuzmicet al.[24]conducted solution polymerization of various chain length alkyl acrylates with styrene,acrylic acid and 1-vinyl-2-pyrrolidone in solution to act on heavy oil located in northern Croatia, and found that the efficiency of additives depends on the composition,polarity and characteristics of heavy oil.Furhermore, compounds with short or un-branched chains were applied for reducing the viscosity of heavy oils.Castroet al.[25]prepared a terpolymer made up of styrene,n-butyl acrylate and vinyl acetate through semi-continuous emulsion polymerization for viscosity reduction of Mexican heavy oil, and proved that the molecular weight of the terpolymer has a great impact on the viscosity reduction.Al-Sabaghet al.[26]synthesized a spiro compound, SB1, which acted on waxy crude oil from Egypt, to reduce the formation of wax mesh and enable heavy oil to possess a higher flow capacity.Generally,a certain viscosity reducer is only effective for oils with specific physical properties [22].Due to the significant differences in heavy oil components from different regions, the characteristics of the viscosity reducer are dissimilar.

Based on the composition characteristics of Chenping heavy oil,a new type of oil-soluble viscosity reducer POAN was prepared by radical polymerization with three monomers, octadecyl acrylate(O), 2-allylphenol (A) andN-methylolacrylamide (N) respectively,and was used for heavy oil in different blocks.Taking Chenping heavy oil as the research object,the effects of some factors on viscosity reduction, including shear behavior, temperature, and monomer ratio, molecular weight, the concentrations of POAN,were analyzed and evaluated.Meanwhile, the interaction mechanism between heavy oil and viscosity reducer was discussed by different characterization and calculations.

2.Experimental

2.1.Material

Octadecyl acrylate(97%,by mass),2-Allylphenol(97%,by mass)andN-methylolacrylamide (98%, by mass) were purchased from Heowns Biochem LLC.Toluene (analytically pure) was purchased from Tianjin Jiangtian Chemical Technology Co.LTD.2,2′-Azobis(2-methylpropionitrile) (98%, by mass) and kerosene (reagent grade) were obtained from Chemart.Ethanol (analytically pure)was obtained from Real & Lead chemical Co.LTD.Three kinds of heavy oil from China were employed in this work.The physical and chemical properties of the oil are listed in Table 1.Of the three heavy oils tested,Chenping heavy oil was systematically analyzed.

Table 1Physical and chemical properties of the heavy oil

2.2.The preparation of the samples

2.2.1.Preparation of POAN

Certain amounts of octadecyl acrylate, 2-allylphenol,Nmethylolacrylamide and toluene/ethanol mixture were added to a three-necked flask equipped with a condenser and a thermometer.Some 2,2′-azobis (2-methylpropionitrile) dissolved in toluene in advance was added to the constant pressure dropping funnel.Nitrogen was injected to replace the oxygen in the flask, then the heating and stirring devices were turned on.When the mix reached the target temperature, the initiator 2,2′-azobis (2-methylpropionitrile) was added.At the end of the reaction, the product was transferred to a beaker and cooled to room temperature.The copolymer was purified with methanol and dried in a vacuum at 40 °C until the mass remained unchanged, and the white granular product was obtained.The polymer molecular structure is illustrated in Fig.1.This is a typical comb-like structure surrounded by many long carbon chains.The aromatic units and polar parts alternately appear on the main chain.

Fig.1. The molecular structure of POAN.

2.2.2.Asphaltene extraction

n-Heptane, as a poor solvent for asphaltene, can be used to extract asphaltene from heavy oil [27].A certain amount of heavy oil, mixes with excessn-heptane (the volume ratio of heavy oil ton-heptane is 1:30),stirring fairly constantly with a glass rod.After resting at room temperature for two days,the mix filtered through a sand core funnel, and repeatedly washing the filter cake withnheptane.Finally, the asphaltene mixture was placed in a vacuum oven for drying.

2.3.Characterization

2.3.1.Rheological measurements

The viscosity value can intuitively reflect the performance of flow.The viscosity of all samples was measured by a rheometer CP-5000,which has a velocity range of 0.3-1500 r·min-1,a torque range of 0.05-30 mN·m and a viscosity range of 1-540,000,000 mPa·s.Viscosity as a function of shear rate could be obtained at a constant temperature.The relationship between temperature and viscosity could be measured at a fixed shear rate.The viscosity of heavy oil was tested at 50 °C and recorded as η0.The heavy oil mixed with 5% (mass) kerosene, subsequently, stirring thoroughly and keeping it in a water bath at 50°C for a while.Then the viscosity of the heavy oil was determined and recorded as η1.Under the same conditions, the viscosity of the heavy oil added with POAN(pre-dissolved in kerosene)was tested and denoted as η2.The viscosity reduction rate (VRR) can be calculated as follows:

whereAηandNηindicate apparent viscosity reduction rate and net viscosity reduction rate.

