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Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Quadripolymers as Viscosity Reducers for Heavy Oil Jincheng Mao,* Jiawei Liu, Yukun Peng, Zhaoyang Zhang, and Jinzhou Zhao State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China ABSTRACT: A series of oil-soluble quadripolymers (denoted as AOSVs) have been synthesized to reduce the viscosity of heavy oil. The quadripolymers were characterized by 1H NMR spectroscopy, GPC, and TGA to determine the structures and properties. By comparing the apparent viscosities of two different kinds of heavy oil before and after the addition of the quadripolymers, we found that the efficiency of the AOSVs for viscosity reduction was markedly elevated for almost the same molecular weight and mole ratio. Thermogravimetric tests and pumping transportation simulations showed that the AOSVs have good thermal stability up to 320 °C and strong shearing resistance. Furthermore, a heavy-component model and a copolymer model were built, and simulations of the interaction process were conducted. The results indicated that the copolymers changed the density of the crude oil by breaking down asphaltene and resin agglomerates to achieve a viscosity reduction.
1. INTRODUCTION The demand for petroleum resources is still increasing in the global energy market, and heavy oil plays a significant role in total oil production.1,2 However, the high viscosity of heavy oil is the main hindrance to the productivity and recovery of this oil. Thus, the processes of exploitation and transportation encounter enormous challenges in flow assurance.3−5 Currently, one common and effective method for achieving flow assurance is the use of chemical additives,6−10 including oilsoluble viscosity reducers and water-soluble emulsifiers. Chemical additives not only can be used independently, but also can be combined with thermal recovery technology in the heavy-oil production process. The addition of oil-soluble viscosity reducers is much more convenient in oilfield applications,11,12 because the use of water-soluble additives would require a large amount of water for dissolution and emulsification, in addition to treatment of the flowback liquid. Furthermore, oil-soluble viscosity reducers can still maintain wide ranges of conditions under which they are applicable while conforming to ecofriendly requirements. Oil-soluble viscosity reducers have been developed on the basis of the pour-point depression of crude oil,13−15 the introduction of new groups (rigid groups, polar groups, etc.), and the interaction and dispersion of asphaltene and resin molecules to reduce the viscosity of heavy oil.16−20 These additives are very selective, meaning that one copolymer product is not necessarily effective for all kinds of heavy oil. The relationship among molecular weight, molar ratio, and dispersion efficiency has been investigated.8,16 The results confirmed that a low molecular weight and dispersity of the copolymer are key factors that determine their performance in viscosity reduction. Amphiphilic molecules have good dispersity and interfacial activity, which leads to their wide use as oilfield chemical additives. They are also used in combination with oil-soluble copolymers to improve the viscosity reduction efficiency.12,21 For these reasons, in this work, a synthesized amphiphilic monomer, namely, 2-(acrylamido)octanesulfonic acid (AMCS), with a short molecular chain and low steric hindrance, was © XXXX American Chemical Society
introduced into a quadripolymer [composed of AMCS, octadecyl acrylate (O), styrene (S) and vinyl acetate (V) and denoted as AOSVs] by free-radical polymerization. The research purposes of this work were (1) to improve the performance and practicability of the viscosity reducer and (2) to analyze the mechanism of molecular interaction between the heavy component and the copolymers by building corresponding models in Materials Studio software.
