Characterization of bitumen and a novel multiple synergistic method for reducing bitumen viscosity with nanoparticles and surfactants

Abstract

This paper is concerned with the formation of bitumen during the drilling of the H oilfield in Iraq. The high viscosity and strong adhesion properties of bitumen can influence the drilling operations. Some complex problems include paste screening, and drill pipe sticking, which cause huge economic losses. Therefore, it is necessary to effectively reduce the bitumen viscosity. The contribution of a single subcomponent of bitumen to the viscosity can vary, and the combined effect of different components of bitumen on the viscosity remains unclear. Furthermore, the mechanism of viscosity reduction remains unclear. In this study, the effects of organic solvents on the viscosity of bitumen were studied, and toluene was selected as the best organic solvent. The results showed that aromatics/resins, aromatics/asphaltenes, and resin/asphaltenes can help increase the bitumen viscosity. Novel methods, including the use of nanoparticles, ethyl cellulose, and the quaternary ammonium salt of heptadecenyl hydroxyethyl imidazoline (QASHI), were proposed to decrease the viscosity. TiO₂ and CuO nanoparticles were chosen, and the main factors influencing the viscosity, such as the particle type, concentration, particle size, temperature, and shear rate, were analysed. The results show that the bitumen viscosity decreases with the increase in the concentrations of ethyl cellulose and QASHI. A synergistic effect between ethyl cellulose and QASHI was found with an optimal concentrations of ethyl cellulose and QASHI (1000 and 1600 mg L⁻¹). A synergistic effect was also observed when nanoparticles, ethyl cellulose, and QASHI were used in combination. This paper reports the micro-mechanism whereby the viscosity of bitumen is decreased.

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1. Introduction

During the drilling of the H oilfield in Iraq, highly viscous bitumen was encountered. With plastic creep characteristics and strong cohesion ^1–4, bitumen deteriorates the properties of the drilling fluid and increases the risk of drill pipe sticking.^5,6 This could reduce the rate of penetration (ROP) and extend the non-productive time (NPT), causing significant economic losses. Fig. 1 shows the bitumen circulated back with the drilling fluid in the drilling field.

Fig. 1 Bitumen encountered during the drilling of the H oilfield: (a) schematic of bitumen formation in the drilling process; (b) different scales of bitumen mixtures in drilling fluid; (c) bitumen collected from the drilling fluid; (d) rheological property test of drilling fluid contaminated with bitumen pollution (bitumen easily adhere and stuck the drill pipe).

The macroscopic phenomenon is controlled by microscopic mechanisms. To understand the high viscosity of bitumen, it is important to analyse the basic chemical components and their effect on viscosity. A saturate, aromatic, resin, and asphaltene (SARA) analysis has shown that the bitumen formed mainly includes saturates, aromatics, resins, and asphaltenes.^5–7 Different subfractions have shown different density and viscoelasticity.^5,8,9 Early studies¹⁰ have found that increasing the content of asphaltene can increase the viscosity of bitumen. Asphaltene has a high percentage of heteroatoms (N, O, S, and metals), and its self-aggregation effect will further increase the structural complexity of bitumen.^11–13 However, studies on the influence of other components on the viscosity of bitumen, including saturates, aromatics, and resins, are lacking. In addition, the relationship between the different subfractions in increasing the bitumen viscosity is unclear.

The most straightforward method of solving the bitumen formation problem is reducing its viscosity.^14–18 Many researchers have designed novel viscosity reducers to decrease the viscosity, with good effect.^17,19–25 However, the synthesis methods are complex and difficult to be applied to industrial production. Nik et al. prepared oxidised asphaltene as a viscosity reducer for bitumen. Although the viscosity reduction effect is good, the preparation process is complicated and unsuitable for large-scale industrial applications. Mortazavi-Manesh and Shaw prepared a novel copolymer as a bitumen viscosity reducer; however, it is difficult to apply this method to industrial applications. Recently, nanoparticles have been widely used in the petroleum industry for applications such as enhanced oil recovery (EOR),^26–28 emulsion stability and demulsification,²⁹ and heat conducting processes.^30,31 The viscosity of bitumen is mainly associated with the formation of a viscoelastic network between asphaltene nanoaggregates or asphaltene and other bitumen subfractions. Nanoparticles have a strong adsorption affinity with asphaltenes and are expected to help decrease bitumen viscosity owing to their small size, high surface area-to-volume ratio, dispersibility, and high adsorption affinity. However, nanoparticles have rarely been employed in this field.