In order to acquire the viscoelastic behavior and deformation ability of heavy oil, the oscillatory frequency test was charactered by the Huck Mars III Rotation Rheometer under constant tempera-ture, and the storage/loss modulus were measured in the frequency range of 0.1-10 Hz.

2.3.2.Fourier transform infrared spectroscopy analysis (FTIR)

The chemical structures of POAN,untreated and treated asphaltene were characterized by FTIR spectrometer ALPHA (BRUKER,Germany).The study on the structural changes of asphaltene, the key viscosity-causing component,is beneficial to learn the function of additive and further understand the mechanism of viscosity reduction.

2.3.3.Thermogravimetric analysis (TG)

The thermal stability of the POAN was determined by thermogravimetric analysis (TG, Mettler Toledo, Switzerland) in the presence of nitrogen gas, which was applied to judge the instability range of the additive and thereby to select a reasonable temperature in the viscosity reduction process.The test was carried out at a range from 0 to 800 °C with a heating rate of 10 °C·min-1.

2.3.4.Multi-detection gel chromatography analysis (GPC)

The molecular weight of the POAN was obtained by a multidetection gel chromatograph TDA305, with the concentration of 5 mg·ml-1of samples and polystyrene as the standard, using tetrahydrofuran (THF) as the mobile phase at a flow rate of 1 ml·min-1.The influence of viscosity reducers with different molecular weights on viscosity of heavy oil was analyzed, and the optimal effect range was determined.

2.3.5.Differential scanning calorimetry (DSC) analysis

2.3.6.Field emission scanning electron microscope analysis (SEM)

The morphology of asphaltene before and after POAN treatment was observed at room temperature by scanning electron microscope (SEM, Apreo S LoVac) at a magnification of 50,000.The arrangement and structure change of asphaltene molecules can be observed after the addition of viscosity reducer.

3.Results and Discussion

3.1.Characterization of POAN

The infrared spectrum results of POAN are presented in Fig.2.The stretching vibration peaks of -CH3and -CH2- appeared at 2921 cm-1and 2850 cm-1.The stretching vibration peaks of C=O and C-O-C in octadecyl acrylate were located at 1734 cm-1and 1170 cm-1.At 721 cm-1, the characteristic peak of long-branched chains of multi-carbon alkyl groups existed.The stretching vibration peak on the benzene ring skeleton in 2-allylphenol was at 1468 cm-1, and the stretching vibration peak of C-O bond connected with benzene ring appeared at 1267 cm-1.A peak at 3355 cm-1was attributed to the O-H and N-H in the associated state.The stretching vibration peak of C=O on the amide group inN-methylolacrylamide was at 1664 cm-1, and the bending vibration absorption peak of N-H and stretching vibration peak of C-N simultaneously were at 1542 cm-1.The results of infrared analysis point out that the copolymer, POAN, has been successfully synthesized by three monomers.

Fig.2. FTIR analysis of POAN.

The thermogravimetric spectra of the POAN illustrates the properties of polymer below 300°C were stable(Fig.3).In tandem with an increase of temperature, a certain degree of mass loss occurred, and the mass loss accelerated rapidly between 300 °C and 450°C.The polymer has decomposed entirely as the temperature exceeded 500°C.Thermogravimetric result reveals that POAN will not decompose under 300 °C and can play a stable role in heavy oil.

3.2.Rheological properties

It is necessary to investigate the rheological properties of heavy oil for viscosity reduction.Taking Chenping heavy oil as an instance, the shear stress of heavy oil nonlinearly changed with the shear rate at 30°C(Fig.4(a)).When the temperature exceeded 40 °C, the shear stress increased with the shear rate linearly, and the heavy oil began to follow characteristics of Newtonian fluids.The change caused by temperature is related to the structure of heavy oil, which varies greatly with different temperature, such as the dissolution of wax crystals and the dispersion of aggregates with elevated temperature.

Fig.3. TG profile of POAN.