2. EXPERIMENTAL SECTION 2.1. Materials. Heavy crude oils were obtained from the Tahe oilfield and the Shengli oilfield in China. The properties of the heavy oils, as determined according to saturates, aromatics, resins, and asphaltenes (SARA) analysis, are reported in Table 1. Acrylonitrile, 1-
Table 1. Characterization of the Crude Oils parameter
Tahe oil sample
Shengli oil sample
density at 25 °C (g·cm−3) wax content (wt %) viscosity at 50 °C (mPa·s) water and sediment (wt %) saturates (wt %) aromatics (wt %) resin (wt %) asphaltene (wt %)
0.953 1.84 3892 7.05 21.09 25.22 20.13 24.67
0.927 3.57 1867 3.62 23.45 38.94 19.10 9.32
octene, fuming sulfuric acid (50%), octadecyl acrylate, styrene, and vinyl acetate were purchased from Aladdin (Shanghai, China). Azobis(isobutyronitrile), 1-dodecanethiol, toluene, n-hexane, and ethanol were purchased from Kelong (Chengdu, China). All reagents were analytical reagents. 2.2. Synthesis of Amphiphilic Monomer (AMCS) and Quadripolymer (AOSVs). 2-(Acrylamido)octanesulfonic acid (AMCS) is the amphiphilic monomer to be introduced as one of the monomers of the quadripolymers (AOSVs). AMCS was prepared using 1-octene (0.10 mol), acrylonitrile (0.5 mol), and fuming sulfuric Received: September 5, 2017 Revised: November 9, 2017
A
DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Structural unit of AR and optimized AR structural model. (Resin/asphaltene a/asphaltene b ratio = 4:1:1, where R represents a random fluorenone and carboxylic acid resin.)
Table 2. Experimental Conditions and Copolymer Characteristics content (mol %) copolymer
monomer/CTA ratio (w/w)
AMCS
octadecyl acrylate
styrene
vinyl acetate
Mn (Da)
Mw (Da)
Te AOSV-Q1 AOSV-Q2 AOSV-Q3
18:1 15:1 15:1 15:1
− 3.0 4.0 5.0
56.0 54.0 53.5 53.0
11.0 10.5 11.0 10.0
33.0 32.5 31.5 32.0
7326 6471 5983 6218
12708 13147 11587 12188
acid (0.12 mol) as raw materials. The fuming sulfuric acid was dripped slowly into the solution with rapid stirring at −5 °C. After 40 min, the reaction mixture was heated to 25 °C and heeld at this temperature for 24 h. Diethyl ether was then added to the reaction mixture, and the mixture was cooled to allow precipitation to occur. Random AOSVs with various molar ratios of amphiphilic monomer (AMCS), octadecyl acrylate (O), styrene (S), and vinyl acetate (V) were synthesized by free-radical polymerization in a toluene/ethanol (2:3, v/v) solution under an argon atmosphere, using azobis(isobutyronitrile) (0.8 wt %) as the initiator and 1-dodecanethiol as the chain-transfer agent at 75 °C for 5 h. The reaction mixture was precipitated with n-hexane, and the precipitate was a white solid at room temperature. 2.3. Characterization of Products. The structures of the amphiphilic monomers (AMCS) and quadripolymers (AOSVs) were confirmed by 1H NMR analysis (Bruker AVANCE III HD 400). The compositions and molecular weights of the quadripolymers were measured by gel permeation chromatography (GPC) (PerkinElmer Corporation) at 35 °C, using tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL·min−1 and a series of polystyrene standards with low dispersity. The thermal performance of the AOSVs was measured by thermogravimetric analysis (TGA) (TGA/SDTA85/e, Mettler). The tested temperature range used for TGA was 20−550 °C, with N2 as the fluid at a flow rate of 50 mL·min−1 and a heating rate of 10 °C· min−1. 2.4. Rheological Behavior of Crude Oil. The investigation of rheological properties was carried out using an MCR-302 rheometer (Anton-Paar Physica, Graz, Austria) equipped with a variety of concentric cylinders. Copolymers were added to the oil samples at a concentration of 0.5 wt %, and all samples were examined under a shear rate range of 1.0−200.0 s−1 at 50 °C. 2.5. Laboratory Test to Simulate the Pumping Transportation. In the pumping process, strong shear is likely to break the molecular structures of polymers. Therefore, it is commonly assumed that good performance in terms of viscosity reduction can be achieved only in the initial stage of transportation. To analyze the process in detail, transportation conditions were simulated. According to fluid mechanics theory, the total pressure drop of a horizontal pipeline results from friction loss in the pipeline (hf), and it can be expressed by the Darcy−Weisbach formula22
