Effect of Polyethylene-Vinyl Acetate (EVA) Pour Point Depressant on

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Fossil Fuels

Effect of Polyethylene-Vinyl Acetate (EVA) Pour Point Depressant on the Flow Behavior of Degassed Changqing Waxy Crude Oil before/after scCO Extraction 2

Shuang Yang, Chuanxian Li, Fei Yang, Xiaoteng Li, Guangyu Sun, and Bo Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00561 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Effect of Polyethylene-Vinyl Acetate (EVA) Pour Point Depressant on the

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Flow Behavior of Degassed Changqing Waxy Crude Oil before/after scCO2

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Extraction

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Shuang Yang1, Chuanxian Li1,2, Fei Yang 1,2,*, Xiaoteng Li1, Guangyu Sun1,2, Bo Yao1,2

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1College

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Shandong 266580, People’s Republic of China

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2Shandong

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266580, People’s Republic of China

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*Corresponding Author: Fei Yang E-mail: [email protected]

of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao,

Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao, Shandong

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Abstract: For the pipeline transportation of waxy crude oil, pour point depressant (PPD) has been

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developed and used to improve its flowability at low temperature. As supercritical CO2 (scCO2)

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flooding becomes a common technique to improve oilfield recovery, how the sensitivity of PPD to

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the degassed crude oil treated by CO2 changes should be studied. In this work, the effect of scCO2

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extraction on the performance of EVA in improving the flowability of Changqing degassed crude oil

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is investigated by means of a self-developed scCO2 treating equipment, SARA/HTGC analysis, SEM

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analysis, pour point test, rheological measurement, asphaltenes precipitation test, particle size

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analysis, DSC test and microscopic observation. The results show that without EVA addition, scCO2

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extraction aggravates the flowability of the crude oil at low temperature. The scCO2 extraction

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extracts light components from asphaltene aggregates causing asphaltenes to precipitate, which

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inhibits the asphaltenes on playing a role as the natural PPD. As a result, it leads to the formation of

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smaller needle-like wax crystals with large amount, worsening the flowability of the crude oil. When

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the EVA is present, however, scCO2 extraction favors the functioning of EVA on improving the

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flowability of the crude oil at low temperature. The addition of EVA can slightly enhance the

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solubility of wax molecules thus decreasing the crude oil WAT to some extent, while can

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significantly change the morphology of precipitated wax crystals from small needle-like to

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agglomerated clusters, thus improving the flowability of the crude oil. Due to the synergistic effect

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of EVA and asphaltene aggregates after scCO2 extraction, a more compact EVA-asphaltene-wax

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ternary crystal structure will form thus further decreasing the pour point, elastic modulus and

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equilibrium viscosity at low temperature.

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1. Introduction

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Most of the crude oil produced in China belongs to waxy crude oil. The high content of wax, high

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pour point and wax deposition rate are common problems in the production and transportation of

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waxy crude oil, which brings huge economic losses to the oil industry

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solubility of wax is highly dependent on temperature. Below an appropriate thermodynamic

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temperature, i.e., the wax appearance temperature (WAT), wax crystals will be continuously

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precipitated from the crude oil and suspended in the oil in the form of crystal particles. They make

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the crude oil become a colloidal dispersion or solid-liquid suspension system with wax particles as

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the main dispersed phase

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crystals in crude oil will increase continuously. Because the precipitated wax crystal particles have

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irregular morphology (needle or plate-like), they are prone to be interlocked with each other and

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form complex spatial network structure, which makes the crude oil lose its flowability and eventually

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become a solid-like gel

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consuming and even danger to the pipeline transportation.

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[3-5].

[6,7].

[1,2].

For waxy crude oil, the

If the temperature further lowered, the amount of precipitated wax

The aggravated flow behavior of waxy crude oil may cause energy

To improve the operating safety and economy of the pipelines transporting waxy crude oils,


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polymeric pour point depressants (PPDs) are often applied to effectively assist waxy crude oil

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pipelining

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change the precipitated wax crystal morphology (size and shape), thus impeding the wax crystals to

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form a three-dimensional network structure. Therefore, a small dosage of polymeric PPDs can

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significantly reduce the pour point and improve the flow behavior of waxy crude oil at low

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temperature [10.11].

[8,9].

The PPD molecules can participate in the wax crystallization process and greatly

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Polyethylene-Vinyl Acetate (EVA) is one kind of co-polymer PPDs widely used in crude oil

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pipelines. Like surfactants, the EVA can solubilize a small amount of wax molecules thus slightly

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decreasing the WAT of waxy crude oil. Meanwhile, due to the similar structure between the ethylene

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segment of EVA and wax molecules, EVA can enter the wax crystals to replace the wax molecules

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(n-alkyl chain molecule) in the lattice through the co-crystallization effect. In addition, the difference

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in polarity between the vinyl acetate (VA) segment of EVA and wax molecules inhibits the growth

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of wax crystals into a two-dimensional plane. The growth speed of the wax crystal in the direction

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perpendicular to the plane is accelerated, as well as the shape of the wax crystal is also changed [12-14].

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Normally, the wax crystal morphology can be summarized as needle-shaped one-dimensional

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structure, sheet-like or plate-like two-dimensional structure, spherical or granular three-dimensional

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structure

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large spherical or granular-like and decreases the specific surface area/surface energy of wax crystal,

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thus making it difficult to aggregate into a three-dimensional network structure.

[15,16].

