Extra-Heavy Crude Oil Viscosity Reduction Using and Reusing Magnetic Copper Ferrite Nanospheres

: The main objective of this study is the synthesis, use, and reuse of magnetic copper ferrite nanospheres (CFNS) for extra-heavy oil viscosity reduction. The CFNS were synthesized using a solvothermal method resulting in mean particle size of 150 nm. Interactions of CFNS with the crude oil were evaluated through asphaltene adsorption isotherms, as well as static and dynamic rheology measurements for two cycles at 25 ◦ C. Adsorption and desorption experiments corroborated that most of the asphaltenes adsorbed can be removed for nanoparticle reuse. During the rheology tests, nanoparticles were evaluated in the ﬁrst cycle at different concentrations from 300 to 1500 mg/L, leading to the highest degree of viscosity reduction of 18% at 500 mg/L. SiO 2 nanoparticles were evaluated for comparison issues, obtaining similar results regarding the viscosity reduction. After measurements, the CFNS were removed with a magnet, washed with toluene, and further dried for the second cycle of viscosity reduction. Rheology tests were performed for a second time at a ﬁxed concentration of 500 mg/L, and slight differences were observed regarding the ﬁrst cycle. Finally, changes in the extra-heavy oil microstructure upon CFNS addition were observed according to the signiﬁcant decrease in elastic and viscous moduli.


Introduction
The current petroleum reserves worldwide are mainly attributed to heavy (HO) and extra-heavy (EHO) crude oils, which represent the double regarding for the conventional oils [1,2]. In particular, Colombia is not distant from the worldwide panorama, where EHO and HO represent around 60% of the total production [3,4]. However, different processes in the oil and gas (O&G) industry remain challenging due to these crude oils' complexity, including the production, transport, and refining. Crude oils have a low API gravity and high viscosity generally attributed to the high content of asphaltenes and resins and the possible viscoelastic network between these compounds [5][6][7][8][9][10][11][12]. Various fluids such as light crude oil, naphtha, diesel, and others, as well as gases (mainly CO 2 ), are commonly used to reduce the viscosity of crude oils [13][14][15][16]. However, these solvents can generate asphaltenes' instability, further blocking the formation and oil pipelines [13,14]. In Colombia, for example, between 2005 and 2016 the crude oil transport increased from 150,000 to 450,000 bpd, with naphtha the solvent most often used for diluting HO and EHO to reach conditions for pipeline transport (400 cSt @ 311 K, 18 • API, and BSW < 0.5%) [17][18][19][20][21].

Physicochemical Characterization Methods
The morphology of CFNS was assessed with a transmission electron microscope (TEM) and a scanning electron microscope (SEM). TEM experiments were carried out with a JEOL JEM-1010 microscope (JEOL Europe SAS, Croissy, France), and SEM micrographs were obtained with Carl Zeiss SMT equipment (Carl Zeiss SMT, Oberkochen, Germany). The Brunauer-Emmett-Teller surface area (S BET ) was determined fromN 2 adsorptiondesorption isotherm at -196 • C, obtained with an Autosorb 1 from Quantachrome (Boynton Beach, FL, USA). X-ray photoelectron spectroscopy (XPS) was performed using an EscaLab 200R system (Thermo Fisher Scientific, East Grinstead, UK) equipped with an MgKα X-ray source and a hemispherical electron analyzer [47].
The crystalline structure of CFNS was characterized by X-ray diffraction (XRD) and collected on an X-ray Empyrean diffractometer with a PIXcel3D detector (Malvern Panalytical, Almelo, The Netherlands) under the following experimental conditions: CuKα radiation, 40 kV and 30 mA, in a 2θ range between 10 and 70 • with 0.01 • step size and an integration time of 100 s. Identification and quantification of the different phases present in each sample were assessed using a Rietveld refinement method [48,49] using HighScore Plus software [50] and the Crystallography Open Database [51].
A Superconducting Quantum Interface Device (SQUID) magnetometer (Quantum Design model MPMS-XL, San Diego, CA, USA) was used to record the magnetization (M) of CFNS at room temperature as a function of the magnetic field applied (H). The magnetic hysteresis (M-H) curve describes the magnetic response of the material and permits measurement of the saturation (M S ), remnant magnetization (M R ), and coercivity (H C ).