hf = λ
l υ2 d 2g
(1)
with λ = f (Re , ε)
(2)
where hf is the friction loss in the pipeline, λ is the friction coefficient, l is the length of the pipe, d is the diameter of the pipe, υ is the velocity of the flow, Re is the Reynolds number, and ε is the roughness of the pipe wall. In accordance with the standards of the China petroleum industry, the flow pattern in a heavy-oil pipeline is laminar flow. From the definitions of the friction coefficient and the Reynold’s number, one can write
λ=
64 Re
(3)
and
Re =
υρd υd = ν μ
(4)
where ν is the kinematic viscosity and μ is the dynamic viscosity. Then, the expression for hf can be rewritten as hf =
32 μυ ρd 2 g
(5)
Samples were transported to a pipeline (0.04-m i.d., 10-m length) by screw pump (JWNB-85D). The flow rate (5 m3/h) and pump pressure (0.50 MPa) were set to be the same in all experiments. From eq 5, one can conclude that dynamic viscosity (μ) is the core factor in the pressure drop, and we also performed an analysis of the rationality of the measured data. 2.6. Molecular Simulation of the Viscosity Reduction Mechanism. Molecular simulations are a very important tool in the study of the quantitative relationships within molecular systems in the field of polymers. In this work, simulation models were built using the Amorphous Cell module of Materials Studio 2017. The ab initio force field COMPASS23 was used for all atomistic simulations with a time B
DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Optimized Te and AOSV structural models. The numbers of molecules of octadecyl acrylate, styrene, and vinyl acetate in a unit of Te are 5, 1, and 3, respectively; the numbers of molecules of amphiphilic monomer, octadecyl acrylate, styrene, and vinyl acetate in a unit of AOSV are 1, 10, 2, and 6,, respectively. step of 1 fs at 323 K. Density functional theory (DFT) and molecular dynamics (MD) calculations were carried out using the DMol3 program and the Forcite module in Materials Studio, respectively. Asphaltene and resin are the main objects to be addressed regarding viscosity reduction of heavy oil.16,17,24 According to the characteristics of heavy oil and the average molecular structural models for some oilfields in China,25−27 a heavy-component model (asphaltene resin agglomerates, denoted as AR) was built, with the optimized structural model shown in Figure 1. Based on the copolymer parameters (molar ratio, molecular weight) in Table 2, the number of monomers in the molecular model was determined. Optimized structural models of the terpolymer (Te) and quadripolymers (AOSVs) are shown in Figure 2. Using the Forcite module for correlation calculations, we compared the energies and densities of heavy-component systems between untreated and treated heavy-oil samples.
H, NHCH], 5.62−5.67 [d, H, CH2CHCO], 6.03−6.21 [m, 2H, CH2CHCO]}. Nineteen H atoms in AMCS were confirmed. Copolymers with uniform distributions and lower molecular weights (Mn values of up to 20000 Da) showed better viscosity reduction performance, which was confirmed from the work of the Castro team and our team.8,12,16 The molar ratios of various monomers and the molecular weight were optimized. The effective parameters of the designed copolymers are reported in the Table 2. A typical result for the 1H NMR spectrum of an AOSV quadripolymer is shown in Figure 4, and the assignments of the different peaks are listed in Table 3.
3. RESULTS AND DISCUSSION 3.1. Characterizations of AMCS and AOSV. The 1H NMR (400 MHz, D2O) spectrum of AMCS is shown in Figure 3 {δ 0.76 [t, 3H, (CH2) 5CH3], 1.13−1.73 [m, 10H, (CH2)5CH3], 2.95−3.09 [d, 2H, CH2SO3H], 4.21−4.29 [m,
Figure 4. 1H NMR spectrum of AOSV with CDCl3 as the solvent.