The addition of EVA changes the morphology of the precipitated wax crystal into

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During the last two decades, supercritical CO2 enhanced oil recovery (scCO2-EOR) technique has

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been widely used in many oilfields of the world because this technique can not only efficiently

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improve the crude oil reservoir recovery, but also sequestrate large amount of CO2 in the reservoir


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thus alleviating the greenhouse effect

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scCO2-EOR technique is more adequate in low/ultra-low permeability reservoirs and heavy oil

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reservoirs. The scCO2-EOR normally has two modes: miscible flooding and non-miscible flooding

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[18].

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swelling crude oil, miscibility with crude oil, reduction of viscosity, and acid unblocking technique

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[19-22].

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application of the technique in oilfields. The researches mainly focused on improving scCO2-EOR

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efficiency [23], reducing the minimum miscible pressure (MMP) [24,25], mitigating corrosion problems

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[26]

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been launched about the effect of scCO2-EOR technique on the flow behavior of degassed crude oil

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at surface, which is crucial for the safe and economic transportation of crude oil in pipelines.

[17].

Compared to the traditional water flooding technique,

The oil recovery mechanisms of scCO2-EOR include the extraction of light components,

Much research work has been carried out on the scCO2-EOR technique to guide the

and asphaltene deposition problems caused by CO2 flooding

[27].

However, few researches have

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Changqing oilfield is the biggest oilfield in China and the crude oil produced there is a specific

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waxy crude oil. Most of the oil reservoirs in Changqing oilfield belong to low/ultra-low permeability

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reservoirs and the traditional water flooding technique is no longer efficient with time

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2017, scCO2-EOR technique has been applied in several blocks of Changqing oilfield. The on-site

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testing results showed that (a) the oil recovery efficiency is improved clearly by the scCO2-EOR, and

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(b) the flow behavior of the degassed crude oil recovered by scCO2-EOR is quite different form that

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recovered by traditional water flooding. The changed flow behavior of the degassed Changqing waxy

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crude oil will certainly affect the latter pipelining operation and even affect the performance of added

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polymeric PPDs.

[28].

Since

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In this paper, the effect of EVA (100ppm) on the flow behavior of degassed Changqing crude oil

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before/after scCO2 extraction was studied. The degassed crude oil samples before scCO2 extraction


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(oil sample A) and after scCO2 extraction (oil sample B) were first prepared and compared. Then, the flow behavior of the two oil samples with/without adding EVA was measured by pour point test and rheological test. Finally, the influencing mechanism of EVA on the flow behavior of the two degassed crude oil samples was discussed based on the DSC analysis, microscopic observation, particle size distribution analysis and asphaltenes stability test. 2. Experimental section 2.1 Materials 2.1.1 Chemical agents The pressurized CO2 gas was purchased from Tianyuan Gas Producer Co., Ltd. in China with the purity > 99 wt%. The ethanol, n-heptane, toluene and petroleum ether were all analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The EVA was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. in China. The VA content of the EVA is 28 wt% and the average molecular weight of the EVA is around 20000. The dosage of the EVA in degassed crude oil samples was fixed at 100 ppm. 2.1.2 Preparation of the degassed crude oil samples before/after scCO2 extraction The degassed crude oil before scCO2 extraction (oil sample A) was recovered by the traditional water flooding technique and contained little water (< 0.1 wt%) after gravity sedimentation. The density of oil sample A is 840.02 kg/m3 at 20 °C, and the wax content is 18.10 wt%. The degassed crude oil after scCO2 extraction (oil sample B) was obtained by treating oil sample A with a self-made scCO2 extraction equipment. Figure 1 shows the sketch map of the scCO2 extraction equipment, which is mainly composed of the CO2 gas cylinder, temperature control system, piston tank, PVT device, high pressure sampler and metering pump


[29].

The treating procedures are as

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follows: (a) a certain amount of oil sample A was extracted by scCO2 in the equipment at 80 °C and 25 MPa (the average temperature and pressure in oil reservoirs of Changqing oilfield) for 6 h; (b) by slowly cooling the oil and lowering the pressure of the oil in the equipment to ambient conditions, the oil sample B was obtained. Table 1 shows the SARA composition of the two oil samples. For oil sample A, the contents of saturates, aromatics, resins and asphaltenes are 64.88 wt%, 24.72 wt%, 8.97 wt% and 1.43 wt%, respectively. For oil sample B, the content of saturates slightly decreases to 64.15 wt%, while the contents of resins and asphaltenes slightly increases to 9.27 wt% and 1.51 wt%, respectively. The carbon number distribution of the alkanes in the two oil samples was also analyzed through a high temperature gas chromatograph (Varian Co., US). As seen in Figure 2, the lighter hydrocarbon components (C7-C15) in oil sample B are slightly reduced. Obviously, the scCO2 extraction extracts a small amount of light hydrocarbon components from oil sample A, but the composition change of the two oil samples is relatively small. 2.2 Methods 2.2.1 Rheological tests of the degassed oil samples The two degassed oil samples with/without adding 100 ppm EVA were first preheated statically at 60 °C for 20 min. Then, the rheological properties of the oil samples were measured. (a) Pour point test

The pour point of the oil samples was measured according to the method

given by standard ASTM D5853-11

[30].

