Asphaltene Precipitation/Deposition Estimation and Inhibition through Nanotechnology: A Comprehensive Review

Abstract

Asphaltene precipitation/deposition is considered a problem of formation damage, which can reduce the oil recovery factor. In addition, asphaltenes can be deposited in pipelines and surface installations, causing serious complications in guaranteeing runoff, decreasing the production of oil wells. The precipitation of asphaltenes can be minimized by reducing the oil production flowrate or by using chemical inhibitors. Analyzing the stability and precipitation trend of asphaltenes in petroleum is vital for the guarantee of flow. For this purpose, several experimental and numerical methods have been proposed. Once the risk of precipitation is established, strategies can be formulated for the prevention and diagnosis of deposition problems in production or production training. The tests can be performed with dead oil, available in the wellhead, and help in understanding the behavior of the asphaltenes. This review aims to present (i) the problem related to the precipitation of asphaltenes; (ii) thermodynamic models of asphaltene precipitation; and (iii) asphaltene inhibition, control, and removal techniques using nanoparticles.

1. Introduction

Oil can be subdivided into four fractions, namely saturated, aromatic, resins, and asphaltenes (SARA). The characterization of the asphaltenic fraction is the most complex, as this fraction consists of the heaviest molecules, with higher polarity and molecular structures containing individual and/or condensed aromatic rings [1,2].

Asphaltenes, due to the variation in oil composition, pressure, and temperature, can precipitate and form solid deposits in the porous media, pipelines, production lines, or treatment equipment, bringing several operational problems [3,4]. Figure 1 outlines and identifies the problems related to the precipitation and deposition of asphaltenes in oil fields.

These inconveniences generate economic damage, as they can alter petrophysical properties causing severe damage to the reservoir; for example, the presence of asphaltenes in the solid form can lead to the blocking of pores in the production formation, a loss of effective porosity, changes in permeability, changes in wettability, and an increase in oil viscosity [3,5,6,7].

On the other hand, during the refining process, asphaltenes pose significant challenges and can lead to various issues. One major concern is the formation of coke in equipment, which negatively impacts its operational lifespan and increases maintenance costs. Asphaltenes tend to thermally decompose and form solid carbonaceous deposits, known as coke, on the surfaces of refining equipment. The accumulation of coke restricts the flow of fluids and heat transfer, leading to reduced efficiency and operational problems [8,9].

The asphaltene agglomeration process can be distinguished into four stages: precipitation, flocculation, aggregation, and deposition [10]. Among these steps, precipitation is the most difficult process to predict and can be identified when an insoluble phase composed of solids appears in the hydrocarbon [11,12,13]. It is described that, when in equilibrium, asphaltenes are found in petroleum as a colloidal dispersion. When destabilized, insoluble asphalt material is detected [1]. The pressure at which this occurs, at a given temperature, is named precipitation onset pressure. Knowing this value accurately provides more tools for reservoir and production engineers to make decisions to propose alternatives and solutions to overcome the problem related to the destabilization of asphaltenes [14].

Many works involving the determination of asphaltene precipitation at atmospheric pressure are available [15,16,17,18,19,20,21,22,23,24,25]. Some others investigate the behavior of asphaltenes at reservoir conditions [14,26,27,28,29,30,31,32].

2. Asphaltenes

Asphaltenes are complex molecules or structures that exist in oils, changing their density and viscosity [11,33]. They can be defined, in terms of solubility, as the fraction of petroleum insoluble in n-alkanes (such as n-pentane and n-heptane) and soluble in aromatic solvents (such as toluene) [11,33,34]. In terms of molecular structure, the most recent and most impactful work was performed by Gray et al. [35]. In this work, the asphaltenes are conceived as associated supramolecules by means of non-covalent bonds.

Over time, different classical molecular models of asphaltenes have been developed. In 1940, Nellensteyn [36] introduced the possibility that asphaltenes were colloids. This concept lasted for 16 years and was maintained in the work of Pfeiffer and Sall (1940) [37]. Later, Dickie and Yen (1967) [38] suggested that asphaltenes have a micellar structure. Years later, this model was taken as a basis by Wiehe (1994) [39] for the construction of the pendant–core and building block models, which in 2010 would undergo some substantial modifications proposed by Mullins [40].

Table 1. Summary of publications related to molecular models of asphaltenes [41].

Authors	Relevance
Nellensteyn [36]  (1924)	Conceptual outline of the colloidal behavior of asphaltenes. Definition of the bases of the main asphaltene separation method by insolubility in heptane and pentane.
Pfeiffer and Saal [37]  (1940)	Definition of the micellar structure of asphaltenes in oil.
Dickie and Yen [38]  (1967)	Justification for the different values of molecular masses of asphaltenes obtained by different techniques, assigning the highest values to the existence of micelles.
Wiehe [39]  (1994)	Introduction of the idea of compositional continuity of oil and its fractions.
Mack [42]  (2002)	Relationship between viscosity and concentration of asphaltenes.
Mullins [40]  (2010)	Improvement of the Yen model (1961). Effectiveness of critical concentration concepts for asphaltene aggregation.

presents the relevance and considerations of each of these models [41].

Several studies involving asphaltenes involve their isolation by solubility difference, the characterization of the isolated fraction, and the evaluation of the phase behavior of model systems. However, different procedures are proposed for the separation of asphaltenes from petroleum.

Table 2. Different asphaltene extraction methodologies reported in the scientific literature.