According to Fig.4(b), before the viscosity of heavy oil became stable,it decreased gradually with the increase of shear rate within the first 30 s-1,which is the shear thinning behavior,also known as pseudoplastic flow.Under static state or low shear rate, heavy oil presents high viscosity due to the entanglement caused by many macromolecules tangled each other.The higher shear stress pares back on the links of entangled macromolecules, and finally heavy oil presents as shear thinning phenomenon.

The storage modulus and loss modulus of the heavy oil sample decreased with the increase of temperature, whereas those increased with the increase of frequency(Fig.4(c)).The higher values ofG′andG′′at low temperature are obtained as a result of the existence of a large number of aggregates and strong threedimensional structure of heavy oil,which has solid-like properties[28]and strong elastic deformation.The lower the loss modulus at high temperature is, the lower the internal friction during flow is,leading to the decrease in the viscosity of heavy oil, which is consistent with the viscosity curve.

3.3.POAN as additives in heavy oil

The polymers,POAN,were prepared with different molar ratios of the monomers.Then the viscosity of the heavy oil treated with POAN was measured according to the method described in Section 2.3.1.Table 2 lists the results.For Chenping heavy oil, with the fixed amount ofN-methylolacrylamide,the viscosity reduction rate increased until the ratio of O:A reached 5:2 and then declined with the molar ratio of O:A increasing further to 7:2.When the molar ratio of O:A was 5:2, the viscosity of the heavy oil remarkably decreased from 15632 mPa·s to 5066 mPa·s.The apparent viscosity reduction rate and the net viscosity reduction rate reached the maximum,67.59%and 35.46%,respectively.Table 2 also shows the effect of POAN on the other two types of heavy oil and the viscosity reducer had little effect on Jinxie oil.According to the compositions of the heavy oil shown in Table 1, the key to this difference lies in the asphaltene.The asphaltene existed in Jinxie oil only takes up 0.93%,so it can be inferred that the viscosity reducer designed in this work is more effective for oils with more asphaltenes.For more pertinence research,the following will continue to analyze Chenping heavy oil.

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Table 2Influence of monomer ratio on viscosity reduction of different heavy oil

Fig.4. Rheograms of Chenping heavy oil at different temperature (30-60 °C).(a)Shear stress vs.shear rate.(b) Viscosity vs.shear rate.(c) Storage modulus (G′) and loss modulus (G′′) vs.frequency.

A proper amount of octadecyl acrylate can improve the effect of viscosity reduction, since the augment of long-branched chain strengthens the space dimension degree,which prevents the association and aggregation between the resin and asphaltene efficaciously as well as dispersing the wax molecules to form the solvated layer.But the excessive content of octadecyl acrylate will obstruct the interaction between polymer and heavy oil due to the increase of steric hindrance of polymer.The benzene ring from 2-Allylphenol can provide certain rigidity.Simultaneously, the aromatic part can dismantle the stacked structure in the heavy oil based on the principle of ‘‘similar miscibility” and ‘‘π-π stacking”.Besides, the phenolic hydroxyl group located on the inside of the main chain can promote the penetrator into asphaltene and resin by forming hydrogen bonds.Subsequently, the amount of N containing polar groups was adjusted based on the optimal ratio of O:A being 5:2.N-methylolacrylamide possesses superior ability to form hydrogen bonds owing to hydroxyl and amide groups.It is worth noting that the polarity of POAN should not be too strong,otherwise the solubility of POAN in heavy oil will become too poor to keep it from reaching full potential.Therefore, the proper ratio of O:A:N was determined as 5:2:1.

The efficacy of the viscosity reducer has a great relevancy with its molecular weight [29].With the monomer ratio fixed (O:A:N=5:2:1),polymers with different molecular weight were synthesized by adjusting the dosage and dropping-speed of initiator.Table 3 summarizes the final molecular weight and the viscosity reduction rates.The polydispersity of all samples was less than 2,demonstrating the homogeneity of the product.In the case of the weight-average molecular weight of POAN from 18,000 to 20,000, the viscosity reduction rates were stable around 30.00%.The smaller the molecular weight of the polymer is, the lower the existence of branched chain and aromaticity is, leading to the worse the viscosity reduction effect to heavy oil is.At appropriate reaction conditions, free radicals can transfer to macromolecular chains and further monomer polymerization is induced.Consequently, the molecular weight and the degree of branched chain are increased.Therefore, when the molecular weight of the PONA can be controlled in an appropriate range, the effect of each functional group is fully exerted.