In this study, the contributions of the basic components of bitumen to its viscosity were analysed. Nanoparticles were innovatively introduced to reduce the viscosity with surfactants, and a systematic optimisation of viscosity decreasing additives formula is provided. Ethyl cellulose and QASHI were chosen to decrease the viscosity. The synergistic micro-mechanism between ethyl cellulose (QASHI surfactant) and the nanoparticles in reducing the bitumen viscosity was revealed. This is a fundamental study for the development of a novel anti-bitumen drilling fluid formula.

2. Materials and methods

2.1 Materials

Various chemical reagents were used in the experiments. Table 1 lists the names of the chemical additives and of their manufacturers.

Table 1 Various chemical additives used in the experiments and their corresponding manufacturers

Chemical additives	Manufacturers	Purity
TiO2 nanoparticles (with diameters of 10, 20, 50, and 100 nm)	Aladdin Chemistry Co. Ltd.	≥99%, analytical purity
CuO nanoparticles (with diameters of 10, 20, 50, and 100 nm)		≥99%, analytical purity
Al2O3		30 nm, analytical purity
Toluene, n-heptane, methanol, p-xylene, cyclopentane, trichloroethylene, ethanol, chloroform, QASHI, et al.	Qingdao Macklin Biochemical Co., Ltd	Chemically purity

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Bitumen samples (85.6% content) were obtained from the Mishrif Formation in the H oilfield, Iraq. The depth of this stratum is in the range of approximately 3200–4500 m, thus falling in the limestone stratum range. Bitumen is formed at many locations in this layer, and its distribution is irregular. 20 of different depth bitumen samples were took, and then the samples were mixed, and the bitumen were extracted from the mixed samples. Table 2 lists the detailed properties of bitumen.

Table 2 Elemental and SARA analyses, including the density and viscosity of bitumen

Elemental analysis (wt%)	C	H	O	N	S
	83.4	9.6	0.1	0.5	6.4

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SARA analysis (wt%)	Saturate	Aromatic	Resin	Asphaltene
	34.7	26.8	19	19.5

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Density (g /cm^−33)

1.2

Viscosity (mPa ·s)

128 000

Softening point (°C)

117.9

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2.2 Methods

Fig. 2 shows a flowchart of the research conducted in this study.

Fig. 2 Flowchart of the research procedure.

2.2.1 Bitumen extraction and SARA analysis. Fig. 3 shows the detailed experimental procedures for bitumen recovery and SARA analysis. The experimental procedure was in accordance with those in previous studies,^32,33 and the (SARA) fractionation of bitumen was performed as per ASTMD4124.⁵

Fig. 3 Experimental procedure of bitumen extraction and separation of saturate, aromatic, resin, and asphaltene components from bitumen.

First, 10 g of the crude bitumen sample was mixed in 60 mL of toluene. Ultrasound extraction was carried out for 1 h. The soluble components were sent to a rotary evaporator; the insoluble components were solids. After toluene extraction, 7.62 g of bitumen was extracted from the crude bitumen samples. This bitumen was used for the viscosity reduction experiment.

Second, 3 g of bitumen was poured into a 500 mL beaker, and 200 mL of n-heptane was added. A sonicated extraction was conducted at 50 °C for 30 min. The insoluble and soluble components were separated through centrifugation conducted at 8000 rpm for 10 min. The insoluble solids were asphaltenes, and the soluble components were saturate, aromatic, and resin (SAR) components. Thereafter, the soluble components were filtered, and the SAR components were then loaded onto the Al₂O₃ column. The SAR components were collected individually by elution with 100 mL of heptane and 100 mL of mixed organic solvent (methanol/toluene (V/V, 1 : 1)).