The preheated temperature and the cooling rate were

consistent with the standard. (b) Viscosity-temperature characteristics test

In the dynamic test process, a fixed shear rate 50 s-1

was applied to the oil samples with the aid of a DHR-1 rheometer (TA Instruments Co., US) and the


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oil samples were cooled at a constant rate of 0.5 °C/min. The datas were ploted with the Arrhenius methodology [31]. (c) Viscoelasticity test

In the static test process, a small amplitude oscillation was applied to the

oil samples with the aid of a DHR-1 rheometer and the oil samples were also cooled at 0.5 °C/min. The oscillation frequency was fixed at 1 Hz, while the oscillation amplitude was fixed at 0.0005, which is so small that it has little influence on the formation of wax crystal network structure during cooling. The viscoelastic parameters, including the storage modulus G′, loss modulus G″, and loss angle δ were measured and recorded during cooling. The gelation point of the oil samples could also be obtained when the G′equals G″. 2.2.2 Exothermic characteristics tests of the degassed oil samples The exothermic characteristics of the two oil samples with/without adding 100 ppm EVA were measured through a DSC821e differential scanning calorimeter (Mettler-Toledo Co., Switzerland). The cooling rate of the oil samples was fixed at 10 °C/min. Based on the DSC curves, the wax appearance temperature (WAT) of the oil samples could be identified. Meanwhile, the wax content of the oil samples could also be calculated from the DSC curves according to the method given in our former work [32]. 2.2.3 Microscopic observation of the precipitated wax crystals The precipitated wax crystals in the two oil samples with/without adding 100 ppm EVA were observed through an Olympus BX51 microscope (Japan). Firstly, a drop of the oil samples was loaded on a slide and then covered by a transparent cover. Secondly, the slide containing oil sample was placed in a thermal stage (Motic China Group Co., China) fixed in the microscope. Thirdly, the oil sample was preheated to 60 °C for 20 min. Finally, the oil sample was cooled slowly to 15 °C,


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then the morphology of the wax crystals in the oil sample could be observed with the aid of polarized light. 2.2.4 Asphaltenes particle size distribution tests of the degassed oil samples The size distribution of asphaltene particles in the two oil samples with/without adding 100 ppm EVA was measured with the aid of a Malvern Mastersizer 2000 Particle Analyzer (Malvern Co., Britain). Before tests, the oil samples were diluted by a mixed solvent of n-heptane and toluene at the oil sample/mixed solvent mass ratio of 1:9. The n-heptane/toluene mass ratio in the mixed solvent was 3:1 based on the saturates/aromatics mass ratio of the crude oil samples. The size distribution was tested at 50 °C. 2.2.5 Asphaltenes stability tests of the degassed oil samples The asphaltenes stability in the two oil samples with/without adding 100 ppm EVA was evaluated by n-heptane precipitation. The oil samples were first diluted by n-heptane at the mass ratio of 1:9. Then the diluted oil samples were centrifuged under 8000 r/min for 20 min to gain the precipitates at 50 °C. The precipitates were dried under vacuum to drive off n-heptane, and then weighted. The percentage of precipitates from the oil samples was calculated to evaluate the stability of asphaltenes. Each test was repeated three times to assure the reproducibility. Meanwhile, the morphology of the precipitates was also observed through a S4800 Scanning Electron Microscope (Hitachi Co., Japan). The real asphaltenes content in the precipitates was tested by traditional SARA test. 3. Results and Discussion 3.1 Effect of EVA and scCO2 extraction on the flow behavior of the oil samples. 3.3.1 Pour point.


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The most intuitive and effective indicator to evaluate the performance of a PPD is the pour point. As is shown in Table 2, the pour points of oil sample A and oil sample B are 23 °C to 25°C, respectively. Obviously, scCO2 extraction raises the pour point of Changqing degassed crude oil by 2 °C. Regardless of whether the oil sample is treated by scCO2 or not, the pour points of the oil samples are evidently reduced after adding 100 ppm EVA. It indicates that EVA has strong interaction with the oil samples, and it can greatly weaken the gel structure of the oils. Compared with the pour point of oil sample A benefited with EVA (13 °C), the pour point of oil sample B benefited with EVA can be further lowered to 10 °C. Therefore, scCO2 extraction improves the performance of EVA on reducing the pour point of Changqing degassed crude oil. 3.3.2 Viscosity-temperature correlation. The effect of EVA on the viscosity/apparent viscosity of the two oil samples is demonstrated in Figure 3. The viscosity/apparent viscosity of oil sample B is somewhat larger than that of oil sample A, indicating that scCO2 extraction increases the viscosity/apparent viscosity of Changqing degassed crude oil to some extent. Adding EVA equals to the addition of macromolecule compounds, which makes the oil sample viscosity increase slightly at higher temperature. As the oil temperature decreases further, wax molecules precipitate in large quantities and gradually associate with each other to form 3D structures. It makes the apparent viscosity of the oil samples increase rapidly. At this point, the addition of EVA can greatly weaken the structural strength of the precipitated wax crystal network, thus outstandingly reducing the apparent viscosity of the oil samples. Compared with oil sample A doped with EVA, adding EVA can further decrease the apparent viscosity of oil sample B. It can be concluded that scCO2 extraction improves the performance of EVA on reducing the