3.2. Thermal Stability of AOSV. Figure 5 presents the thermal properties of an AOSV sample measured by thermogravimetric analysis (TG-DTG). As the temperature increased, the quality of the sample changed by evaporation, decomposition, and other factors. Under an inert atmosphere, AOSV starts its thermal decomposition rapidly at 320 °C. At 425 °C, AOSV is almost fully decomposed with little residue, indicating that the structures of AOSV have been destroyed.
Figure 3. 1H NMR spectrum of AMCS. C
DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 3. Peak Assignments for Functional Groups of AOSV characteristic signal(s)
shift (ppm)
assignment
a b, c d e f g h
0.8−0.9 1.2−1.8 2.1 3.1 4.0 4.5 7.1
−CH3 −CH2− −CO−CH3 −CH2−SO3H −OCH2− −CH− linked to −NH− aromatic ring
Figure 7. Apparent viscosities of Shengli oil containing copolymers as a function of shearing rate at 50 °C.
are all more than 66.3% at a shear rate of 50 s−1; For Shengli crude oil, AOSVs still performed well, and the viscosity reduction rate has been improved from 48.7% to 61.2% by AOSV-Q1 at the shear rate of 50 s−1. Overall, AOSV-Q3 exhibited the most considerable reduction of apparent viscosity. This can be explained by the capacity of the monomers: The amphiphilic monomers in AOSVs can increase the solubility and dispersity of the copolymers in heavy oil, which can promote the interaction of the soluble structure with the crudeoil molecules and disperse agglomerates of resin and asphaltene. Furthermore, AOSVs contain more branched chains and multiple attachment points than Te of similar molecular weights. 3.4. Pumping Transport Experiments. Crude-oil samples and additives were circulated through the whole flow system, as shown in Figure 8. The inlet pressure and the outlet pressure (three cycles) of the unit pipeline were recorded at 50 °C, with the results listed in Table 4. The inlet pressure was kept constant by proper setting of the pump temperature and the flow rate. In contrast to heavy-oil transportation, the friction losses of oil samples with added Te and AOSV-Q3 decreased sharply, and the mean storage pressures of both systems were more than 0.045 MPa. The mean outlet pressure of AOSV-Q3 was about 0.0137 MPa higher than that of Te. After three successive cycles, the value of the outlet pressure confirmed that the synthesized AOSV-Q3 sample had an obvious viscosity reduction effect on heavy crude oil and that AOSV-Q3 was stable under the simulated transportation conditions. 3.5. Analysis of the Mechanism of Viscosity Reduction. The density analysis of the trajectories was performed using the Analysis module, and Figure 9 shows that the densities of the AR, AR + Te, and AR + AOSV systems tend to be approximately stable after 45 ps. The average molecular weight of the asphaltene was far greater than that of the heavy oil, so the density of the heavy-component model (AR) exceeded that of the heavy oil with an approximate value of 1.06 g/cm3. After addition of an oil-soluble copolymer (Te or AOSV), the density analysis of the heavy component showed a significant downward trend, and the AR + Te and AR + AOSV systems present density fluctuations of about 1.03 and 1.01 g/ cm3, respectively. This is due to the interactions of the copolymers with the asphaltene and resin. Long-chain alkanes of the copolymer extension and multiple attachments, π−π
Figure 5. TG and DTG curves of AOSV-Q3 quadripolymer.
Therefore, the practicality of using AOSVs is feasible at high temperature. 3.3. Rheological Behavior of Crude Oil. The flow behavior of crude oils was investigated over a wide range of shear rates (0.1−200 s−1) (see Figures 6 and 7). In the initial
Figure 6. Apparent viscosities of Tahe oil containing copolymers as a function of shearing rate at 50 °C.
stage of shear process, fluctuations in the Tahe rheological curve are notable. With increasing shear rate, the viscosity of the crude oil showed a slow declining trend in both figures. For Tahe crude oil, the viscosity reduction rate of copolymer Te was up to 55.0% at a shear rate of 50 s−1. In contrast, the AOSV samples (Q1, Q2, and Q3), with varying amphiphilic monomer compositions, demonstrated that AOSVs have better effect on flowability of heavy oil than Te, and the viscosity reduction rate D
DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 8. Flowchart of simulated transportation.