Table 3Influence of molecular weight on viscosity reduction

The viscosity reduction rate of the heavy oil with different concentration of POAN was tested at 40 °C and 50 °C, the results are shown in Fig.5.Obviously, the optimal performance can be reached at the dosage of 800 mg·kg-1.With the concentration of POAN increasing from 200 mg·kg-1to 800 mg·kg-1,the net viscosity reduction rate increased to 26.42%at 40°C and 40.45%at 50°C.Since most of the wax in heavy oil is deposited [30]and adsorbed on the resin and asphaltene at low temperature, lower concentration of additive has no obvious effect on reducing viscosity.After the concentration exceeded 800 mg·kg-1, the net viscosity reduction rate began to decrease.This phenomenon derives from the high reactive amine and hydroxyl groups existing in heavy oil[31-33],which POAN can interact with by means of its own special comb structure.The higher the concentration, the stronger the interaction will be.If the concentration of POAN exceeds the optimal amount, molecules of POAN will intertwine and aggregate with each other due to the nature of high polymer, resulting in adverse effect ultimately.This result has also been confirmed by other scholars [29].

Fig.5. The viscosity reduction rate of POAN at(a)40°C and(b)50°C with different concentrations.

Compared with three commonly used oil soluble viscosity reducers, the viscosity of heavy oil was measured in the shear range of 3-90 s-1, as shown in Fig.6.Under the same test conditions, POAN showed the best performance, followed by POMA,while EVA and POMS had similar results with almost coincidence curves.This result shows that it is necessary to design the suitable viscosity reducers for one specific oil.

Fig.6. The viscosity of heavy oil as a function of shear rate under different viscosity reducers (POMS: Octadecyl methacrylate-maleic anhydride-styrene viscosity reducer, POMA: Octadecyl methacrylate-maleic anhydride-acrylamide viscosity reducer, EVA: Ethylene-vinyl acetate viscosity reducer).

3.4.The properties of heavy oil related with temperature

The Arrhenius equation (Eq.(3)) is the most common model to describe the relationship between fluid viscosity and temperature.

where η,A,Ea,R,Trepresent viscosity, the front factor, activation energy, general gas constant and temperature, respectively.

Enriqueet al.[34]proposed that the Arrhenius linear model can be used to identify the transition temperature of heavy oil.The viscosities of heavy oil at different temperatures were tested followed by plotted,the test and calculated values in the form of ln[η]versus1/Tin Fig.7.There was a deviation occurring at 1/T=0.0031(when the temperature was about 50°C),Since heavy oil as a kind of non-Newtonian fluid with complex composition is viscoelastic at low temperature.As the temperature rises, the aggregate structure and macromolecular sequence of the heavy oil are destroyed after the wax is dissolved,then the heavy oil gets viscous.The variation of this character results in the misfit between the actual viscosity and the calculated value at 50 °C.

Fig.7. The experimental and calculated values of viscosity of heavy oil.

Differential scanning calorimetry (DSC) is commonly used to analyze the temperature characteristics of heavy oil.DSC results show that the glass transition temperature (GTT) of heavy oil was about 8 °C, while it dropped slightly to 5 °C after adding viscosity reducer.It indicates that the viscosity reducer can enhance the viscous flow characteristics of the heavy oil molecules and make the viscous deformation of the compounds occur at a lower temperature.The wax appearance temperature (WAT) is defined as the maximum temperature at which wax crystallization occurs in waxy crude fluids[35],as it observed in the DSC diagram where the curve deviates from the baseline (Fig.8).WAT should be the apex position of the endothermic peak of the DSC curve.The result shows that the WAT of the heavy oil appeared at about 50 °C,which was consistent with the transition temperature predicted by the Arrhenius model(Fig.7).This obvious phase transition process is caused by the crystallization of wax.As the temperature drops, the wax molecules crystallize from the heavy oil and form an interlock gel network.The melting point of pure wax is between 58-62 °C, but the wax crystals in the heavy oil start to melt at about 45 °C, since the crystallization behavior of wax is affected by other components in heavy oil[36].After the heavy oil was treated with the POAN,the endothermic peak disappeared.Crystallization theory [37]states that only after the radius of the crystal nucleus of wax is larger than the critical radius, can the wax continue to grow and remain stable.The co-crystallization between the long-branched chain of POAN and the wax molecule increases the solubility of the wax crystal and reduces the interfacial tension between the wax crystal and the oil phase.As a result, the critical radius increases, which is larger than the size of wax crystal nucleus.Meanwhile, the additive strongly destroys the meshwork wax crystal structure and finally leads to the disappearance of endothermic peak.