Extra-Heavy Crude Oil
The EHO sample has a 20% content of asphaltene, as well as an API gravity of 6.4 • at 15.6 • C. n-C 7 asphaltenes were obtained as described elsewhere [52][53][54]. The elemental analysis showed a content of 81.7, 7.8, 0.3, 6.6, and 3.6% wt. of C, H, O, N, and S, respectively. In addition, n-C 7 asphaltene's molecular weight of 907.3 g/mol was obtained through vapor pressure osmometry.

Adsorption/Desorption Tests
The adsorption and desorption cycles were evaluated in this manuscript, as described below. n-C 7 asphaltenes were dissolved in toluene at different concentrations up to 3000 mg/L. A fixed amount of 100 mg of CFNS was added to 10 mL of solution and kept under 200 rpm for 24 h [53,55]. The amount adsorbed "N ads " (mg/g) is obtained as follows: where C and C E are the concentrations of the n-C 7 asphaltenes before and after adsorption onto de CFNS, W (g) is the dry mass of CFNS, and V (L) the solution volume. C E is obtained through absorbance changes using a GENESYS 10S UV-vis spectrophotometer (Thermo Scientific, Waltham, MA, USA), as reported in previous studies [53,[55][56][57]. This test was performed in triplicate. CFNS with adsorbed n-C 7 asphaltenes were dried (120 • C and 24 h) before evaluating the desorption tests. Then, the dried material was added to pure toluene and stirred for 72 h at 200 rpm, and the remaining adsorbed n-C 7 asphaltenes "N ads,rem " (mg/g) is calculated as follows [58,59]: where C E,rem (mg/L) is the asphaltene concentration in toluene after the desorption process.
2.6. Rheological Measurements 2.6.1. Steady-State Rheology The effect of CFNS on crude oil rheological properties was assessed through steadystate and dynamic rheological measurements, as reported in previous studies [44,45,60,61]. For this, a Kinexus Pro rotational rheometer (Malvern Instruments, Worcestershire, UK) was employed. The effect of the concentration was evaluated by adding CFNS to the EHO matrix at different concentrations (300, 500, 700, 1000, and 1500 mg/L). The rheological measurements were evaluated in a shear rate range 1-100 s −1 at 25 • C, which was done in triplicate. Nanoparticles were added to the crude oil matrix by stirring at 500 rpm for 30 min until homogenization [44]. To evaluate the reuse of the CFNS in the process of reducing the viscosity of EHO, we conducted a new measurement labeled as the second cycle of viscosity reduction by adding CFNS. The nanoparticles previously added to the EHO matrix were removed from the crude oil, taking advantage of their magnetic properties by adding a magnet and manually stirring the crude oil. Subsequently, the magnet with the nanoparticles adhered to the surface was manually removed. The CFNS nanoparticles were then removed and reconditioned for use by means of a rigorous 10 h wash with toluene. The first cycle step was repeated, but only 500 mg/L were added (the best dosage obtained from the first round of rheological tests).

Dynamic Rheology Measurements
Dynamic rheology (oscillometry tests) were conducted with amplitude and frequency sweep tests. A strain-sweep test was performed to evaluate the linear viscoelasticity region. In addition, an oscillatory amplitude test was made for viscoelastic moduli estimation of EHO and first-and second-cycle EHO with 500 mg/L CFNS nanoparticles were performed using a rotational rheometer Kinexus Pro (Malvern, UK). A fixed frequency (ω) of 10 rad/s was employed, and the percentage of strain (γ) varied between 0.1% and 100% at a. A strain value of 2% was employed for dynamic rheological tests [62][63][64][65]. Finally, a frequency range of 0.1-100 rad/s at 25 • C was used for oscillatory frequency tests.

Solid-Liquid Equilibrium (SLE) Model
The solid-liquid equilibrium (SLE) model was employed to describe the adsorption of self-associative molecules such as asphaltene over the CFNS [66,67]. In addition, The Herschel-Bulkley (H-B) model was employed to describe the effect of CFNS in the rheological behavior of EHO [68,69]. The correlation coefficient (R 2 ) and root-mean-square error (RMSE) were used to estimate the suitability of fit using the Solver feature in Excel package 2015 [70][71][72][73][74]. Expressions both the SLE and H-B model are shown below, and related parameters are summarized in Table 1. Table 1. Expressions and related parameters of the solid-liquid equilibrium (SLE) and Herschel-Bulkley (H-B) models.