Method	Precipitating Agent	Conditions	Rate Solvent/Sample  (mL/g)	Methodology
ASTM D893  [46]	n-C5 commercial	65 ± 5 °C. Filter solids with 150 mL n-C5 at room temperature	10	Centrifuge at 600–700 rpm for 20 min. To decant until only 3 of solution in the tube. Centrifuge again under the same conditions. Dry at ±105 °C for 30 min.
ASTM D2006  [47]	n-C5 commercial	No heating required	50	Leave to stand for 15 h, filter, and wash three times with 10 mL of n-C5 in each wash.
Bulmer et al.  [48]	n-C5 analytical grade and commercial benzene	Heat to dissolve if necessary	40 mL  n-C5 and  1 mL benzene	Dissolve in benzene and heat if necessary. Add n-C5 and shake for 5 min. Leave to stand for 2 h. Filter under vacuum. Wash the balloon where the test was performed. Dry at 105 °C.
ASTM D2007 [49]	n-C5 commercial	Requires heating	10	Add n-C5 and shake well. Heat for a few seconds until dissolved. Leave to stand for 30 min. Wash with 10–20 mL of n-C5.
ASTM 6560  [50]	n-C7 and toluene	Requires reflow	100	Add n-C7 and reflux for 1 h. Cool for 1.5 to 2.5 h under light. Filter on Whatman No. 42 paper. Rinse the filter paper with hot n-C7 for 1 h. Keep under reflux with 30–60 mL of toluene in a water bath. Dry at 100–110 °C for 30 min.
ASTM D3279  [51]	n-C7 with purity > 99%	Requires reflow	1000	Add n-C7 and reflux for 15–20 min. Cool for 1 h. Filter under vacuum. Wash three times with 10 mL of n-C7 in each wash. Dry at 107 °C for 15 min.

summarizes the main asphaltene extraction methodologies as well as the considerations necessary to ensure effectiveness in the process [41,43,44,45].

2.1. Asphaltene Precipitation

Several authors agree that the predominant variables that determine the precipitation of asphaltenes are pressure and composition, but temperature can also have an influence on the trend of the asphaltene precipitation of an oil [52,53,54].

A clear example of the effect of composition on asphaltene precipitation can be observed in processes that require the injection of some type of fluid into the reservoir, such as advanced recovery or well stimulation, where considerable amounts of substances are injected that can destabilize asphaltenes in petroleum. This occurs because such substances generate a change in the oil solubility parameter, and therefore, the precipitation of asphaltenes can occur when the oil is mixed with incompatible substances, with the operations of injection of CO₂, gases, or solvents being critical [54,55,56,57,58].

This pressure variation causes, at some stage of production, the pressure of the asphaltene precipitation envelope (APE) to be reached, which is the pressure at the start of the asphaltene precipitation. At this point, all dissolute asphalt material in the oil will be precipitated and possibly deposited in the reservoir or in the pipes. Figure 2 shows the critical points for precipitation to begin. In general, when the pressure is reduced, the amount of precipitated asphaltenes increases, constituting an inverse relationship [55,59,60,61].

Additionally, Hartmann (2016), using the UV–visible technique and propane as an asphaltene flocculant solvent, reported a decrease in precipitation onset value with increased pressure. This behavior was attributed to the fact that the propane’s solubility parameter changes when subjected to pressure. In

Table 3. Effect of pressure on solution precipitation onset for 0.5% by weight of C7 asphaltenes with propane at 56 °C at different pressures: from 362.59 Psi to 5801.51 psi [34,59].

Pressure	Translucence (%)
580,151	110
290,075	123
145,038	143
72,519	161
36,259	190

, it can be observed that the onset at 5802.51 psi is practically one-fifth of that measured at 362.59 psi [34,59].

2.2. Thermodynamic Models of Asphaltene Precipitation

To better understand the phenomenon of asphaltene precipitation, it is important to establish the different variables that influence the process. In this way, the identification of thermodynamic conditions brought knowledge about the behavior of asphaltenes [62].

In 1942, the Flory–Huggins [63] polymeric solution theory was applied to asphaltenes. The asphaltene components were a non-ideal solution, and the amount of precipitated asphaltenes was tested by adding asphaltene mass to a previously prepared polymeric solution [19,64]. The objective of the work was to demonstrate that the heavy fraction was not soluble in the polymeric solution, thus corroborating that the two substances were not compatible, even having high molar mass. Years later, other researchers included the micellar nature of asphaltene and again tested the theory introduced by Flory and Huggins, obtaining similar results [65,66,67]. On the other hand, Leontaris and Mansoori (1987) [68] proposed a colloidal thermodynamic model, where it was considered that the asphalt molecules were surrounded by resins. Based on the Flory–Huggins polymeric solution model, the researchers determined the potential of an oil to precipitate asphaltenes, considering thermodynamic conditions.

The solid model proposed by Gupta (1986) [69] and Thomas et al. (1992) [70] considered the asphaltic component as a solid and the oily phase as a liquid modeled with cubic state equations, and the precipitation of asphaltenes was represented as a multicomponent solid phase [71]. Similarly, Chung et al. (1991) [72] modeled two fractions present in heavy petroleum: one of these corresponded to a precipitated fraction, the asphaltene; the other corresponded to a totally soluble and non-precipitable resin. This model aimed to identify the parameters that influenced the stability of asphaltenes and, like the models of Gupta [69] and Thomas [70], also considered this fraction as pure solids.

In 1993, Nghiem et al. [71] developed a state equation (EOS) to predict the precipitation of asphaltenes using thermodynamic considerations. The model consisted of the identification and division of two parts: one would be a material that would tend to precipitate (asphaltene); the other would be a phase that would not precipitate. This EOS was based on the Peng–Robinson equation [73] and, using diffuse logic and binary interaction coefficients, allowed statistical analysis of asphaltene precipitation in the non-precipitated phase. This work did not consider the micellar nature of asphaltenes. Years later, in 1998, Nghiem and Coombe [74,75] would complement this equation with calculations of flash vaporization in three phases.

Hirschberg et al., in 1984 [59], presented a work on a liquid thermodynamic model, which describes the behavior of asphalt molecules in the petroleum reservoir, considering variations in pressure, temperature, and the compositional gradient. This model also included the concept of reversibility in asphaltene precipitation described by Fussel in 1979 [76]. However, Hirschberg based his model on the Redlich–Kwong state equation [77].

A constant in the models presented above concerns the asphaltene fraction being represented as a solid or a mass conglomerate, until, in 1996, Victorov and Firoozabadi [78] inserted the thermodynamic micellization model, which presented asphaltenes as micelles contained in oil. This work applied the Peng–Robinson state equation to describe the oily phase that included mixtures of petroleum, with heavy and light components, and employed the Gibbs free energy concept to minimize the size of the micelles, thus ensuring that their molar mass did not exceed the mass value of an oligomer.