To explore the influence of temperature on viscosity reduction,net viscosity reduction rates at 99 different temperatures (30-100°C) were measured (Fig.9).The viscosity reduction rates did not decrease monotonously with the increase temperature, a sharp leap arising from 45 °C to 50 °C.This is consistent with the DSC analysis, in other words, this change located in a range which the wax crystals from start to finish melting in Fig.8.The increase of viscosity reduction rate at this stage indicates that with the wax crystal melting gradually, the viscosity reducer can better penetrate into the aggregates.It is possible that the melting of wax may cause the change of viscoelastic properties of the heavy oil,which makes the additive enter into the tangled asphaltene-resin through π-π stacking, hydrogen bonding, van der Waals forces,and so on.The above results confirm the micellar structure proposed by predecessors [38]from the side, asphaltenes are suspended in the heavy oil as solid particles, forming micelles with resins and dispersing steadily in the liquid oil.Quanet al.[23]pointed out that wax molecules would be inserted into the aggregates formed by asphaltene and resin to make heavy oil flowable worse.As the temperature rising continually, the viscosity reduction rate gradually declined in fluctuation as a consequence of the influence of temperature on viscosity at this time tremendously beyond the effect of viscosity reducer.Therefore, the properties of heavy oil have impact on the applicable conditions.

Fig.8. DSC of heavy oil before and after POAN addition.

3.5.Molecular dynamics simulation

The energy can be calculated by molecular simulation.The resin and asphaltene models(RA)were established in the Materials Studio software (Fig.10), and the system energy with and without POAN was calculated.The construction tool was used to build the model system,and then the interaction process was calculated through DMol3 and Dynamics module.Andersen method was selected to control the temperature, and molecular dynamics simulation with the time step of 150 ps was carried out in compass[39]forcefield.

Fig.9. Relationship of Net VRR and temperature.

Potential energy is the energy stored in the system, and low potential energy means more stable intermolecular structure.The potential energy of the system after POAN addition was significantly reduced(Fig.11(a)),proving that the aggregation structure formed by resin and asphaltene was destroyed to form a new stable system.The potential energy remained low below 50 °C,while the random movement of molecules in the system accelerated as the temperature rise, thereby, the potential energy of the system was increased.The non-bond energy of the system increased to the negative direction after POAN action, indicating that new hydrogen bonds and van der Waals forces formed between additive and heavy component stacking structure(Fig.11(b)).The absolute value of the non-bond energy reached its highest at 50 °C.At this time, the intermolecular interactions were most stable and the destructive effects of the additive were therefore strongest.The simulation results agree with the fact that the net viscosity reduction rate reaches the maximum value at 50°C.

3.6.Interaction between POAN and asphaltene

Asphaltene with the strongest polarity is the heaviest component of heavy oil,surrounded by light component,resin and hydrocarbon[40],and contribute the most to the viscosity of heavy oil.It is crucial to study the effect of additive on asphaltene.Fig.12 illustrates the chemical structure changes of asphaltene with and without POAN.The peaks at 1596 cm-1and 1369 cm-1of asphaltene were assigned as the in-plane bending vibration absorption of N-H and O-H respectively, the peak of 1452 cm-1contributed to the skeleton vibration of the aromatic ring C=C bond (Fig.12(a)).The C-H flexural vibration absorption peaks of the ortho and meta substitution of the aromatic ring appeared at 806 cm-1and 742 cm-1, respectively.In Fig.12(b), the peak at 1596 cm-1in asphaltene disappeared after being treated, suggesting that POAN permeates into the heavy components through ester chain groups, hydroxyl groups, amine groups, combining with the H exposed on the outside of the asphaltene sheets to form a strong hydrogen bond and change layered complex structure.And the disappearance of peaks at 806 cm-1and 742 cm-1indicates that part of the connection among benzene rings is broken and the tight structure of the components is destroyed,thus reducing the degree of order.

In Fig.13(a), asphaltene stacked crossing each other have an irregular structure,with rough surface and many macropores,providing good sites for wax adsorption and deposition.Meanwhile,large specific surface area is conducive to enhance the mutual stacking with resin, thus forming a firm structure.The asphaltene after treatment (Fig.13(b)) have a relatively smooth surface with reduced roughness due to that POAN enters into the asphaltene through its aromatic rings and polar groups.The long chain forms a protective layer outside the asphaltene molecules to change the polarity of the outer layer.Owing to the steric hindrance, it is difficult for the resin to entangle with asphaltene after adding POAN.