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viscosity/apparent viscosity of Changqing degassed crude oil. 3.3.3 Viscoelastic properties. As can be seen in Figure 4, because little amount of wax has been precipitated out at higher temperature, both the storage modulus G′ and the loss modulus G′′ of the oil samples are very low. Meanwhile, G′ is far lower than G′′, and the loss angle δ is nearly 90°. At this point, the oil samples are Newtonian fluids. As the temperature decreases, the oil samples will undergo different processes towards gelation. The gelation process of oil sample A and oil sample B (without adding EVA) can be clearly divided into two processes (see Figure 4a and b). Process I is a fast gelation process. Large amount of wax crystals is precipitated from the crude oil samples. The precipitated wax crystals form loose wax crystal flocs relying on the Van der Waals force. The liquid oil phase is absorbed in these loose flocs. With further cooling, the liquid phase among the crystal branches gradually solidifies into the crystallization structure. The precipitated wax crystals gradually crosslink to form a 3D network structure and bind the light oil [15,16]. At this stage, G′ is increasing much faster than G′′, resulting in the more and more obviously elastic response of the system. For oil sample A, G′ equals to G′′ at the gel point of 23.8 °C, while the gel point is 25.7 °C for oil sample B. In Process II, the precipitated wax crystals have formed the firm network structure. The elastic response of the oil samples is more significant than the viscous response. The loss angle δ decreases slowly with cooling, and the system shows the characteristics of a gelled crude oil. The G′ of oil sample B is larger than that of oil sample A. As seen from Figure 4c and d, the gelation process of the oil samples beneficiated with EVA can be divided into three processes. The synergistic effect of EVA and asphaltenes can improve the


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microstructure of wax crystals, inhibiting the ability of wax crystals of developing a three-dimensional network structure. This is Process I of forming EVA-asphaltene-wax ternary crystals to delay the gelation of crude oil

[12-14].

As the temperature decreases further, the amount of

precipitated wax crystals grows to more than 6 wt% (see Figure 5b). The new formation of wax crystals gradually formed, which makes G′ increase rapidly. The elastic response is enhanced, and this is Process II. Eventually, the system reaches Process III, which is similar to the Process II in Figure 4a. Adding EVA can dramatically improve the flow behavior of the crude oil (see Figure 4a and c). For oil sample A, the gel point falls to 13.3 °C after EVA addition, as well as G′ dramatically falls to a lower value. Moreover, scCO2 extraction improves the effect of EVA on the flow behavior (see Figure 4b and d). For oil sample B, the gel point lowers to 8.9 °C after EVA addition, as well as G′ further falls. According to the results mentioned above, it is clear that scCO2 extraction enhances the performance of EVA on improving the flow behavior of Changqing degassed crude oil. 3.2 Effect of EVA and scCO2 extraction on the exothermic characteristics of the oil samples. Changqing crude oil is a colloidal dispersion system, which contains asphaltenes as the solid dispersion phase. Thus, wax precipitation is no longer a homogeneous nucleation of amorphous wax molecules due to thermal fluctuations. Wax precipitation is a process in which the asphaltene particles act as heterogeneous nucleation points and the wax crystals precipitate at the surface of the asphaltene aggregates

[33-35].

Compared with homogeneous nucleation which requires a greater

degree of undercooling, heterogeneous nucleation at the phase interface can occur at higher thermodynamic temperature, elevating the WAT of Changqing crude oil.


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Figure 5a exhibits the influence of EVA and scCO2 extraction on the DSC curve of Changqing crude oils. It can be seen that the WAT of oil sample A is 39 °C. Since the scCO2 extraction destroys the solvation structure of asphaltenes, the phase interface between asphaltene aggregates and hydrocarbons is clearer. It is advantageous for the asphaltene aggregates acting as heterogeneous nucleation points, thus resulting in the increase of the WAT to 40 °C. On the contrary, EVA plays the role of disperser of asphaltene aggregates. It disperses and protects asphaltene aggregates, inhibiting the heterogeneous nucleation of solid dispersion phases [27,34,35].

So it can be seen in Figure 5a that the WAT lowers to 37 °C after adding EVA to the oil

samples. In Figure 5b, the precipitated wax amount is shown. It is obviously that scCO2 extraction hardly changes the precipitated wax amount. The cumulative precipitated wax amount is 18.10 wt% and 18.25 wt% at -20 °C for the crude oil samples before and after scCO2 extraction, respectively. Adding EVA into the oil sample A can reduce the amount of precipitated wax at higher temperature, while the cumulative amount of precipitated wax is still 18.09 wt% at -20 °C. It indicates that EVA can increase the solubility of wax molecules, as well as it can improve the undercooling required for wax crystal precipitation. 3.3 Effect of EVA and scCO2 extraction on the precipitated wax crystal morphology in the oil samples. Figure 6 shows the polarized micrograph of the oil samples at 15 oC. As can be seen in Figure 6a and b, the size of the wax crystals precipitated from the crude oils without adding EVA is small but the amount is large. Thus, it is easy to overlap with each other, forming a three-dimensional network structure. Due to the worse co-crystallization effect of the less active asphaltene aggregates caused


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by scCO2 extraction, the wax crystals become even smaller after scCO2 extraction. Moreover, the larger-in-amount and smaller-in-size wax crystals after scCO2 extraction mean larger interface between wax crystals and liquid oil. It indicates that scCO2 treatment can worsen the flow behavior of the oil sample at lower temperature through larger interface between wax crystals and liquid oil. In Figure 6c and d, the morphology of wax crystals changes from needle-like to clusters after adding EVA into the oil samples. The strong co-crystallization effect of EVA greatly increases the average wax crystal size. As the wax crystals co-crystallize with EVA to form clusters, the total crystal-oil interface decreases rapidly. At this point, the main factor determining the flow behavior of the oil samples is no longer the area of crystal-oil interface but the size of the space not occupied by wax crystals [28]. The larger the space not occupied by wax crystals is, the more liquid oil there is in the oil samples that can flow. ScCO2 extraction can peel off the solvation layer of asphaltene aggregates. Thus, it needs more EVA to surround the asphaltene aggregates closely as a substitute of the natural colloidal solvents. It leads to less liquid oil in the EVA-asphaltene-wax ternary crystals, making the precipitated wax crystals form a more compact structure. Moreover, due to more EVA acts as the disperser to raise the stability of asphaltene aggregates weakened by scCO2, the amount of free EVA in the oil sample B doped with EVA is less than that in the oil sample A doped with EVA. The free EVA bridges the wax crystal clusters together through polar attraction. Therefore, the wax crystal clusters in the oil sample A benefited with EVA are large and loose, while the wax crystal clusters in the oil sample B benefited with EVA are small and compact. 3.4 Effect of EVA and scCO2 extraction on the asphaltenes particle size distribution in the oil samples.