SLE
N ads (mg/g): amount adsorbed N ads,m (mg/g): maximum adsorption capacity C E (mg/g) is the equilibrium concentration of n-C 7 asphaltenes.  where, ; ξ = N ads,m · N ads N ads,m − N ads And for H-B model:

Results and Discussion
Previous results indicate that SiO 2 nanoparticles (8 nm) can selectively adsorb asphaltenes and resins on their surface [53], leading to changes in the asphaltenes aggregates and further viscosity reduction. Hence, these particles are selected to compare the performance of CFNS on the viscosity change of the EHO. The results are divided into four parts: (1) the physicochemical characterization of the CFNS sample, (2) rheology studies at a steady state in the presence and absence of CFNS for the first and second use cycles, (3) dynamic oscillometry of HO in the presence and absence of CFNS for the first and second use cycles, and (4) the asphaltene adsorption isotherms on CFNS.  Figure 1c shows the XRD patterns of CFNS, which was composed of three phases: cubic CuFe 2 O 4 (67.5%), Cu 2 O (17.1%), and Cu (15.4%), with average crystallite sizes of 7.7, 9.8, and 90.0 nm, respectively. The presence of metallic Cu and Cu + ions was due to the reduction by ethylene glycol of some Cu 2 + ions present in the solution [75][76][77][78][79][80].

Characteristics of the CFNS Sample
XPS was used to investigate the oxidation state and coordination number of the metal ions on the outermost surface of the CFNS sample. Figure 1d,e depicts the XPS profiles of Fe and Cu 2p core-level regions, respectively. Deconvolution of the broad Fe 2p3/2 peak showed two more peaks. The first peak occurred at a BE of 709.8 eV, which was accompanied by a satellite peak at a BE of around 718.5 eV, indicating the presence of Fe 3+ cations [6]. The second peak occurred at 711.3 eV, between the first peak and its satellite, indicating the presence of Fe3+ cations in more than one coordination environment, that is, A sites at a higher BE (second peak) and B sites at a lower BE (first peak) [81]. The Cu 2p3/2 profile showed two peaks at 932.3 and 933.5 eV, assigned to reduce copper species (Cu 0 /Cu + ) and Cu 2+ , respectively, and a shake-up satellite at around 942 eV, indicating the presence of Cu 2+ [82,83]. The Cu/Fe surface atomic ratio was 0.22, which was lower than the theoretical value of 0.5, indicating that Fe ions were segregated to the CFNS surface. Figure