2.2.2 Viscosity measurements. The viscosity was measured using the MCR 301 rheometer (Anton Paar Germany GmbH, Graz, Austria). The rheological measurements were conducted in a temperature range of 20–60 °C and a shear rate range of 0–80 s⁻¹. Every experiment was repeated at least three times; the final value was the average of the three.
(1) Role of organic solvents in decreasing bitumen viscosity. Organic solvents, including toluene, p-xylene, and cyclopentane, were used to decrease the viscosity of bitumen. One gram of bitumen was diluted in different amounts (0–1 mL) of organic solvents. The viscosity was measured at 20 °C and at a shear rate of 20 s⁻¹.
(2) Effects of bitumen components on viscosities of SARA-toluene solutions. To study the role of saturate, aromatic, resin, and asphaltene components in decreasing the bitumen viscosity, the viscosity measurements of the saturate, aromatic, resin, and asphaltene-toluene solutions were repeated by preparing samples with different mass concentrations (0, 0.1, 0.2, 0.3, 0.4, and 0.5 g mL⁻¹) at 20 °C and at a shear rate of 20 s⁻¹. Moreover, the synergistic effect between the components was studied. The saturate/aromatic, saturate/resin, saturate/asphaltene, aromatic/resin, aromatic/asphaltene, and resin/asphaltene mixtures were used to prepare the mixture-toluene solutions at different concentrations. The viscosities were measured at 20 °C and at a shear rate of 20 s⁻¹.
(3) Effect of temperature on viscosities of 0.5 g mL⁻¹ SARA-toluene solutions. The viscosity measurements of the saturate, aromatic, resin, and asphaltene-toluene solutions were repeated for a 0.5 g mL⁻¹ sample in the temperature range of 20–60 °C and at a shear rate of 20 s⁻¹.
(4) Effects of temperature and nanoparticle concentration on bitumen viscosity. To study the role of nanoparticles in decreasing the bitumen viscosity, the nanoparticles were mixed with bitumen at a stirring rate of 300 rpm for 1 h. The nanoparticles dispersed into the bitumen uniformly. In this study, 20 nm TiO₂ and 20 nm CuO were selected as the nanoparticles. The effect of different nanoparticle concentrations (0, 100, 500, 1000, 2500, 5000, and 10 000 mg L⁻¹) on the viscosity of bitumen at different temperatures (20, 30, 40, 50, and 60 °C) and at a shear rate of 20 s⁻¹ was studied.
(5) Effects of nanoparticle size and shear rate on bitumen viscosity. Because different nanoparticle sizes can influence the surface area, the nanoparticle size could influence the bitumen viscosity. In this study, 2500 mg L⁻¹ of TiO₂ and 1000 mg L⁻¹ of CuO were chosen, because at this concentration, the nanoparticles showed the highest viscosity reduction effect. To study the nanoparticle size effect on the bitumen viscosity, 10, 20, 50, and 100 nm TiO₂ and CuO nanoparticles were used. Because bitumen is a non-Newtonian fluid, and the viscosity is not linear with the shear rate, the variation in the bitumen viscosity at different shear rates (20, 40, 60, and 80 s⁻¹) was studied.
(6) Effects of ethyl cellulose and QASHI concentration on bitumen viscosity. Ethyl cellulose has been widely used for decreasing the viscosity of emulsions.^34–36 However, reports on the use of ethyl cellulose for decreasing bitumen viscosity are limited, but many researches focused on the ethyl cellulose help bitumen recovery.^37,38 In this study, ethyl cellulose and QASHI were added to bitumen with different concentrations and stirred at 300 rpm for 1 h. The viscosity was measured at 20 °C and at a shear rate of 20 s⁻¹. The concentrations of ethyl cellulose and QASHI were in the range of 0–2000 mg L⁻¹.
(7) Synergistic effect of ethyl cellulose (QASHI) and nanoparticles on decreasing bitumen viscosity. The nanoparticles and ethyl cellulose (QASHI) were mixed in bitumen at optimal concentrations. The stirring rate was 300 rpm, and the stirring time was 1 h. The bitumen viscosity was measured at 20 °C and at a shear rate of 20 s⁻¹.

3. Results and discussion

3.1 Effect of organic solvent on decreasing bitumen viscosity

Fig. 4 shows the viscosities of bitumen diluted with different organic solvents at 20 °C and at a shear rate of 20 s⁻¹. Toluene, p-xylene, and cyclopentane help decrease the bitumen viscosity. When the organic concentration is lower than 0.3 mg L⁻¹, the viscosity reducing capacity is as follows: toluene > p-xylene > cyclopentane. This is because toluene and p-xylene are aromatic solvents and have better solubility with the aromatic, resin, and asphaltene components in bitumen.^39,40 When the organic solvent concentration is 0.1 mg L⁻¹, the bitumen viscosities decrease by one order of magnitude, indicating that the organic solvents have a good viscosity reducing effect.