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As is shown in Figure 7, the average particle size of the asphaltene aggregates in the oil sample A is 407.5 nm. No matter scCO2 extraction or not, adding EVA will make the average particle size larger. Due to the strong interaction between EVA and asphaltenes, adding EVA to the oil sample A can increase the particle size of the asphaltene aggregates to 416.1 nm. ScCO2 extraction leads to the association of asphaltenes. It forms asphaltene aggregates with the average particle size increasing to 433.2 nm. Moreover, since the solvation layer of asphaltene aggregates has been damaged by scCO2 extraction, more EVA acts as disperser to improve the stability of the asphaltene aggregates. It can further increase the average particle size to 447.8 nm. It makes clear that the asphaltene aggregates induced by scCO2 extraction can still or even stronger interact with EVA. 3. 5 Effect of EVA and scCO2 extraction on the asphaltene stability of the oil samples. 3.5.1 Amount of the precipitates. Figure 8 shows the stability changes of asphaltene aggregates in oil samples with/without adding EVA. It can be seen that scCO2 extraction makes the asphaltene aggregates less stable. ScCO2 extraction damages the solvation layer of asphaltenes, causing asphaltene aggregates more easy to precipitate. However, EVA can improve the stability of asphaltene aggregates no matter whether the oil samples have been treated by scCO2 or not. EVA can interact with asphaltene aggregates to act as disperser. The asphaltene aggregates, whose solvation layer has been damaged by scCO2 extraction, can be better dispersed with the aid of EVA. Compared with that from the oil sample B without adding EVA, the percentage of precipitates decreases dramatically from 0.939 wt% to 0.646 wt% after adding EVA.


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3.5.2 The SEM images. As is shown in Figure 9, it is obvious that scCO2 extraction can greatly change the morphology of the precipitates and the mass ratio of asphaltenes in the precipitates, while the addition of EVA hardly influence the morphology of the precipitates. Due to the strong π−π electron interaction

[36],

the asphaltenes can aggregate with the solvation

liquids after centrifugation. The existence of a larger amount of solvation liquids in the precipitates before scCO2 extraction results in the large blocks in Figure 9a and c. However, the scCO2 extraction can stepwise extract nonpolar and light polar components from asphaltene micelle, which is disadvantage of protecting asphaltenes from precipitation

[36-38].

Thus, the morphology of the

precipitates after scCO2 extraction is flocculated asphaltene particles (around 1 μm) due to the less solvation liquids. The real asphaltene content in the precipitates before scCO2 extraction (around 92.8 wt%) is smaller than that in the precipitates after scCO2 extraction (around 97.2 wt%), meaning that the scCO2 extraction destroys the solvation layer around asphaltene aggregates, which is disadvantage to asphaltene aggregates on playing a role as the natural PPD. 3.6 Mechanism explanation. 3.6.1 Influence of scCO2 extraction For oil sample A, the dispersed asphaltene particles in the crude oil are regarded as natural PPDs, which can change the structure of the precipitated wax crystals through the heterogeneous nucleation and co-crystallization effects

[39-41].

The soluble wax molecules in the crude oil precipitate and

co-crystallize with asphaltene aggregates to form asphaltene-wax crystals. The microstructure of the crystals is slightly changed from needle-like to plates (see Figure 6a), thus improving the flowability


Energy & Fuels 1 2 3 4 331 5 6 7 332 8 9 333 10 11 12334 13 14 335 15 16 17336 18 19 20337 21 22338 23 24 25339 26 27 340 28 29 30341 31 32 33342 34 35343 36 37 38344 39 40 345 41 42 43346 44 45 46347 47 48348 49 50 51349 52 53 350 54 55 56351 57 58 59352 60

Page 16 of 32

of the crude oil at low temperature. For oil sample B, scCO2 extraction can damage the solvation layer of asphaltenes by extraction effect. It makes the asphaltenes associate with each other, which causes difficulties to form co-crystallization structure with wax crystals, thus preventing the asphaltenes from playing the role of natural PPDs [36,42]. As a result, the wax crystals become smaller (see Figure 6b). 3.6.2 Influence of EVA addition. We consider that the scCO2 extraction changes the dispersing state of the asphaltenes in oil phase, and then alters the EVA-asphaltene-wax interactions. As seen in Figure 10a, for the oil sample A doped with EVA, the solvation layer of asphaltenes is thick and can inhibit the adsorption of EVA on the asphaltene aggregates. Therefore, the formed EVA/asphaltene composite PPD has relatively low efficiency and the formed EVA-asphaltene-wax ternary crystals entrap much liquid oil in the crystal structure, i.e., the wax crystal structure is loose. As seen in Figure 10b, for the oil sample B doped with EVA, the asphaltene solvation layer is destroyed and then favors the EVA adsorption on the asphaltene aggregates. Therefore, the formed EVA/asphaltene composite PPD has higher efficiency and the formed EVA-asphaltene-wax ternary crystals entrap less liquid oil in the crystal structure, i.e., the wax crystal structure is compact. Thus, the synergistic effect of EVA and asphaltenes can improve the flow behavior of the crude oil further. 4. Conclusions In this paper, the effect of EVA on the flow behavior of the degassed crude oil treated by scCO2 is studied in detail based on SARA/HTGC analysis, SEM analysis, pour point test, rheological test, asphaltenes precipitation test, particle size analysis, DSC analysis and microstructure observation. The main conclusions are as follows:


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Energy & Fuels

(1) ScCO2 extraction can break the solvation layer of asphaltenes, leading to the formation of asphaltene aggregates. It prevents asphaltenes from playing a role as the natural wax crystal modifier. Thus, it results in more and smaller precipitated wax crystals, which increases the pour point and apparent viscosity of the degassed crude oil at low temperature. (2) The addition of EVA enhances the solubility of wax thus decreasing the crude oil WAT to some extent, while can change the precipitated wax crystal morphology. The wax crystals morphology of the degassed Changqing crude oil transforms from needle-like ones to clusters after adding EVA. The more compact wax crystal cluster structure can significantly reduce the pour point and improve the flow behavior of the Changqing crude oil at low temperature. (3) The asphaltene aggregates induced by scCO2 can still interact with EVA. The asphaltene solvation layer is destroyed by scCO2 and then favors the EVA adsorption on the asphaltene aggregates. Therefore, the formed EVA/asphaltene composite PPD has higher efficiency, making the precipitated wax crystal structures more compact. It results in more liquid oil left in the oil samples. Thus, the synergistic effect of EVA and asphaltenes can improve the flow behavior of the crude oil further. 5. Acknowledgement This work was financially supported by project of the Flow Behavior of Produced Fluid by scCO2 Flooding from Changqing Oilfield in China. Support from the National Natural Science Foundation of China (Grant No. 51704315) and the Fundamental Research Funds for the Central Universities (Grant No. 18CX02004A) is gratefully acknowledged. References [1] Xu, J., Xing, S., Qian, H., et al. Effect of polar/nonpolar groups in comb-type copolymers on cold flowability and paraffin crystallization of waxy oils. Fuel 2013, 103: 600-605.


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[2] Oliveira, M. C. K., Teixeira, A., Vieira, L. C., et al. Flow Assurance Study for Waxy Crude Oils. Energy & Fuels 2011, 26 (5): 2688-2695. [3] Yao, B., Yang, F., Li, C., et al. Performance improvement of the ethylene-vinyl acetate copolymer (EVA) pour point depressant by small dosages of the Amino-functionalized Polymethylsilsesquioxane (PAMSQ) microsphere: An experimental study. Fuel 2017, 207: 204-213. [4] Yang, F., Zhao, Y., Sjöblom, J., et al. Polymeric wax inhibitors and pour point depressants for waxy crude oils: a critical review. J. Dispersion Sci. Technol. 2015, 36 (2): 213-225. [5] Cao, K., Zhu, Q., Wei, X., et al. Influences of the Molecular Weight and Its Distribution of Poly (styrene-alt-octadecylmaleimide) as a Flow Improver for Crude Oils. Energy & Fuels 2016, 30 (4): 2721-2728. [6] Oliveira, L. M. S. L., Nunes, R. C. P., Melo, I. C., et al. Evaluation of the correlation between wax type and structure/behavior of the pour point depressant. Fuel Process. Technol. 2016, 149: 268-274. [7] Yao, B., Li, C., Yang, F., et al. Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate. Fuel 2016, 166: 96-105. [8] Litvinets, I. V., Prozorova, I. V., Yudina, N. V., et al. Effect of ammonium-containing poly alkyl acrylate on the rheological properties of crude oils with different ratio of resins and waxes. J. Pet. Sci. Eng. 2016, 146: 96-102. [9] Binks, B. P., Fletcher, P. D. I., Roberts, N. A., et al. How polymer additives reduce the pour point of hydrocarbon solvents containing wax crystals. Phys. Chem. Chem. Phys. 2015, 17 (6): 4107-4117. [10] Yao, B., Wang, L., Yang, F., et al. Effect of Vinyl-Acetate Moiety Molar Fraction on the Performance of Poly (Octadecyl Acrylate-Vinyl Acetate) Pour Point Depressants: Experiments and Mesoscopic Dynamics Simulation. Energy & Fuels 2017, 31 (1): 448-457. [11] He, C., Ding, Y., Chen, J., et al. Influence of the nano-hybrid pour point depressant on flow properties of waxy crude oil. Fuel 2016, 167: 40-48. [12] Yao, B., Li, C., Yang, F., et al. Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 1. Effect of Wax Content and the Synergistic Mechanism. Energy & Fuels 2018, 32 (2): 1567-1578. [13] Yao, B., Li, C., Yang, F., et al. Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 2. Effect of Asphaltene Content. Energy & Fuels 2018, 32 (5): 5834-5845. [14] Yao, B., Li, C., Yang, F., et al. Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 3. Vinyl Acetate Content. Energy & Fuels 2018, 32 (8): 8374-8382. [15] Jost, R., Sebastien, S., Jens, N., et al. Wax-Inhibitor Interactions Studied by Isothermal Titration Calorimetry and Effect of Wax Inhibitor on Wax Crystallization. Energy & Fuels 2017, 31 (7): 6838-6847. [16] Yi, S. and Zhang, J. Shear-Induced Change in Morphology of Wax Crystals and Flow Properties of Waxy Crudes


