Full Length ArticleRheological implications of the inclusion of ferrofluids and the presence of uniform magnetic field on heavy and extra-heavy crude oils
Introduction
Heavy crude oils are defined as oils with specific gravity scale developed by the American Petroleum Institute (API) and viscosity below 22.3 and 10000 cP, respectively; while extra-heavy oils are those with API gravity under 10 and viscosity above 10000 cP at reservoir conditions [1]. Generally, they consist in complex mixtures of hydrocarbon families (hydrogen 8–12 wt% and carbon 80–88 wt% [2]); heteroatoms like sulfur (0–9 wt% [3]), nitrogen (0–2 wt% [3]) and oxygen (0–2 wt% [3]); and traces of heavy metals (V, Ni, Fe) contained in metalloporphyrins [2], [4], [5], [6]. Because of the chemical complexity of the heavy and extra-heavy crude oils, industries and academy generally employ a practical fractionation technique based on solubility and polarity parameters known as SARA analysis, which divides their components into Saturates, Aromatics, Resins, and Asphaltenes.
From the microscopic structural viewpoint, heavy crude oils are also defined as viscoelastic colloidal suspensions, whose dispersed elastic phase is attributed to asphaltene aggregates over a semi-continuous matrix of maltenes (saturates, aromatics and resins) [7], [8], [9], [10]. Under certain thermodynamic conditions (i.e., modifications of the composition, temperature or pressure [11]), these components are prone to self-associate and to precipitate due to their compositional complexity that varies from single aromatic cores (island) to bridged aromatic moieties (archipelago), with short and long alkyl units, and multi-heteroatom functionalities [12]. One of the well-known consequences of the agglomeration and precipitation of asphaltenes consists of dramatic increases in viscosity and non-Newtonian rheological behavior (i.e., viscoelasticity [7], [13], thixotropy [14]), as well as, the formation of deposits on steel surfaces [15]. This complex behavior hinders their mobility and flowability through pipelines, causing several operational and economic problems in oil industries. As a result, the oil transportation industry has implemented conventional technological solutions, mainly based on viscosity and friction reduction, to reach the required viscosity specifications, which must be in the range of 250–400 cSt at 37.8 °C [16]. Martínez-Palou et al. [5] and Hart [17] describe in detail some of these techniques and discuss their advantages and disadvantages. As an important conclusion from the reports, it is observed that industry has not yet developed an effective technique to be implemented at operational level; these conventional solutions imply high energy consumption, high operational cost, and technical difficulties (i.e., flow turbulence), evidencing the need to propose and study alternative approaches.
Over the last decades, the non-conventional magnetic and electric technologies have shown to be effective in reducing viscosity and flow turbulence (mainly when using waxy crude oils) [18], showing lower energetic and economic costs. Several studies have concluded that the magnetic field may reduce viscosity and modify wax crystallization mechanisms of waxy crude oils without affecting their thermodynamic equilibrium [19], [20], [21], [22], [23]. It has also been reported that magnetic fields influence the resinous-asphaltic fraction [21], [24] and can even modify the rheology due to the presence of paramagnetic ions in the water phase [25]. Some researchers have explained that the magnetic/electric fields improve the flow properties by induction of an anisotropic viscosity, which is the result of aggregation phenomena of the colloidal phase [18], [20], [26], [27]. In contrast, other authors have described the influence of magnetic fields as promoters of disaggregation [22], [23]. Equally important, various studies have also shown the high selectivity of the magnetic treatment over the nature of crude oils [25], [28], and in several cases, the non-functionality of this method; which have been justified by the well-known diamagnetism of these fluids [19], [25], [28], [29]. Certainly, so far, it has not been possible to establish the interaction mechanism between the crude oil and the magnetic field, as well as its role in preventing or promoting the association of heavy organic materials, such as asphaltenes, in the case of heavy crude oils. Thus, it is fundamental to propose experimental strategies to facilitate the elucidation and functionality of this approach, seizing their technical and operational benefits.
Additionally, another useful innovative approach for improving the rheological properties of heavy crude oils is nanotechnology, which harnesses the considerably high ratios of surface area/volume as well as the functionalizable surface area of nanoparticles (NPs) for adsorption of asphaltenes [11], [15], [30], [31], [32], [33], [34], [35]. Among the NPs successfully employed are included, iron oxides (Fe2O4 [36], Fe3O4 [30], [35], [36], γ-Fe2O3 [11],α- Fe2O3 [11]), aluminum oxides (γ-Al2O3 [15], Al2O3 [32]), nickel oxide (NiO [30]), cobalt oxide (Co3O4 [30]), titanium oxide (TiO2 [30]), silica ( [32], [34]), among others; and several theoretical assumptions about asphaltene adsorption mechanisms have been proposed, including both chemisorption [37] and mostly, physisorption [15], [30], [38], [39], [40]. Furthermore, it has been concluded that asphaltenes are mainly adsorbed on the surface of NPs forming single layers, i.e., the adsorption isotherms are typically Langmuir Type I [11], [15], [36], [41].