Asphaltene Adsorption/Desorption Experiments
The viscosity reduction is mainly driven by the interaction between the heavy components of the EHO (mainly, the aggregates of n-C 7 asphaltenes) and the CFNS. To understand this interaction, we have evaluated the adsorption equilibrium based on the adsorption isotherms. Figure 2 shows the adsorbed amount of n-C 7 asphaltenes regarding the equilibrium concentration when CFNS is added into the solution model. Figure 2 shows the adsorption (cycles 1 and 2) and desorption isotherms of n-C7 asphaltene on CFNS at 25 • C obtained from the batch mode, obtaining a deviation lower than 5%, considering both curves nearly equal. Adsorption isotherms are Type I according to the International Union of Pure and Applied Chemistry, where the adsorbed amount of n-C 7 asphaltenes rises as C E increases until a plateau is reached. The obtained SLE parameters support this behavior; mainly, the maximum adsorbed uptake ( Table 2). Iron has a high-affinity asphaltene adsorption according to the selectivity toward heteroatoms such as nitrogen in the forms of amine, Processes 2021, 9, 175 6 of 16 quaternary-N, pyridine, and pyrrolic [84]. Among these, due to its aromatic nature and the increase in the atomic ratio, pyridines could be the main source for interaction with CFNS due to both σ and π bonding [85,86]. Note that the adsorptive capacity of CFNS in cycle 1 is slightly higher than cycle 2, especially at low concentrations that also indicate higher adsorption affinity and is corroborated by the SLE model's estimated H parameter. This is confirmed by the values of the h parameter of the SLE model. These results agree with findings reported by Nassar et al. [55], Montoya et al. [66], and Arias-Madrid et al. [85].
Processes 2021, 9, x FOR PEER REVIEW 6 of 17 presence of Cu 2+ [82,83]. The Cu/Fe surface atomic ratio was 0.22, which was lower than the theoretical value of 0.5, indicating that Fe ions were segregated to the CFNS surface. Figure    al. [66], and Arias-Madrid et al. [85].
From Figure 2, it is possible to conclude that the adsorption and desorption process on CFNS at 25 °C is reversible. The loops of adsorption and desorption for the CFNS are similar, showing a clear case of reversibility in this process, which favors their reuse in the EHO reduction process. This opens the landscape for the use and reuse of these materials for applications to improve EHO mobility.    From Figure 2, it is possible to conclude that the adsorption and desorption process on CFNS at 25 • C is reversible. The loops of adsorption and desorption for the CFNS are similar, showing a clear case of reversibility in this process, which favors their reuse in the EHO reduction process. This opens the landscape for the use and reuse of these materials for applications to improve EHO mobility. Figure 3 shows the rheological evaluation through the flow curve of the EHO and its mixtures with CFNS at different concentrations (300, 500, 700, 1000, and 1500 mg/L). The behavior described for the evaluated samples is typical of this class of fluids, showing a shear-thinning behavior, that is, oil viscosity decreases as the shear increases. Mortazavi-Manesh and Shaw [21,34], Bazyleva et al. [62], Mozaffari et al. [87], Tao and Xu [88], and Taborda et al. [44] found similar results. Clearly, viscosity reductions are detected when adding up to a concentration of 700 mg/L of the material, obtaining the largest viscosity change at 500 mg/L. However, a shear-thinning effect of EHO is visible (viscosity dependent on the shear rate), which is expected behavior in a non-Newtonian fluid. These phenomena (viscosity reduction) are due to the interaction between CFNS and aggregates of asphaltene in the EHO structure. The asphaltene adsorption over the CFNS promotes a breakdown of the viscoelastic network that favors the reduction of their viscosity. The explanation for this phenomenon is widely explained in previous works [38,[43][44][45][46][89][90][91].

First CFNS Cycle for Viscosity Evaluation
adding up to a concentration of 700 mg/L of the material, obtaining the largest viscosity change at 500 mg/L. However, a shear-thinning effect of EHO is visible (viscosity dependent on the shear rate), which is expected behavior in a non-Newtonian fluid. These phenomena (viscosity reduction) are due to the interaction between CFNS and aggregates of asphaltene in the EHO structure. The asphaltene adsorption over the CFNS promotes a breakdown of the viscoelastic network that favors the reduction of their viscosity. The explanation for this phenomenon is widely explained in previous works [38,[43][44][45][46][89][90][91]. As can be seen in Figure 3, when the concentration exceeds 1000 mg/L, the effect on viscosity reduction is negligible, and a slight increase in viscosity may appear in the mixture of EHO and CFNS. This is due to an agglomeration effect of the nanoparticles themselves inside the highly viscous fluid. The agglomeration causes the particles to behave as larger solids, reducing the interaction with asphaltenes and promoting an increase in viscosity, according to Einstein's theory of viscosity [92]. Figure 4 shows the degree of viscosity reduction for all CFNS dosages evaluated in EHO, which was performed at low shear rates (7.1 s −1 ) [93][94][95]. Notably, at 500 mg/L, a viscosity reduction of 18% was obtained, showing the best performance of CFNS over a wide range of concentrations evaluated. This is a promising result in the search for a nanofluid design capable of enhancing transport processes. Moreover, the process may eventually generate a reduction in the consumption of diluents with economic and energy savings.
For comparison, our previous results indicated the best concentration obtained for the use of nanoparticles in EHO was 1000 mg/L, mainly for SiO2 nanoparticles. As can be seen in Figure 3, when the concentration exceeds 1000 mg/L, the effect on viscosity reduction is negligible, and a slight increase in viscosity may appear in the mixture of EHO and CFNS. This is due to an agglomeration effect of the nanoparticles themselves inside the highly viscous fluid. The agglomeration causes the particles to behave as larger solids, reducing the interaction with asphaltenes and promoting an increase in viscosity, according to Einstein's theory of viscosity [92]. Figure 4 shows the degree of viscosity reduction for all CFNS dosages evaluated in EHO, which was performed at low shear rates (7.1 s −1 ) [93][94][95]. Notably, at 500 mg/L, a viscosity reduction of 18% was obtained, showing the best performance of CFNS over a wide range of concentrations evaluated. This is a promising result in the search for a nanofluid design capable of enhancing transport processes. Moreover, the process may eventually generate a reduction in the consumption of diluents with economic and energy savings.