After this study, other models were presented as complex equations. In 2004, Chapman et al. [79] modeled a state equation for phase equilibrium prediction based on the statistical association of fluid theory (SAFT). This model gives importance to the impact of the molecular form, the intermolecular association, and the van der Waals interaction forces. Three years later, in 2007, Pedersen and Hasdbjerg [80] expanded the equation using test results from fluid extracted from reservoirs of condensed gas with asphalt content and heavy oil. This model introduces a more realistic behavior of asphaltenes since it allows the modeling of the thermodynamic balance between three phases (liquid, vapor, and solid) [62].

3. Nanotechnology for the Inhibition of Asphaltene Precipitation/Deposition

Crude oils are considered as colloidal dispersions in which the asphaltenes’ stability depends on the fluid composition, the pressure and temperature conditions, and the production parameters [3,4]. Asphaltenes have a self-assembly behavior that is prompted by their particular chemical structure [81]. In this regard, asphaltenes molecules are generally described as island-shaped structures composed of a polyaromatic core with alkyl ramifications and with the presence of heteroatoms such as sulfur, oxygen, and nitrogen [82,83]. These heteroatoms enable the asphaltenes’ self-assembly by different mechanisms such as acid–base interactions and H-bonding when having H-terminal groups. These aggregation mechanisms are commonly related to the interaction of N-containing (pyrrolic, pyridine, quinoline) and O-containing (carbonyl, carboxyl, hydroxyl) functional groups [84,85,86]. The phenomena are governed by the N-containing functional groups that could be found as positively charged species with the ability to interact with the negatively charged oxygen groups due to the difference in electronegativity between the O atoms and other elements such as H [87]. Additional aggregation mechanisms associated with π–π stacking due to the stacking of the polyaromatic cores of the asphaltenes and metal coordination complexes derived from the presence of vanadium, iron, and nickel in the asphaltene structures are also observed [88,89].

The mentioned aggregation mechanisms were revealed by Gray et al. [87] and are schematized in Figure 3. Even though these intramolecular forces are weak, the authors stated that the combination of the mechanisms is the factor governing the asphaltene aggregation phenomena [87]. Nonetheless, the crude oil composition plays a major role in the aggregation phenomena, as in different studies it was proven that the resins interact with the asphaltene nanoaggregates, limiting their growth and keeping them in their original colloidal form [90,91].

The main difference between resins and asphaltenes is their polarity, which is lesser for the resins as these molecules usually have a smaller core with a lower amount of heteroatoms and a higher presence of alkyl substituents [92,93]. This last characteristic is commonly related to the greater solubility exhibited by the resins, which surround the asphaltene aggregates, keeping them in their colloidal form [94]. Thus, it is considered that a lack of this fraction enables the asphaltene precipitation/deposition [95]. Therefore, controlling the asphaltene precipitation/deposition in crude oils with a natural lack of stabilizing agents has been a major concern for the oil and gas industry to avoid productivity losses which are mostly presented in light crude oils [96,97,98].

Previously in this manuscript, the thermodynamical models of the asphaltenes were introduced; the next sections will present a discussion of novel nanotechnology-based treatments for the inhibition of the formation damage associated with asphaltene precipitation/deposition. At the nanoscale, it is feasible to develop materials to capture the asphaltenes at reservoir conditions without inducing additional damage due to the small nanoparticle sizes (1–100 nm) [99]. The nanoparticles adsorb the asphaltenes in their colloidal state, preventing their aggregation, which provides an efficient technique for avoiding productivity losses and additional interventions for the mitigation of the formation damage process which often are not cost-effective operations [100].

In this way, many authors have worked on the synthesis and evaluation of different types of nanomaterials with the purpose of determining their adsorptive capacity towards asphaltenes and improving the flow conditions through coreflooding experiments after the asphaltene destabilization is fomented [101]. A summary of the mentioned studies is presented in

Table 4. Summary of the scientific publications reporting the application of nanoparticles for the inhibition of the formation damage associated with asphaltene precipitation/deposition.