Table 4. Pressures of the Unit Line at 50 °C medium heavy oil heavy oil dosed with Te heavy oil dosed with AOSV-Q3
inlet (MPa)
outlet no. 1 (MPa)
outlet no. 2 (MPa)
outlet no. 3 (MPa)
0.498 0.498
0.417 0.461
0.416 0.462
0.416 0.462
0.498
0.475
0.475
0.476
Figure 10. Potential energies of the AR, AR + Te, and AR + AOSV systems at 323 K.
Figure 9. Densities of the AR, AR + Te, and AR + AOSV systems at 323 K.
stacking, and polar functionalities of small molecules have a dispersing effect on asphaltene and resin, which slightly decreases the density of heavy oil. By comparison of the potential energies and nonbond energies of the three systems in Figures 10 and 11, it can be observed that the potential energy of AR dosed with copolymers was lower than that of pure AR and that the nonbond energy of AR dosed with copolymers increased markedly compared to that before being dosed. The reduction of the potential energy confirms that the copolymers interacted with the heavy component, broke the aggregation structure of asphaltene and resin, and formed a new homogeneous structure. The value of the nonbond energy is negative because of large numbers of hydrogen bonds in heavy-oil colloids.24,28 The increase of the nonbond energy can be explained by the fact that AR dosed with copolymers creates stronger-binding hydrogen bonds, which improves the raw aggregated asphaltene and resin molecules. Further, the AR + AOSV system exhibits more significant changes than the AR + Te system in terms of potential energy and nonbond energy, and this could lead to a better viscosity reduction effect.
Figure 11. Nonbond energies of the AR, AR + Te, and AR + AOSV systems at 323 K.
4. CONCLUSIONS A series of oil-soluble quadripolymers denoted as AOSVs were designed, synthesized, and evaluated as a new type of viscosity reducer. Owing to the amphiphilic monomer embed in their structure, AOSVs present a better viscosity reduction effect than Te of a similar molecular weight. Compared with Te, AOSV’s increasing range of viscosity reduction is over 20.55% for Tahe oil and over 25.67% for Shengli oil in the shear rate range of 0.1−200 s−1 at 50 °C. Thermogravimetric analysis of AOSV exhibited good thermal stability up to 320 °C. After three successive cycles of laboratory tests to simulate pipeline transportation, the pressure of the outlet decreased markedly upon addition of AOSV, with the viscosity reduction of heavy E
DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
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oil caused by a lower friction loss. Thus, good thermal and shear stability confirm that the practical application of AOSV is feasible. Furthermore, the mechanism of viscosity reduction of oil-soluble copolymers was analyzed by molecular simulations at 323 K with the consideration of various parameters of the synthesized copolymers. The results showed that the density of the AR + AOSV system decreased from 1.06 to 1.01 g/cm3. A substantial increase of the nonbond energy and a decrease of the potential energy confirmed that the interactions between the copolymers and the heavy oil changed the aggregated asphaltene and resin structure, which reduced the frictional resistance between the molecular layers and improved the flowability of the heavy oil.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 134 0145 5236. Fax: +86 28 83033546. E-mail:
[email protected]. ORCID
Jincheng Mao: 0000-0003-0301-1322 Funding
The research was supported by the Sichuan Youth Science & Technology Foundation (2017JQ0010), the National High Technology Research & Development Program (2016ZX05053), the Key Fund Project of Educational Commission of Sichuan Province (16CZ0008), the Explorative Project Fund (G201601) of the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), the Major Program of the National Natural Science Foundation of China (51490653), and the 973 Program (2013CB228004). Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.7b02631 Energy Fuels XXXX, XXX, XXX−XXX