Fig. 4 Viscosities of bitumen diluted with different concentrations of organic solvents at 20 °C and at a shear rate of 20 s⁻¹.

3.2 Effect of SARA components

To study the individual roles of the SARA components in reducing the bitumen viscosity, saturate, aromatic, resin and asphaltene-toluene solutions were prepared. Fig. 5 shows the viscosities for concentrations in the range of 0–0.5 mg L⁻¹ at 20 °C, and at a shear rate of 20 s⁻¹. For saturate, aromatic, resin, and asphaltene components, the SARA-toluene viscosity increases with the concentration of the SARA components. For the same concentration of the SARA components, the viscosity values were in the following order: saturate < aromatic < resin < asphaltene. As the saturate, aromatic, resin, and asphaltene components have different polarity and density, their influence on the bitumen viscosity is different. Although the asphaltene content in the toluene solution was low, asphaltene precipitants increase the viscosity, and asphaltene agglomerates increase with increasing asphaltene concentration.^12,41–43 This is consistent with the results obtained by Hemmati-Sarapardeh et al.⁴⁴

Fig. 5 Viscosities of (a) saturate; (b) aromatic; (c) resin; (d) asphaltene-toluene solutions with different concentrations at 20 °C and at a shear rate of 20 s⁻¹.

3.3 Effects of SARA composite concentrations on viscosities of SARA-toluene solutions

Fig. 6 shows the effect of SARA composite concentrations on the viscosities of the SARA-toluene solutions.

Fig. 6 Viscosities of (a) saturate, aromatic; (b) saturate, resin; (c) saturate, asphaltene; (d) aromatic, resin; (e) aromatic, asphaltene; (f) resin, asphaltene-toluene solutions with different concentrations at 20 °C and at a shear rate of 20 s⁻¹.

The saturate/aromatic, saturate/resin, saturate/asphaltene, aromatic/resin, aromatic/asphaltene, and resin/asphaltene solutions were mixed and diluted in toluene with different concentrations. As shown in Fig. 6(a), when the concentration is in the range of 0.1–0.5 mg L⁻¹, the viscosity of the saturate/aromatic component is higher than that of the saturate component but lower than that of the aromatic component. The same is true for the saturate/resin (Fig. 6(b)) and saturate/asphaltene (Fig. 6(c)) components. However, Fig. 6(d)–(f) show that the viscosities of the mixed aromatic, resin, and asphaltene components are higher than those of the individual components at the same concentration. For example, the synergistic effect between the aromatic, resin, and asphaltene components in increasing the bitumen viscosity is obvious. This could be because all the aromatic, resin, and asphaltene components are aromatic organic compounds, and their mixture would aid resin and asphaltene aggregation. The more the aggregation, the higher the viscosity.

3.4 Effect of temperature on viscosities of 0.5 g mL⁻¹ SARA-toluene solutions

The influence of temperature on the viscosities of the SARA-toluene solutions was studied, as shown in Fig. 7. Saturate-toluene, aromatic-toluene, resin-toluene, and asphaltene-toluene solutions at a concentration of 0.5 mg L⁻¹ were used. The viscosity decreases with the increase in the temperature from 20 to 60 °C. The degree of viscosity reduction of the different solutions with temperature is as follows: asphaltene > resin > aromatic > saturate. This is because asphaltene and resin do not aggregate as much at higher temperatures.

Fig. 7 Viscosities of saturate, aromatic, resin, and asphaltene-toluene solutions (0.5 g mL⁻¹) with different temperatures and at a shear rate of 20 s⁻¹.