Authors	Year	Nanoparticle	Summary	Methodology	Experimental Ensamble	Source/Synthesis Method	Results
Rezakazemi et al. [101]	2018	γ-Al2O3	Separation of the asphaltene using a ceramic membrane.	Dynamic light scattering (DLS)	Membrane cells	Commercial γ-Al2O3 (size, purity, and the specific surface area are 30 nm, 99.99%, and 90–160 m2/g)	Raman spectroscopy results revealed a significant rise in the estimated asphaltene molecular sheet diameter from 5.4818 to 13.7866 A.
			The impact of adding alumina nanoparticles on expanding asphaltene’s molecular size.				According to DLS data, the addition of nano-alumina increased the molecular size of asphaltenes from 512.75 nm to 2949.55 nm.
Parsaei et al. [102]	2020	Iron oxide	The effect of nanoparticles on asphaltene precipitation was studied by measuring surface tension in the presence of CO2 at different temperatures and pressures.	IFT measurement by using pendant drop method in capillary tubes	Capillary tube	Commercial nanoparticles obtained from US Research Nanomaterials Inc. Houston, TX, USA	The addition of iron oxide nanoparticles to the oil solution reduces the interfacial tension at higher pressures with a steeper slope, showing that nanoparticles can decrease asphaltene precipitation.
							The presence of nanoparticles reduced the amount of asphaltene that precipitated at 50 °C by 16.34% and at 70 °C by 19.65% depending on the temperature.
Ahmadi, Aminshahidy [103]	2018	CaO and SiO2	The impact of CaO and SiO2 nanoparticle concentration on asphaltene precipitation in the presence of CO2 at different temperatures.	PVT cells to perform natural depletion	PVT cells	SiO2 was bought from Houston Brand company. For the CaO, 10 g CaCO3 was mixed with 5 mL acid solutions of succinic acid, tartaric acid, and citric acid (0.5 g acids dissolved in 5 mL water) and left to rest for 24 h. This mixture was then dried at 100 °C for 2 h.  The samples were  heated separately for 2 h at 900 °C.	Temperature increased from 90 °C to 100 °C during pressure reduction from 2500 Psi to 1500 Psi.
							CaO decreased asphaltene precipitation from (0.32 wt%, 0.62 wt%) to (0.096 wt%, 0.214 wt%); SiO2 decreased asphaltene from (0.56 wt%, 1.10 wt.%) to (0.27 wt.%, 0.52 wt.%).
Oliveira et al. [104]	2013	Cobalt ferrite	The use of modified cobalt ferrite nanoparticles as a flocculant agent for asphaltenes.	Nanoparticles are annealed in an effort to modify their structural phase.	UV-Vis spectrophotometer Varian	Through the homogeneous precipitation method, cobalt ferrite nanoparticles were created using deionized water, a 2.0 mol/L solution of FeCl3, and a 1.0 mol/L solution of CoCl2.	The system’s asphaltene precipitation is unaffected by the presence of modified nanoparticles, indicating that the particles can help the asphaltene aggregate.
Cortés et al. [105]	2012	SiNi	Analyze the effect of temperature and NiO content on the asphaltene uptake by a hybrid nanomaterial composed of nickel oxide nanoparticles supported on a nanoparticulated matrix of silica gel.	UV-Vis technique to determine the asphaltene adsorption on the nanoparticles.	Nanoparticulated matrix of silica gel	Silica nanoparticles were synthesized by the sol–gel method following an acid route. The gel was prepared from TEOS (tetraethoxysilane), ethanol, water, and HNO3.	Asphaltene adsorption increased with increasing nickel oxide concentration in the hybrid nanomaterials at constant temperature. Regardless of asphaltene concentration, the hybrid nanomaterials’ ability to absorb asphaltene decreased as the temperature rose.
						The synthesized nanosilica was impregnated with aqueous solutions of nickel nitrate Ni(NO3)2 in different concentrations (5 and 15 wt%) for 3 h and then dried at 120 °C for 6 h and cured for 6 h at 450 °C.
Kazemzadeh et al. [106]	2014	Fe3O4	Examining the effect of Fe3O4 nanoparticles on asphaltene precipitation	Bond number measurement and IFT measurement using VIT technique.	The high-pressure chamber was made out of a capillary tube at the top.	Commercial nanoparticles. No provider reported.	The intensity of the asphaltene precipitation would be reduced as the the mass fraction of Fe3O4 nanoparticles increased.
Shojaati et al. [107]	2017	γ-Al2O3  Fe3O4  NiO	The impact of Fe3O4, NiO, and -Al2O3 metal oxide nanoparticles on synthetic oil was explored in this study in order to reduce the danger of asphaltene deposition and postpone the commencement of asphaltene precipitation.	An indirect technique as opposed to other onset measurement techniques.	Test tubes	Nanoparticles obtained from U.S. Research Nanomaterials, Inc., Houston, TX, USA.	Metal oxide nanoparticles showed a great effect on inhibition of asphaltene precipitation and can be applied as an inhibitor.
							The instability of asphaltenes and the amount of asphaltene deposits were reduced in the presence of nanoparticles, in the following order of effectiveness: γ-Al2O3 > NiO > Fe3O4.
Mohammadi et al. [108]	2011	TiO2  ZrO2  SiO2	Study the effect of metal oxide nanoparticles in organic-based nanofluids for stabilizing asphaltene particles in oil.	Oil titration method, making use of the polarized light microscopy technique, to check their potential in stabilizing or destabilizing asphaltene nanoaggregates.	The titration procedure is performed by gradual and step-by-step addition of n-heptane. Then, precipitation of asphaltene was investigated using a polarized light microscope.	TiO2 Nanoparticles: two different solutions were prepared.	Rutile (TiO2) fine nanoparticles can effectively enhance the asphaltene stability in acidic conditions and act inversely in basic conditions. It was found that the required amount of n-heptane for destabilizing the colloidal asphaltene is considerably higher in the presence of TiO2 nanofluids at pH below 4. FTIR spectroscopy shows the changes in n-heptane-insoluble asphaltenes when acidic nanoliquid TiO2 is used as an inhibitor. According to the results of FTIR spectroscopy, TiO2 nanoparticles can increase the stability of asphaltene nanoaggregates by forming hydrogen bonds in an acidic medium. At this time, the other materials used in this experiment, as well as the TiO2 nanoparticles, are not able to form a hydrogen bond under alkaline conditions; hence, they are not able to prevent the precipitation of asphaltenes.
						Solution prepared by mixing 10 mL of tetraisopropyl orthotitanate with 25 mL of ethanol and 2 mL of ethylenediamine as template under vigorous stirring.
						3 mL HCl, 20 mL distilled water, and 10 mL ethanol. Then, it was slowly injected into solution 1 under 40–50 °C and stirred for about 4 h.
						Zirconium oxychloride (ZrOCl2.8H2O) was used as the Zr source. The stock solution was prepared by mixing the metal salt solution with a solution of 1.5 g urea and 9 mL LNH3 (25 wt%) at a temperature of 60–80 °C and a pH between 9 and 10. As a surfactant, 2 g ethoxylated nonylphenol (20 mol) was added to form a nanoemulsion.