Second CFNS Cycle for Viscosity Evaluation
Evaluation of the second cycle of CFNS performance was performed by reusing the particles after the first cycle of viscosity reduction. For this process, it was decided to use the dosage that showed the best performance in the first cycle, that is, 500 mg/L. Figure 5 shows the performance of the CFNS in its second cycle of use. It can be seen how the reduction of viscosity is statistically equal to that obtained in the first cycle, which shows the reuse capacity of the particles for reducing EHO viscosity. Note that the performance of the CFNS for the two cycles evaluated showed a similar behavior over the shear rate evaluated. There is a slight deviation between the two measurements, mostly at a shear rate lower than 10 s −1 . For comparison, our previous results indicated the best concentration obtained for the use of nanoparticles in EHO was 1000 mg/L, mainly for SiO 2 nanoparticles.

Second CFNS Cycle for Viscosity Evaluation
Evaluation of the second cycle of CFNS performance was performed by reusing the particles after the first cycle of viscosity reduction. For this process, it was decided to use the dosage that showed the best performance in the first cycle, that is, 500 mg/L. Figure 5 shows the performance of the CFNS in its second cycle of use. It can be seen how the reduction of viscosity is statistically equal to that obtained in the first cycle, which shows the reuse capacity of the particles for reducing EHO viscosity. Note that the performance of the CFNS for the two cycles evaluated showed a similar behavior over the shear rate evaluated. There is a slight deviation between the two measurements, mostly at a shear rate lower than 10 s −1 .

Second CFNS Cycle for Viscosity Evaluation
Evaluation of the second cycle of CFNS performance was performed by reusing the particles after the first cycle of viscosity reduction. For this process, it was decided to use the dosage that showed the best performance in the first cycle, that is, 500 mg/L. Figure 5 shows the performance of the CFNS in its second cycle of use. It can be seen how the reduction of viscosity is statistically equal to that obtained in the first cycle, which shows the reuse capacity of the particles for reducing EHO viscosity. Note that the performance of the CFNS for the two cycles evaluated showed a similar behavior over the shear rate evaluated. There is a slight deviation between the two measurements, mostly at a shear rate lower than 10 s −1 .  Notably, nanoparticles have magnetic and adsorptive characteristics that can change over time, so it is necessary in future work to evaluate different operating conditions, for example, testing more than two cycles, evaluating the system at different temperatures, and even changing the type of fluid. Table 3 shows the H-B parameters for EHO samples with CFNS evaluated. It is seen that µ ∞,γ and K H decreased as the viscosity decreases, that is, for the addition of CFNS at dosages up to 700 mg/L. Meanwhile, n H approximates to 1.0 as the amount of CFNS is increased up to 500 mg/L, indicating that the sample tends to be more Newtonian. This result is in agreement with the reported by Mortazavi-Manesh and Shaw [34].  Figure 6 presents the experimental comparison of the viscosity reduction capacity between the SiO 2 nanoparticles and CFNS. SiO 2 particles are taken as a reference point as previous publications demonstrated it had the best performance in reducing heavy crude oil viscosity [44][45][46]. Although this figure shows an evaluated concentration of 500 mg/L, the best concentration for this type of nanoparticle has been reported at 1000 mg/L [43,44,46]. This finding can be understood from Figure 7, which shows the results quantitatively. The figure shows that the reduction in viscosity of the SiO2 particles is 22%, slightly higher than the 18% produced by CFNS (evaluated at 7.1 s −1 ). However, the added value of using CFNS particles lies in the reuse they can have, which considerably reduces the consumption of new material. This can positively impact the economic aspect of the process, which is reflected due to the low consumption of CFNS and diluents compared with conventional methods.