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Energy & Fuels

Modified with the Pour-Point Depressant. Energy & Fuels 2011, 25 (12): 5660-5671. [17] Katherine, Y., Hornafius, J., and Scott, H., Carbon negative oil: A pathway for CO2 emission reduction goals. Int. J. Greenhouse Gas Control 2015, 37: 492-503. [18] Tran, T. Q. M. D., Neogi, P., and Bai, B. Stability of CO2 Displacement of an Immiscible Heavy Oil in a Reservoir. SPE J. 2017, 22 (2): 539-547. [19] Lei, H., Yang, S., Zu, L., et al. Oil Recovery Performance and CO2 Storage Potential of CO2 Water-Alternating-Gas Injection after Continuous CO2 Injection in a Multilayer Formation. Energy & Fuels 2016, 30 (11): 8922-8931. [20] Ali, A. and Farshid, T. Oil Recovery Performance of Immiscible and Miscible CO2 Huff-and-Puff Processes. Energy & Fuels 2014, 28 (2): 774-784. [21] Ali, E. B., Radzuan, J., Shahab, H., et al. Application of CO2-based vapor extraction process for high pressure and temperature heavy oil reservoirs. Petrol. Sci. Eng. 2015, 135: 280-290. [22] Sharbatian, A., Abedini, A., Qi, Z., et al. Full characterization of CO2–oil properties on-chip: solubility, diffusivity, extraction pressure, miscibility, and contact angle. Analytical chemistry 2018, 90 (4): 2461-2467. [23] Ding, M., Wang, Y., Liu, D., et al. Mutual interactions of CO2/oil and natural gas/oil systems and their effects on the EOR process. Pet. Sci. Technol. 2015, 33 (23-24): 1890-1900. [24] Bian, X. Q., Han, B., Du, Z. M., et al. Integrating support vector regression with genetic algorithm for CO2-oil minimum miscibility pressure (MMP) in pure and impure CO2 streams. Fuel 2016, 182: 550-557. [25] Hawthorne, S. B., Miller, D. J., Sorensen, J. A., et al. Effects of reservoir temperature and percent levels of methane and ethane on CO2/Oil MMP values as determined using vanishing interfacial tension/capillary rise. Energy Procedia 2017, 114: 5287-5298. [26] Barker, R. J., Hu, X., Neville, A., et al. Empirical Prediction of Carbon-Steel Degradation Rates on an Offshore Oil and Gas Facility: Predicting CO2 Erosion-Corrosion Pipeline Failures Before They ccur. SPE J. 2014, 19 (3): 425-436. [27] Lu, T., Li, Z., Fan, W., et al. Nanoparticles for Inhibition of Asphaltenes Deposition during CO2 Flooding. Ind. Eng. Chem. Res. 2016, 55 (23): 6723-6733. [28] Yao, B., Li, C., Yang, F., et al. Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant. Fuel 2016, 184: 544-554. [29] Li, C., Yang, F., Yang, S., et al. Supercritical CO2 Treating Equipment for Crude Oil. CN Patent 2015. NO. 201510387232.1 [30] ASTM D5853-11, Standard Test Method for Pour Point of Crude Oils. 2011. West Conshohocken, United States: ASTM.


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[31] Kristofer, P., Anne, S., Geir, S., et al. Characterization of the Formation, Flowability, and Resolution of Brazilian Crude Oil Emulsions. Energy & Fuels 2009, 23 (1): 471-480. [32] Yang, F., Cai, J., Cheng, L., et al. Development of Asphaltene-Triggered Two-Layer Waxy Oil Gel Deposit under Laminar Flow: An Experimental Study. Energy & Fuels 2016, 30 (11): 9922-9932. [33] Gabrienko, A. A., Martyanov, O. N., and Kazarian, S. G. Behavior of Asphaltenes in Crude Oil at High-Pressure CO2 Conditions: In Situ Attenuated Total Reflection−Fourier Transform Infrared Spectroscopic Imaging Study. Energy & Fuels 2016, 30 (6): 4750-4757. [34] Tinsley, J. F., Jahnke, J. P., Dettman, H. D., et al. Waxy gels with asphaltenes 1: Characterization of precipitation, gelation, yield stress, and morphology. Energy & Fuels 2009, 23 (4): 2056-2064. [35] Tinsley, J. F., Jahnke, J. P., Adamson, D. H., et al. Waxy gels with asphaltenes 2: use of wax control polymers. Energy & Fuels 2009, 23 (4): 2065-2074. [36] Timing, F., Muhan, W., Jiawei, L., et al. Study on the Asphaltene Precipitation in CO2 Flooding: A Perspective from Molecular Dynamics Simulation. Ind. Eng. Chem. Res.2018, 57 (3): 1071-1077. [37] Doris, L. G., Francisco, M. V., George, J. H., et al. Modeling Study of CO2-Induced Asphaltene Precipitation. Energy & Fuels 2008, 22 (2): 757-762. [38] Edris, J., Jim, B., Rod, B., et al. Exploration of the Difference in Molecular Structure of n-C7 and CO2 Induced Asphaltenes. Ind. Eng. Chem. Res. 2018, 57 (26): 8810-8818. [39] Ehsan, M., Fatemeh, S. Z., Vahid, T., et al. Effects of Paraffinic Group on Interfacial Tension Behavior of CO2-Asphaltenic Crude Oil Systems. J. Chem. Eng. Data 2014, 59 (8): 2563-2569. [40] Alcazar-Vara, L. A., Garcia-Martinez, J. A., and Buenrostro-Gonzalez, E. Effect of asphaltenes on equilibrium and rheological properties of waxy model systems. Fuel 2012, 93 (1): 200-212. [41] Lei, Y., Han, S., and Zhang, J. Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions. Fuel Process Technol 2016, 146: 20-28. [42] Li, Y., Han, S., Lu, Y., et al. Influence of Asphaltene Polarity on Crystallization and Gelation of Waxy Oils. Energy & Fuels 2018, 32 (2): 1491-1497.