						20 mL of TEOS was dissolved in a mixture of isopropyl alcohol and ethanol and stirred at 50° C. for about 1 h. To this solution were then added 5 mL of ethylene diamine and 3 g of citric acid. The resulting solution was hydrolyzed to 65% by weight HNO3 solution for 2 h with vigorous stirring and then refluxed for 24 h.
Nassar et al. [109]	2011	NiO  Co3O4  Fe3O4	Asphaltenes have been investigated for their oxidation onto different types of nanoparticles, namely NiO, Co3O4, and Fe3O4.	The asphaltenes containing nanoparticles were separated by centrifugation. The supernatant was decanted and precipitated. Then, the samples were subjected  to thermal analysis for estimating the adsorbed amount of  asphaltenes and oxidation.	Batch adsorption experiments	Commercial nanoparticles purchased from Sigma Aldrich.	All tested nanoparticles showed high adsorption affinity and catalytic activity for the adsorption and oxidation of asphaltenes in the following order: NiO > Co3O4 > Fe3O4. The oxidation temperature of asphaltenes decreased by 140, 136, and 100 °C compared to non-catalytic oxidation in the presence of NiO, Co3O4, and Fe3O4 nanoparticles, respectively.
Tarboush et al. [110]	2012	NiO	Shows that NiO nanoparticles prepared in situ within heavy oil display much higher affinity toward asphaltene adsorption.	Oil characterization, before and after asphaltene adsorption,  was conducted using density and viscosity measurements.	Viscosity measurements were determined using a cone–plate Brookfield viscometer model.	Nickel(II) nitrate hexahydrate (99.9985%, Puratronic) was used as the precursor salt. Commercially available nickel oxide (NiO) nanoparticles (dp < 50 nm, 99.8%) were used for comparison.	An asphaltene absorption of 2.8 g asphaltenes/g nanoparticles was reported. Commercial NiO nanoparticles in the same size range exposed to the same experimental conditions adsorbed only 15% of the above value.
Shayan and Mirzayi [111]	2015	γ-Fe2O3  α-Fe2O3	Synthesized maghemite (γ-Fe2O3) and hematite (α-Fe2O3) nanoparticles were used for the adsorption and removal of asphaltenes from the prepared solution.	UV-vis spectrophotometer to determine the maximum peak of adsorption for asphaltene.	Batch adsorption experiments	FeCl3 (ferric chloride), FeCl2 4H2O (ferric chloride tetrahydrate), HCl (hydrochloric acid, 37%), ammonium hydroxide (NH4OH, 25% ammonia), methylene blue.	This work showed that the synthesized MNPs and HNPs can be considered as nanoadsorbents of asphaltenes, although MNPs are more efficient.
Zabala et al. [112]	2013	γ-Al2O3	Describes the evolution of a fluid containing nanomaterial with high adsorption capacity for asphaltene inhibition.	Upscaling and field trial application	Real in-field conditions	Commercial silica nanoparticles obtained from Petroraza S.A.	Asphaltene content measured in the produced oil increased after the well treatment with the nanofluid containing alumina nanoparticles.
Al-Jabari et al. [113]	2007	Fe3O4	Combination of nanoparticle adsorption and magnetic separation for the removal of asphaltenes from heavy oil by adsorption on colloidal magnetite.	Combination of nanoparticle adsorption and magnetic separation	Magnet and UV-Vis spectroscopy	Obtained from Nanostructured & Amorphous Materials, Inc., (130 Benton St, Garland, TX 75042,TX, USA)	Ultra-dispersed magnetite nanoparticles offer several advantages over conventional ones; for example, they provide a large surface of contact, reduce the distance traveled between the adsorbed species and the surface of the solid particles, and are excellent for phase separation with the aid of a magnetic medium.
Hosseinpour et al. [114]	2013	NiO  CaCO3  Fe2O3  WO3  MgO  ZrO2	Three different categories of metal oxide nanoparticles with acidic, amphoteric, and basic surfaces were synthesized, and their textural, structural, and acid–base properties were characterized. Asphaltenes are extracted from the dead heavy oil sample, and their structure, elemental composition, and acid–base number are determined. The nanoparticles are then used to adsorb asphaltenes from asphaltene–toluene solutions.	Centrifugation followed by UV–vis spectroscopy of the supernatant liquid	The nanoparticles were mixed in tightly sealed vials	Precipitation method employed for obtaining the different nanostructures	The adsorption capacity of asphalt nanoparticles is between 1.23 and 3.67 mg/m2 and decreases in the order NiO > Fe2O3 > WO3 > MgO > CaCO3 > ZrO2, which is accompanied by the synergistic effects of acidity and surface charge.
Li et al. [115]	2018	NiO  SiO2  Fe3O4	Investigated effect of nanoparticles on the inhibition of asphaltene particle aggregation in a water-wet micro-sized pore.	Experimental methodology that directly observed asphaltene aggregation at the pore scale.	Water-wet microsized pore	Commercial nanoparticles. SiO2 nanoparticles (20 nm, ≥99.9%), NiO nanoparticles (40 nm, ≥99.9%), and Fe3O4 nanoparticles (20 nm, ≥99.5%)	The nanoparticles can act as inhibitors of asphaltenes, preventing the aggregation of asphaltenes and increasing the stability of asphaltenes in the microcapillaries. Asphaltene particles can easily aggregate with each other in the absence of nanoparticles. On the other hand, the presence of nanoparticles can prevent asphaltene particles from flocculating. This could be mainly due to the high surface area to volume ratio, good adsorption capacity, and high degree of suspension of the nanoparticles.
Azizkhani et al. [116]	2019	Fe3O4  γ-Al2O3	Focused on the asphaltene precipitation by liquid-free asphaltene inhibitors at reservoir conditions.	The vanishing interfacial tension technique was implemented to evaluate the effect of the nanoparticles on minimum miscibility pressure.	PVT cells	Commercial nanoparticles. No provider reported.	Direct inhibitors of asphaltenes (liquid inhibitors) can be considered excellent candidates for field-scale mixed gas injection. Injection of  CO2/nanoparticles reduced the precipitation of asphaltenes compared to injection of pure CO2 under reservoir conditions.  Mixtures containing Fe3O4 can perform better than Al2O3 solutions as direct inhibitors of asphaltenes.
Varamesh et al. [117]	2019	Fe3O4  NiO	Development of a reliable and simple CPA EoS-based approach to model asphaltene precipitation in the presence of Fe3O4 and NiO nanoparticles.	Asphaltene onset in the  presence and absence of nanoparticles was measured using dynamic light scattering. Cubic plus association equation of state (CPA EoS) was employed to predict the asphaltene precipitation in the presence and absence of the nanoparticles.	FTIR spectrophotometer	NiO and Fe3O4 nanoparticles were synthesized via precipitation from aqueous solutions.	CPA EoS can be used to develop chemical inhibitors of asphaltene precipitation by metal oxide nanoparticles.