Linear Viscoelasticity Region
Evaluating the viscoelastic moduli alteration is of primary importance to determine how the CFNS leads to changes in the EHO internal structure. At low deformations, the EHO shows a linear trend between stress and strain (linear viscoelastic region), which indicates that the internal structure has not been modified [96]. In general, determining the linear viscoelastic region (LVR) of a material is a relatively simple task, using dynamic However, our evaluations have been conducted at the same concentration with the same EHO to observe the rheology behavior for this research. Thus, in Figure 6, both particles produce viscosity reductions, as expected. However, the SiO 2 nanoparticles indicate the same performance, producing a similar change in viscosity for the concentration evaluated. Note SiO 2 nanoparticles at 500 mg/L showed a significant reduction in the same amounts as CFNS. However, it is impossible to reuse SiO 2 nanoparticles due to the difficulty of recovering them from the oil matrix. Hence, the CFNS sample showed promise for being applied in the O&G industry, reducing costs in the dilution process for the transport of EHO.
This finding can be understood from Figure 7, which shows the results quantitatively. The figure shows that the reduction in viscosity of the SiO 2 particles is 22%, slightly higher than the 18% produced by CFNS (evaluated at 7.1 s −1 ). However, the added value of using CFNS particles lies in the reuse they can have, which considerably reduces the consumption of new material. This can positively impact the economic aspect of the process, which is reflected due to the low consumption of CFNS and diluents compared with conventional methods.

Linear Viscoelasticity Region
Evaluating the viscoelastic moduli alteration is of primary importance to determine how the CFNS leads to changes in the EHO internal structure. At low deformations, the EHO shows a linear trend between stress and strain (linear viscoelastic region), which indicates that the internal structure has not been modified [96]. In general, determining the linear viscoelastic region (LVR) of a material is a relatively simple task, using dynamic rheology tests with amplitude sweeps at a given frequency. Figure 8 shows the amplitude sweep test for EHO with and without 500 mg/L CFNS. This finding can be understood from Figure 7, which shows the results quantitatively. The figure shows that the reduction in viscosity of the SiO2 particles is 22%, slightly higher than the 18% produced by CFNS (evaluated at 7.1 s −1 ). However, the added value of using CFNS particles lies in the reuse they can have, which considerably reduces the consumption of new material. This can positively impact the economic aspect of the process, which is reflected due to the low consumption of CFNS and diluents compared with conventional methods.

Linear Viscoelasticity Region
Evaluating the viscoelastic moduli alteration is of primary importance to determine how the CFNS leads to changes in the EHO internal structure. At low deformations, the EHO shows a linear trend between stress and strain (linear viscoelastic region), which indicates that the internal structure has not been modified [96]. In general, determining the linear viscoelastic region (LVR) of a material is a relatively simple task, using dynamic rheology tests with amplitude sweeps at a given frequency. Figure 8 shows the amplitude sweep test for EHO with and without 500 mg/L CFNS.  Figure 8 shows the obtained storage modulus (G′), loss modulus (G′′), and the phase angle (δ). For both EHO in the presence and absence of CFNS, it is observed that as the strain increases up to 2%, the moduli do not change. Hence, the LVR is given for strain values between 0.1 and 10%. G′′ is always higher than G′, indicating that the sample's behavior is more viscous than elastic. The G′ decreases when 500 mg/L of CFNS are added to the EHO, suggesting lower internal resistance and lower viscosity. Meanwhile, δ is always higher than 45°, indicating a liquid-like behavior [96][97][98].
From Figure 8, it is observed that the LVR is slightly higher upon CFNS addition, indicating an internal structure change in the EHO [61]. Further, the frequency analysis is  Figure 8 shows the obtained storage modulus (G ), loss modulus (G ), and the phase angle (δ). For both EHO in the presence and absence of CFNS, it is observed that as the strain increases up to 2%, the moduli do not change. Hence, the LVR is given for strain values between 0.1 and 10%. G is always higher than G , indicating that the sample's behavior is more viscous than elastic. The G decreases when 500 mg/L of CFNS are added to the EHO, suggesting lower internal resistance and lower viscosity. Meanwhile, δ is always higher than 45 • , indicating a liquid-like behavior [96][97][98].
From Figure 8, it is observed that the LVR is slightly higher upon CFNS addition, indicating an internal structure change in the EHO [61]. Further, the frequency analysis is carried out in the LVR to determine the effect of CFNS on the EHO viscoelastic properties.