Particularly, metallic oxide NPs have attracted great interest because of their highly selective physicochemical properties, enabling broad applicability in hydrocarbon Exploration and Production-E&P [42]. Within their unique properties are included, significant surface area and amount of active sites [36], thermal conductivity and heat transfer [42], enhanced catalytic effect [30], [38], and in some cases, such as magnetite (Fe3O4), superparamagnetic behavior (3–15 nm [43]). Fundamentally, because of the latter property, magnetic NPs have been widely used in other fields, such as the development of stable magnetic colloidal suspensions, known as ferrofluids, to solve a wide variety of technical problems related to fluid dynamics. Ferrofluids exhibit an extraordinary high initial susceptibility, and thus, they present high magnetization for moderate magnetic field strength (~50 mT) [44]; accordingly, their flow properties can be controlled by such magnetic fields. For example, some studies have demonstrated increases in viscosity and a magnetoviscous effect (i.e., concentrated magnetic suspensions [44]) under the influence of uniform static magnetic fields [45], [46], but it has also been possible to induce “negative” viscosity responses under the action of alternating magnetic fields [47], [48]; and even, flow appearance using rotational and traveling magnetic fields [49], [50], [51], [52], [53], [54].
It is worth noting that, to the best of our knowledge, there is no research in which magnetic NPs and externals magnetic fields have been simultaneously applied to improve the flow behavior of heavy and extra heavy crude oils. However, this approach has already been evaluated as an alternative for improving crude oil demulsification [55], [56], [57], [58], [59], spill control [60], pavement engineering [61], and Enhanced Oil Recovery (EOR) [62].
Motivated by the possibility of using magnetic fields to control the rheological behavior of heavy oils effectively, this research reports the rheological and magneto-rheological characterization of mixtures of ferrofluids and heavy crude oils (hereinafter, crude oil-ferrofluid models), considering the fundamentals of the induction of magnetoviscous effects. Accordingly, this work is divided into two parts: (1) a rheological characterization of the crude oils and the crude oil-ferrofluid models, including steady-state flow curves and dynamic experiments to generate a baseline for comparison purposes, and (2) a magneto-rheological characterization of the crude oil-ferrofluid models to evaluate modifications in the rheological behavior by the application of a magnetic field. The experiments were carried out under the effect of static uniform magnetic fields, using a magneto-rheometer. The results revealed that the action of a uniform external magnetic field had a direct effect on the rheological properties of the heavy crude oil-ferrofluid models, and this effect was hindered by the complexity of the flow behavior of the samples, particularly, the structural properties and the viscoelastic effects. These results could be the basis for the applicability of this hybrid approach in decreasing the energy consumption, due to the possibility of controlling the flow properties of heavy crude oils by the action of external magnetic fields that overcome the gravitational and viscous forces.
Section snippets
Materials
The samples used corresponded to two Colombian heavy crude oils (hereinafter, C1 and C2), which were characterized by the determination of i) the concentration of asphaltenes (ASTM 6560-12 [63]), ii) the density by the pycnometer method (ASTM D70-18a [64]), and iii) the zero-shear viscosity by rheometry; their properties are shown in Table 1. Commercial magnetite NPs (EMG 1300) were obtained from Ferrotec (USA) Corporation and were used without any further treatment. According to the providers,
Linear viscoelasticity of the crude oils
As previously discussed, heavy crude oils can be described as colloidal viscoelastic suspensions, where the components having the highest molecular weights (i.e., the asphaltenes) are dispersed, forming fractal aggregates with sizes between 2 and 9 nm [2], [8], [69]. According to Lesueur, Behzadfar, and Hatzikiriakos [2], [7], [9], the complexity of the system lies in the role of resins acting as stabilizing agents (resin shell), as well as the wide range of melting temperatures of maltenes, that
Conclusions
According to the viscoelastic characterization and the applicability of the TTS principle, the extra-heavy crude oil C1 showed to be thermorheologically simple, i.e., it did not experience first-order phase transitions with temperature. Although the heavy crude oil C2 exhibited partial applicability of the principle, both samples showed an excellent fit to the generalized Maxwell model in the dynamic flow field. It could be inferred that the elastic structural properties in both samples are
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work has been supported by the Office of Research and Extension of the Universidad Industrial de Santander, project 2507. The authors gratefully acknowledge the help of MSc. Diana Marcela Cañas-Martínez with formatting the final manuscript, and the Microscopy Laboratory-LabMIC (UIS, Colombia) for the assistance with the SEM-EDS recording.
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