Lu et al. [118]	2016	γ-Al2O3	Investigated the adsorption of asphaltenes onto Al2O3 through 2 methods:  (a) by adding a certain mass of nanoparticles in a fixed volume  solution with different initial concentrations of asphaltenes	Coreflooding tests	Core	Commercial nanoparticles purchased from Aladdin Reagents Co. Ltd. (Shanghai, China).	The higher the mass fraction of Al2O3, the lower the precipitation intensity of asphaltenes.  Al2O3 nanofluid injection can reduce the amount of oil  and reduces permeability because nanoparticles can inhibit asphaltene deposition on the sand surface in a porous medium.
			(b) by exposing a certain amount of asphaltenes in a fixed volume of solution with the addition of different amounts of nanoparticles
Ezeonyeka et. al. [119]	2018	Fe2O3  Fe3O4  γ-Al2O3	Investigation of the adsorption of n-heptane-precipitated asphaltenes, C7 asphaltenes, from toluene model solutions onto three metal oxide NPs, Fe2O3, Fe3O4, and Al2O.	UV–vis spectroscopy at three different wavelengths was  compared with thermogravimetric analysis (TGA) results	Sapphire measuring prism	Commercial Fe2O3 (dp <50 nm), Fe3O4 (20–30 nm), and Al2O3 (<50 nm particle size) were used as adsorbents.	Al2O3 showed the highest adsorption capacity with 385 ± 5 mg/g, followed by Fe3O4 and Fe2O3. Referring to mg/m2, however, Fe2O3 showed the highest adsorption capacity. TGA analysis showed that NPs promoted the oxidation of adsorbed asphaltenes in the reverse order of their adsorption capacity, qmax (mg/g) (Al2O3 > Fe2O3 ≈ Fe3O4).
Nassar et al. [120]	2015	SiO2  γ-Al2O3  Fe3O4	Commercial nanoparticles of silica, γ-alumina, and magnetite were used as adsorbents to probe the chemical nature of the nanoparticles for asphaltene growth inhibition and to validate the model.	Experimental data on the kinetics of asphaltene aggregation were obtained using dynamic light scattering (DLS) measurements	UV-Vis spectrophotometer through asphaltene model solution in toluene	Commercial nanoparticles purchased from Sigma Aldrich	Under different conditions tested, all nanoparticles reduce the hydrodynamic diameter of large aggregates in solution to different degrees due to adsorption. The influence of the chemical nature of the nanoparticles, the origin of the asphaltenes, the heptol solution, and the temperature was successfully evaluated with DLS measurements.
Tarboush et al. [121]	2014	Fe2O3	Presentation of the sol–gel/emulsion method for the in situ production of Fe2O3 nanoparticles in heavy oil from their aqueous precursor and comparison of their asphaltene adsorption with commercial Fe2O3 nanoparticles.	In situ prepared nanoparticles were recovered by centrifuging the crude oil for 10 min. The recovered samples were analyzed through TGA experiments.	In situ in heavy oil phase starting from their precursor aqueous iron (III) nitrate solution using a sol–gel/emulsion approach.	Iron (III) nitrate nonahydrate (used as the precursor salt), commercial iron (III) oxide (Fe2O3) nanoparticles (dp = 20–30 nm, 98%, used for comparison), toluene (99.8%), n-heptane (99%), and/or dichloromethane (DCM) (anhydrous, ≥99.8%, used to wash the nanoparticles recovered from the oil phase for microscopy).	The nanoparticles prepared in situ showed a much higher absorption, 2.6 ± 0.12 g/g, and were much more selective than the asphaltenes. Increasing the concentration of in situ generated particles showed a downward trend in absorption compared to the equilibrium concentration of asphaltenes.
Hashemi et al. [122]	2016	NiO	Possible influence of nickel oxide (NiO) nanoparticles on the destabilization of asphaltene deposits in porous media in the presence of carbon dioxide.	Three experiments were designed to analyze the precipitation process of asphaltenes in the oil stream in porous media and the impact of the presence of nanoparticles in this process.	Carbonate porous matrix	The material used for the synthesis of nickel oxide nanoparticles was nickel acetate (C4H6NiO4). First, an appropriate amount of nickel acetate was dissolved in water, and then the solvent, citric acid, was added to the mixture in a stoichiometric ratio to form a homogeneous gel. The droplets of the prepared solution were dispersed in the carrier gas and transported to the reaction medium.	The accumulation of asphaltenes in the heart was reduced from 0.1033 (g) in EXP-2 to 0.0128 (g) in EXP-3 in essentially identical experimental situations.
				The first experiment consisted of injecting live oil into the heart to analyze the effect of injection pressure and velocity, which also includes the mechanism of elimination of organic matter in the natural degradation process.
				In the second experiment, the asphaltene precipitation inside the core was studied by injecting CO2 into the core.
				In a third experiment, nickel oxide nanoparticles were dispersed in CO2 to study the effect of the presence of nanoparticles on asphaltene precipitation.
Betancur et al. [123]	2016	SiO2	Studied the role of the particle size and surface acidity of silica nanoparticles on their interaction and adsorption of asphaltenes	Constructed adsorption isotherms through UV–visible spectrophotometry, as well as estimated the change in the asphaltene aggregation through dynamic light scattering (DLS).	UV–visible spectrophotometer, nanosizer, and core for dynamic tests	Implemented the sol–gel method for the synthesis of silica nanoparticles of different sizes from a tetraethyl orthosilicate (TEOS) precursor. The surface acidity of the nanoparticles was also modified.	It was observed that as the nanoparticle size increased, the adsorption was reduced due to a lesser availability of active sites in the adsorbent surface. Moreover, the acidity had a direct relation to the asphaltene adsorption and disaggregation. In addition, coreflooding tests were carried out with a nanofluid including the best nanoparticles, and the recovery factor had an increment of 11%.
Amin and Nazar [124]	2016	SiO2  γ-Al2O3  TiO2	The influence of effective factors such as nanoparticle types, asphaltene types, nanoparticle-to-solution ratio of the asphaltene model, and temperature on the adsorption size of asphaltenes on metal oxide nanoparticles was evaluated.	The Taguchi design of experiments (DOE) approach, the toluene–asphaltene solution model, and a UV–visible spectrophotometer.	UV–visible spectrophotometer	Commercial nanoparticles purchased from TECNAN.	The nanoparticle type and nanoparticle structure of asphaltenes with an impact of 48.5% and 3.11%, respectively, have the highest and lowest proportions of the amount of adsorbed asphaltenes at selected concentrations. Alumina nanoparticles have the highest adsorption, and silica nanoparticles have the lowest adsorption. The temperature has no statistical significance. Asphaltenes with high aromaticity tend to adsorb more onto nanoparticles.