Frequency Sweep Test for Heavy Crude Oil
For angular frequency analysis, deformation was fixed at 2%, and the sweep was carried out at 25 • C between 0.1 to 100 rad/s. Figure 9 shows G , G , and δ at 25 • C in the absence and presence of CFNS. From Figure 9, it is observed that in the absence of the nanomaterial, G is higher than G , indicating a more viscous behavior than an elastic one. The moduli also increase as the angular frequency increases, attributed to the asphaltene distribution in the EHO matrix and the viscoelastic network formation due to strong colloidal forces [63,[99][100][101]. es 2021, 9, x FOR PEER REVIEW 13 of 17 distribution in the EHO matrix and the viscoelastic network formation due to strong colloidal forces [63,[99][100][101].
In the case of the EHO in the presence of CFNS, the G′′ is also greater than G′. Nevertheless, the moduli decreased by approximately 30% regarding the system without nanoparticles, corroborating the decrease of dynamic viscosity due to the decrease in viscous and elastic contributions. This viscosity reduction upon CFNS addition would positively impact the optimization of operating conditions related to the production, recovery and transport of EHO.

Conclusions
CFNS is composed of nanospheric particles (150 nm) containing three phases: cubic-CuFe2O4 (67.5%), Cu2O (17.1%), and Cu (15.4%). XPS showed that Fe 3+ was placed in A and B sites and the presence of metallic Cu and Cu + ions. CFNS had a high MS value and very low MR/MS ratios, indicating substantial superparamagnetic behavior. The rheological evaluation carried out reflects a viscosity reduction of up to 18% by adding 500 mg/L of CFNS. The rheological curve described by the extra-heavy crude oil sample in the presence and absence of CFNS defines a shear-thinning-type pseudoplastic behavior, typical of this class of fluids, where the viscosity decreases as the shear rate increases. The nanoparticles were evaluated in their optimal concentration in two cycles, representing the use and reuse of the magnetic particles. We identify significant decreases in the elastic and In the case of the EHO in the presence of CFNS, the G is also greater than G . Nevertheless, the moduli decreased by approximately 30% regarding the system without nanoparticles, corroborating the decrease of dynamic viscosity due to the decrease in viscous and elastic contributions. This viscosity reduction upon CFNS addition would positively impact the optimization of operating conditions related to the production, recovery and transport of EHO.

Conclusions
CFNS is composed of nanospheric particles (150 nm) containing three phases: cubic-CuFe 2 O 4 (67.5%), Cu 2 O (17.1%), and Cu (15.4%). XPS showed that Fe 3+ was placed in A and B sites and the presence of metallic Cu and Cu + ions. CFNS had a high M S value and very low M R /M S ratios, indicating substantial superparamagnetic behavior. The rheological evaluation carried out reflects a viscosity reduction of up to 18% by adding 500 mg/L of CFNS. The rheological curve described by the extra-heavy crude oil sample in the presence and absence of CFNS defines a shear-thinning-type pseudoplastic behavior, typical of this class of fluids, where the viscosity decreases as the shear rate increases. The nanoparticles were evaluated in their optimal concentration in two cycles, representing the use and reuse of the magnetic particles. We identify significant decreases in the elastic and viscous moduli through dynamic rheology tests, causing internal changes in the viscoelastic modules, which are responsible for a reduction in the viscosity of the crude oil. The adsorption isotherms constructed demonstrate an existing interaction between asphaltenes from heavy crude oil and CFNS. The main characteristic of CFNS-type materials is their high magnetic power and their ability to reduce the viscosity of heavy crude oils. Both properties offer advantages to the chemical addition processes to enhance the transport and mobility processes of heavy crude oils, allowing the reuse of nanoparticles and the reduction of diluents. This could generate technological, economic and environmental impacts for the oil and gas industry.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.