3.5 Effect of temperature on bitumen viscosity under different nanoparticle concentrations

Fig. 8 shows the bitumen viscosity measurements in the presence of TiO₂ and CuO nanoparticles at concentrations of 0, 100, 500, 1000, 2500, and 10 000 mg L⁻¹, in the temperature range of 20–60 °C, and at a shear rate of 20 s⁻¹. The results show that the viscosity tends to decrease with the increase in the concentration of the nanoparticles in the bitumen. The can be mainly attributed to the absorption of the heavy components of bitumen by the nanoparticles, particularly asphaltene, and the consequent decrease in the aggregation of asphaltene.^45–47 With the increase in the nanoparticle concentration, the amount of asphaltene adsorbed increases, and the bitumen viscosity decreases. However, for the TiO₂ nanoparticles, the optimal concentration is 2500 mg L⁻¹ (Fig. 8(a)). The optimal concentration of the CuO nanoparticles is 1000 mg L⁻¹. When the nanoparticle concentration is higher than the optimal concentration, the bitumen viscosity increases. This is because at higher nanoparticle concentrations, the packing factor of the particles increases, and the nanoparticles can aggregate. The interaction between the nanoparticles and the bitumen particles decreases, and a greater number of nanoparticles aggregate, thus increasing the bitumen viscosity.

Fig. 8 Viscosity of bitumen in the presence of (a) TiO₂ nanoparticles; (b) CuO nanoparticles at different concentrations and temperatures ranging from 20 to 50 °C and at a shear rate of 20 s⁻¹.

For the TiO₂ and CuO nanoparticles, the bitumen viscosity decreases with increasing temperature. The reason for this is as follows. First, at higher temperatures, the viscosity of bitumen subfractions decreases, as shown in Fig. 8. Second, a greater amount of asphaltene is absorbed by the nanoparticles at higher temperatures.⁴⁸

3.6 Effect of shear rate on bitumen viscosity under different nanoparticle sizes

Fig. 9 shows the variation in the bitumen viscosity in the presence of 2500 mg L⁻¹ of TiO₂ nanoparticles and 1000 mg L⁻¹ of CuO nanoparticles and with different sizes at 20 °C and a shear rate range of 20–80 s⁻¹. The viscosity reduction effect decreases with the increase in the nanoparticle size. For larger nanoparticles, the specific surface area decreases, and their ability to adsorb asphaltene becomes less. For a fixed size and concentration, the bitumen viscosity decreases with increasing shear rate.

Fig. 9 Bitumen viscosity in the presence of (a) 2500 mg L⁻¹ of TiO₂ nanoparticles; (b) 1000 mg L⁻¹ of CuO nanoparticles of different sizes at 20 °C and a shear rate range of 20–80 s⁻¹.

3.7 Effect of ethyl cellulose concentration on bitumen viscosity

Fig. 10 shows the bitumen viscosity measurements in the presence of different concentrations of ethyl cellulose (Fig. 11(a)) and QASHI (Fig. 11(b)) at a temperature of 20 °C and a shear rate of 20 s⁻¹.

Fig. 10 Bitumen viscosity with different concentrations of (a) ethyl cellulose; (b) QASHI at 20 °C and at a shear rate of 20 s⁻¹.

Fig. 11 (a) Synergistic effect of aromatic, resin, and asphaltene components on increasing viscosity; (b) mechanisms whereby ethyl cellulose and QASHI help decrease in bitumen viscosity.

The bitumen viscosity decreases with the increase in the ethyl cellulose concentration. The synergistic effect between the aromatic, resin, and asphaltene components in increasing the bitumen viscosity is obvious, as shown in Fig. 11(a). Ethyl cellulose breaks the synergistic effect and decreases the viscosity between the aromatic, resin, and asphaltene components. For example, ethyl cellulose can break the resin or asphaltene aggregates, as shown in Fig. 11(a). When the concentration of ethyl cellulose is higher than 1000 mg L⁻¹, the viscosity remains stable, because ethyl cellulose effectively breaks the resin and asphaltene aggregates, and more ethyl cellulose is redundant. The bitumen viscosity decreases with increasing QASHI concentration. The mechanism here is that QASHI could adsorb asphaltene because of the electrostatic interaction and work as cations, and as asphaltene is negatively charged, the asphaltenes were absorbed to a greater extent, thus decreasing the number of asphaltene aggregates, as shown in Fig. 11(b). However, when the QASHI concentration is higher than 1600 mg L⁻¹, the bitumen viscosity remains stable.