Hosseini-Dastgerdi et al. [125]	2022	SiO2  Polyacrylamide (PAM)	The study assesses how silica–polyacrylamide nanocomposite might be used for the first time to prevent asphaltene precipitation.	Techniques for polarized microscopy, dynamic light scattering, asphaltene dispersion testing, and viscometry	FTIR polarizing microscope (Olympus), FESEM technique	Using the vapor acid process with sulfuric acid over 1300 °C for 48 h, SiO2 nanoparticles were functionalized. The functionalized nanoparticle solution was then combined in a 1:1 mass ratio with polyacrylamide.  By adding a certain quantity of synthesized nanocomposites to distillate water to reach a concentration of 1000 ppm by mass, the silica–PAM nanofluid was created.	The aggregate (asphaltene) size decreases as the dosage of the nanocomposite increases. It is anticipated that the heterogeneities of the nanocomposite surface will produce a number of sites for the adsorption of asphaltene, enhancing adsorption affinity and reducing asphaltene self-association. For the crude oil, the greatest dispersion effectiveness of the nanocomposite was 69% and 79% at doses of 1% and 2.5% nanofluid volume.
López et al. [126]	2020	SiO2  cardanol	Assess how cardanol/SiO2 nanocomposites behave in preventing asphaltene damage using a coreflooding test under reservoir circumstances.	Adsorption curves/desorption isotherms of cardanol onto SiO2 nanoparticles were constructed. Likewise, the relationship between the total surface acidity and the H and K of the SLE model was presented.	Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS)	To eliminate any dampness, SiO2 nanoparticles (SNs) were first dried at 120 °C for 4 h. The beginning wetness technique was used to secure the CDN to the SN surface. The mass of the cardanol sample per gram of SiO2 nanoparticles was then changed to produce three SiO2/cardanol nanocomposites (CSNs).	The developed nanocomposites demonstrate significant asphaltene precipitation/deposition inhibition capacity. Additionally, using nanocomposites improves oil recovery by more than 50% when compared to the scenario with asphaltene damage.
Bagherpour et al. [127]	2023	Carboxylate-alumoxane nanoparticles  functionalized BMA and PBMA	In this study, the application of two types of carboxylate-alumoxane nanoparticles (functionalized boehmite by methoxyacetic acid (BMA) and functionalized pseudo-boehmite by methoxyacetic acid (PBMA)) for asphaltene adsorption and precipitation was investigated.	BMA and PBMA nanoparticle DLS analysis. Pore size distribution and nitrogen adsorption–desorption isotherm for PBMA were presented.	Ultraviolet–visible (UV–Vis) spectroscopy	Boehmite and pseudo-boehmite were functionalized using the acidic technique to create carboxylate-alumoxane nanoparticles, and are now known as BMA and PBMA, respectively. Aluminum oxide-hydroxide with varying concentrations of H2O molecules and variable crystal sizes makes up boehmite. Boehmite is frequently used as the main starting material when creating alumina phases.	In comparison to the onset point of the reference synthetic oil, the use of BMA and PBMA delays the commencement of precipitation by 17 and 26%, respectively. The adsorption of asphaltene on the surface of these functionalized nanoparticles is the most important factor for asphaltene removal in the presence of carboxylate-alumoxane nanoparticles. The carboxylate-alumoxane nanoparticles containing asphaltene are eliminated from the system upon centrifugation.
Mahmoudi Alemi et al. [128]	2021	Fe2O3 and functionalized SiO2 nanoparticles  F-SiO2	In a light live oil with a high danger of asphaltene deposition, this study investigates their impacts on asphaltene precipitation and aggregation. High pressure, high temperature (HPHT) reservoir conditions were used for the studies.	TGA mass loss curves of pure asphaltenes and asphaltenes adsorbed onto Fe2O3 and F-SiO2 nanoparticles	OPS technology with a LEUTERT one-phase sampler to have a representative oil sample;  HPHT filtration experiments	A straightforward chemical precipitation technique was used to create pure iron oxide Fe2O3 nanostructures. In this technique, a quantity of iron(III) chloride hexahydrate (FeCl3, 6H2O) is used.	The results demonstrate that adding 150 ppm of F-SiO2 nanoparticles to live oil before depressurization at 274.9 °F delays the onset of asphaltene by over 600 psi; in contrast, adding the same amount of Fe2O3 nanoparticles before depressurization makes the oil more stable and prevents the precipitation of asphaltenes.
Simin Tazikeh [129]	2022	Fe3O4	Investigate changes in the surface properties of silica during the precipitation of asphaltenes with magnetite (Fe3O4) nanoparticles.	Images captured by an AFM technique of an A2 asphaltene precipitation on a silica substrate.  Changing wettability using the Young–Laplace and modified Wenzel models	Fourier transform infrared spectroscopy (FTIR);  atomic force microscopy (AFM)	Polythiophene-coated Fe3O4 nanoparticles (Fe3O4-PTNP) are synthesized in two steps. First, Fe3O4 nanoparticles are synthesized by coprecipitation. Then, they are coated with polydopene using a chemical polymerization technique.	The results show that heteroatoms (e.g., O, N, and S) and aromatic rings as functional groups can affect the process of asphaltene agglomeration and adsorption onto a silica surface. Atomic force microscopy (AFM) is used to obtain adequate topography information.
Gandomkar and Nasrian [130]	2020	Metal oxide nanoparticles (GO, TiO2, SiO2, and MgO)	As direct asphaltene inhibitors (DAIs) on asphaltene stability over the period of miscible CO2 injection, metal oxide nanoparticles (GO, TiO2, SiO2, and MgO) have been addressed in this study in the liquid-free mode.	Four metal oxide nanoparticles (GO, TiO2, SiO2, MgO) were used as direct inhibitors to stabilize asphaltenes during CO2 injection into reservoir oil. The nanoparticles have acidic (SiO2 and TiO2) and basic (MgO and GO) characteristics.	IFT measurements of chemical properties. Bulk sample and dynamic asphaltene test	Commercial nanoparticles.	The CO2/GO mixture reduces asphaltene aggregation/deposition and improves oil recovery by 6–25% compared to CO2 injection alone. Direct asphaltene inhibitors reduce interfacial tension (IFT) and allow miscible gas injection at reservoir pressure and temperature. Metal oxide nanoparticles increase the solubility of asphaltene particles, keeping them in solution.
Azizkhani and Gandomkar [131]	2020	Fe3O4  Al2O3	This study centered on the inhibition of liquid-free asphaltene precipitation under reservoir conditions. During CO2 injection, the Fe3O4 and Al2O3 nanoparticles were utilized as direct asphaltene inhibitors (DAIs).	The interfacial tension technique was used to evaluate the effect of DAIs on the minimum miscibility pressure during CO2/nanoparticle injection. Asphaltene precipitation in volatile and intermediate oils was studied by varying the concentration of DAI from 500 to 3000 ppm.	IFT (advanced drop shape analysis) PVT Cell	All the nanoparticles are commercially available, so Fe3O4 and Al2O3 were used as received. These nanoparticles were used in different concentrations such as 500, 1000, 2000, and 3000 ppm.	The addition of Fe3O4 and Al2O3 to CO2 reduces MMP in reservoirs. Mixtures with Fe3O4 are better asphaltene inhibitors than Al2O3 solutions. Solubility is more important than aggregation during CO2 nanoparticle injection. DAI concentrations above 2000 ppm are not favorable.