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Figure:

Figure 1.

Schematic diagram of scCO2 treating equipment: 1. CO2 gas cylinder, 2. temperature

control component, 3. piston tank, 4. PVT device, 5. high pressure sampler, 6. metering pump.


Energy & Fuels

Oil sample A Oil sample B

5

Mass fraction / wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

4

3

2

1

0

5

10

15

20

25

30

35

40

45

Carbon number Figure 2.

Carbon number distribution of the hydrocarbons in the crude oil samples.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Viscosity(apparent viscosity) / mPa·s

Page 23 of 32

Oil sample A Oil sample B Oil sample A doped with EVA Oil sample B doped with EVA

100

10

0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 -1

-1

T /K Figure 3.

Viscosity-temperature curves of the oil samples with/without adding EVA.


Energy & Fuels

100 G' G'' δ

10000

G'5°C=61403Pa

10

G''5°C=7534Pa

23.8°C

40

1 0.1

1000

I 60

II

100 10

80

I

G'5°C= 91551Pa

60

G''5°C=10008Pa

25.7°C

1

40

0.1 0.01

20

0.01

II

20

1E-3 10

15 20 Temperature/ °C

100000

G'5°C= 2082Pa

10000

G''5°C= 679Pa

1000

25

100 G' G'' δ

80

100

60

δ/°

13.3°C

II

1

40

III

0.1

1E-3

Figure 4.

10000

10


25

0 30 (c)

5

10


25

0

100

G''5°C= 318 Pa

10 1

30 (b)

G'5°C= 756 Pa

I

G' G'' δ

100

0.1 20

0.01 5

100000

1000

I

10

1E-4

0 30 (a)

80 60

δ/°

5

G' G'' / Pa

1E-3

G' G'' δ

10000

G' G'' / Pa

100

80

δ/°

G' G'' / Pa

1000

100

100000

δ/°

100000

G' G'' / Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

8.9 °C

II

40

III

20

0.01 1E-3 5

10


25

0 30 (d)

Viscoelastic properties of the oil samples: (a) oil sample A, (b) oil sample B, (c) oil sample A doped with EVA, (d) oil sample B doped with EVA.


Page 25 of 32

0.90 0.85 -1

0.80

Heat flow/ W· g

0.75 0.70 0.65 0.60

le A mp B A ple EV sam l i ith O w d ope VA Ad e l th E p i m w sa d ope Oil Bd e l p sam Oil sa Oil

Figure 5.

WAT=40 °C

WAT=37 °C

0.50 0.45

WAT=39 °C

WAT=37 °C

0.55

Precipitated wax crystal amount / wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-20

0

20 40 Temperature/°C

60 (a)

1

10

0

10

-1

10

Oil sample A Oil sample B Oil sample A doped with EVA Oil sample B doped with EVA

-2

10

-3

10

-4

10

-20

-10

0 10 20 Temperature / °C

30

40 (b)

DSC curves (a) and precipitated wax contents (b) of the crude oil samples with/without adding EVA.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6.

Page 26 of 32

(a)

(b)

(c)

(d)

Wax crystal morphology of the oil samples at 15 °C: (a) oil sample A, (b) oil sample B, (c) oil sample A doped with EVA, (d) oil sample B doped with EVA.


Page 27 of 32

Oil sample A Oil sample A doped with EVA Oil sample B Oil sample B doped with EVA

20

Mass fraction / wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

15 Average Size = 447.8 nm Average Size = 433.2 nm Average Size = 416.1 nm Average Size = 407.5 nm

10 5 0 200

Figure 7.

400

600 800 Particle Size/ nm

1000

1200

Size distribution of the asphaltene particles in the crude oil samples diluted by solvent at 50 °C.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Percentage of precipitated asphaltenes / wt%

Energy & Fuels

Page 28 of 32

1.0

0.8

0.6

0.4 Oil sample A Oil sample B

Oil sample A

Oil sample B

doped with EVA doped with EVA

Figure 8.

Asphaltenes precipitation test of the crude oil samples.


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Figure 9.

Energy & Fuels

(a)

(b)

(c)

(d)

The morphology of precipitates centrifuged from (a) oil sample A, (b) oil sample B, (c) oil sample A doped with EVA, (d) oil sample B doped with EVA.


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Page 30 of 32

(a)

(b) Figure 10.

Schematic of the EVA-asphaltene-wax ternary crystals before (a)/ after (b) scCO2 extraction.


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Energy & Fuels

Table:

Table 1.

SARA results of the oil samples before/after scCO2 extraction. Saturates

Aromatics

Resins

/wt%

/wt%

/wt%

Oil sample A

64.88

24.72

8.97

1.43

Oil sample B

64.15

25.07

9.27

1.51


Asphaltenes /wt%

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2.

Page 32 of 32

Pour point of the oil samples with/without adding EVA. Sample

Pour Point /°C

Oil sample A

23

Oil sample B

25

Oil sample A doped with EVA

13

Oil sample B doped with EVA

10


Effect of Polyethylene-Vinyl Acetate (EVA) Pour Point Depressant on

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