4.Viscosity Reduction Mechanism

The asphaltene in Chenping heavy oil has a lot of pore structures,which makes the resin tends to bind it,and the large specific surface area provides more crystallization sites for the adsorption and growth of wax.According to the experiments above,a viscosity reduction model as shown in Fig.14 is proposed.The first are effects on the outermost wax molecules.Below the WAT, the wax molecules precipitate to form a three-dimensional [41]network structure.After reaching the melting temperature of wax crystals, the wax adsorbed on the aggregate dissolves, followed by the number of aromatic sheets of asphaltene association decreasing, and the graphite-like lamellar structure between the asphaltene and the resin becomes loose(Fig.14(b)).Such structure is beneficial for the viscosity reducer to deeply penetrate into the aggregates to rearrange the distribution of macromolecules.The long chain structure of POAN (Fig.1) provides new crystallization sites for free wax molecules, co-crystallizes with wax molecules[42], changes the growth characteristics of wax crystals, and hinders the tendency of interconnection to form a gel network structure.Meanwhile,the solubility of wax is increased,which prevents the deposition of wax crystals in the cooling process,leading to the endothermic peak in the DSC curve disappeared.

Fig.10. Simulation structure of system (a) RA (b) RA with POAN.

Fig.11. Potential energy (a) and non-bond energy (b) of model systems.

Fig.12. IR spectrum of (a) asphaltene, and (b) asphaltene after POAN addition.

The resin and asphaltene in Chenping heavy oil can form hydrogen bonds to make aromatic sheets stack each other.POAN firstly breaks the connection among polycyclic aromatic hydrocarbons through N—H and O—H bonds in polar groups to dismantle the tight planar stacking structure.Then it enters the asphaltene pores through the rigid aromatic groups.On account of asphaltene with certain Lewis base characteristics [20], phenol structures of POAN with weak acidity can interact with asphaltene and bond with N,O and other elements, resulting in partly fracture of asphaltene macrostructure.The asphaltene pores are destroyed and the surface becomes smooth,forming a new structure with more compact and stable feature.The produced surface effect prevents the resin from binding to the asphaltene, and these fragments evenly and vertically dispersed on the comb structure of POAN (Fig.14(c)),then diffusing into the continuous phase.Hence,heavy oil becomes more uniform,and conclusively achieves reduction in viscosity.For Jinxie heavy oil,its asphaltene content is very little.The content of wax and resin is much higher than that of Chenping oil, which makes the greater interaction of wax and resin for Jinxie oil.Resin has the function of the crystal structure due to the covalent bond[43].The van der Waals force between the alkyl side chain in the resin molecule and the wax alkane molecule makes the resin participate in the directional arrangement of the wax molecules, and the synergistic effect will produce the complex crystal structure more compact and continuous, which no longer has the pore for the action of viscosity reducer.The viscosity reducer is unable to break this continuous micelle and therefore has not effective impact on Jinxie oil.

Fig.13. SEM images of (a) asphaltene, (b) asphaltene after POAN addition.

Fig.14. Schematic elaboration of POAN interaction with heavy oil.

5.Conclusions

In this research, the physical and chemical properties of Chenping heavy oil were studied.As a non-Newtonian fluid,the storage modulus and loss modulus of heavy oil decrease with the increase of temperature, and the heavy oil exhibits typical shear thinning behavior.A new oil-soluble polymer, POAN, was synthesized to reduce the viscosity of heavy oil and the structure and molecular weight of POAN have a great impact.The viscosity reducer was applied to three different kinds of heavy oil,and the effect on Jinxie heavy oil with very low asphaltene content was very little.When the concentration of POAN was 800 mg·kg-1,the viscosity of Chenping heavy oil reduced from 15,632 mPa·s to 4675 mPa·s at 50 °C.Then the net viscosity reduction rates were measured at different temperatures between 30 °C and 100 °C, and there was a sudden increase from 45°C to 50°C.The change was caused by the dissolution of wax crystals during this temperature range.The action mechanism of viscosity reducer was analyzed by molecular simulation calculation.The numerical decrease of non-bond energy and potential energy proved that it could form stronger molecular interaction with resin and asphaltene, which caused strong damage to the accumulated structure.Taking into account of the environmental friendliness and no post-treatment character, oilsoluble polymer demonstrates potential prospect in the application of heavy oil viscosity reduction.It should pay attention that the chemical viscosity reducer should match with the heavy oil.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2018YFA0702403).