3.8 Synergistic effect of ethyl cellulose and nanoparticles on decreasing bitumen viscosity

Fig. 12 shows the synergistic effect of the nanoparticles with ethyl cellulose and QASHI in decreasing the bitumen viscosity. Ethyl cellulose along with the TiO₂ nanoparticles helps decrease the bitumen viscosity from 103 802 to 34 892 mPa s, which is lower than 73 402 mPa s (TiO₂). The decrease in the viscosity when ethyl cellulose was mixed with CuO nanoparticles is from 103 802 to 37 823 mPa s. The bitumen viscosity in the case of ethyl cellulose and nanoparticles is lower than that in the case of ethyl cellulose or nanoparticles individually. The reasons are as follows. Ethyl cellulose helps the nanoparticles to adsorb asphaltene, thus decreasing the number of asphaltene aggregates. Moreover, the nanoparticles work together with ethyl cellulose to destroy the synergistic effect of the aromatic, resin, and asphaltene components. QASHI along with TiO₂ nanoparticles could decrease the bitumen viscosity from 128 356 to 26 930 mPa s, which is much lower than 87 234 mPa s (QASHI only) and 73 402 mPa s (TiO₂ nanoparticles only). The synergistic effect between QASHI and CuO nanoparticles is also obvious. This can be attributed to the fact that QASHI and nanoparticles have a synergistic effect in increasing the asphaltene adsorption.

Fig. 12 Synergistic effect of nanoparticles with ethyl cellulose and QASHI in decreasing the bitumen viscosity at 20 °C and at a shear rate of 20 s⁻¹.

4. Conclusions

The highly viscous bitumen formed during the drilling of oilfields can significantly affect the drilling efficiency. To effectively remove bitumen, the reason for its high viscosity was studied, and the effect of SARA components on the bitumen viscosity was explored. Novel methods, including the use of organic solvents, TiO₂ nanoparticles, CuO nanoparticles, ethyl cellulose, and QASHI, were employed to decrease the viscosity of bitumen. The following conclusions can be drawn from this study:

(1) The organic solvent toluene exhibited the best effect in decreasing the bitumen viscosity, followed by p-xylene and cyclopentane. Under the same concentration, the viscosity values were as follows: saturate < aromatic < resin < asphaltene. The viscosity of the SARA-toluene solution increased with increasing SARA concentration. The aromatic, resin, and asphaltene components had a synergistic effect in increasing the viscosity. The viscosity decreased at elevated temperatures.

(2) TiO₂ and CuO nanoparticles were used to decrease the bitumen viscosity. The optimal concentrations of the TiO₂ and CuO nanoparticles were 2500 and 1000 mg L⁻¹, respectively. The viscosity of bitumen decreased with increasing nanoparticle concentration and then decreased. Below the optimal concentration, the bitumen viscosity decreased with increasing nanoparticle concentration, because the nanoparticles adsorbed resins and asphaltene to a greater extent. However, the bitumen viscosity increased with increasing nanoparticle concentration. For TiO₂ and CuO, at the same concentration, the bitumen viscosity decreased with increasing temperature. The viscosity reduction effect decreased with increasing nanoparticle size and increasing shear rate.

(3) The viscosity decreased with increasing ethyl cellulose and QASHI concentrations. Ethyl cellulose could break the resin and asphaltene aggregates and decrease the viscosity of bitumen. QASHI could adsorb asphaltene owing to electrostatic interaction and decrease the number of asphaltene aggregates. The optimal concentrations of ethyl cellulose and QASHI were 1000 and 1600 mg L⁻¹, respectively. The nanoparticles, ethyl cellulose, and QASHI could effectively reduce the viscosity of bitumen with a synergistic effect.

Abbreviations

QASHI:

Quaternary ammonium salt of heptadecenyl hydroxyethyl imidazoline

Authors' contributions

Conceptualisation—LY, QZ; methodology—LY, ZH; validation—NZ, ZX; investigation—LY, ZC; data curation—LS, XX; writing—LY, ZH; funding acquisition—CZ, LS.

Conflicts of interest

The authors declare no conflict of interests.

Acknowledgements

This work was financially supported by the 13th Five-Year Plan National Key Project of China (No. 2017ZX05005005-006), the National Major Science and Technology Project of China (No. 2017ZX05032-004-005, No. 2017ZX05030-001), the National Natural Science Foundation of China (No. 51974354 and No. 51704322) and the CNOOC Project (No. 2019KJ-ZC02).

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