.

To improve the understanding of the applicability of nanoparticles and the phenomena surrounding their interactions with asphaltenes, further sections will be introduced. The order for this manuscript section is as follows: (I) synthesis of nanoparticles for the inhibition of the precipitation/deposition of asphaltenes, (II) phenomenological approaches to the asphaltene–nanoparticle interactions, and (III) inhibition of asphaltene precipitation/deposition.

Table 5. Advantages and disadvantages of the most commonly used nanoparticle synthesis techniques [56,57,58,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157].

Method	Summary	Advantages	Problems
Chemical synthesis	Nanoparticle synthesis involves controlled chemical reactions using precursors and reducing agents. Common techniques include chemical reduction, chemical precipitation, coprecipitation, and microemulsion.	Chemical synthesis of nanoparticles offers precise control of size and shape, has diverse applications, and is easy to implement in the laboratory.	Chemical synthesis of nanoparticles can require toxic or expensive reagents, be a slow process, and be difficult to scale up for large-scale production.
Thermal decomposition method	Thermal decomposition generates nanoparticles by decomposing precursors at high temperatures in a controlled atmosphere to obtain metallic, semiconducting, or ceramic particles.	Allows the synthesis of nanoparticles at high temperatures with high purity, especially in the case of metallic and ceramic particles.	Requires special equipment, but there may be problems with stability, aggregation, and generation of unwanted by-products.
Wet synthesis	Uses an aqueous solution to generate nanoparticles. Methods such as sol–gel synthesis, hydrolysis, and precipitation in aqueous media are employed. This technique allows precise control of the size and shape of the nanoparticles.	Excellent purity and homogeneity, exact control of nanoparticle size and shape, and suitability for high-volume manufacturing.	It could need additional stages and agents, be sensitive to environmental factors, and demand rigorous supervision.
Microemulsion method	Stable colloidal systems of water, oil, and surfactant are used, achieving high uniformity in particle size and shape.	Guarantees high uniformity in size and shape, greater colloidal stability, and the production of very small particles.	The synthesis of nanoparticles with microemulsions is complex, requires specific temperature and pH conditions, and may involve additional purification and separation steps.
Microwave-assisted synthesis	Can be accelerated using microwave radiation, which allows rapid energy transfer to activate the chemical reaction efficiently.	It accelerates nanoparticle synthesis by providing rapid and uniform heating, reducing reaction time and increasing efficiency. It also allows precise control of reaction conditions.	Requires specialized microwave equipment and adjustments to reaction parameters. Production scale may be limited due to equipment constraints.

summarizes the most commonly used systematization techniques, as well as their main advantages and disadvantages. It should be noted that the above advantages and disadvantages are variable depending on the synthesis conditions, materials, and application requirements. It is essential to carefully evaluate each method based on the needs and constraints of the project.

3.1. Synthesis of Nanoparticles for the Inhibition of the Precipitation/Deposition of Asphaltenes

There are several methodologies for nanoparticle synthesis, which can be divided into two broad classifications: (I) bottom-up and (II) top-down. Bottom-up techniques refer to any experimental methodology which enables the construction or assembly of the nanostructure from smaller structures, molecules, or atoms; top-down techniques are related to the nanostructures obtained from micro- and macroscale materials typically by applying high-energy milling. A schematic of both processes is shown below (Figure 4).

Nonetheless, the scientific literature shows minor applications of nanoparticles obtained through high-energy milling processes for the inhibition of the formation damage associated with asphaltene precipitation/deposition. Thus, this manuscript describes the nanoparticle synthesis processes related to bottom-up methodologies such as sol–gel, coprecipitation, and functionalization of nanoparticulated supports [100,105,123,159,160].

3.1.1. Sol–Gel

The sol–gel method has been widely used for the synthesis of different types of materials such as aerogels, ceramics, membranes, and silica nanoparticles [161,162,163]. Silica nanoparticles have great applicability in different fields, including the oil and gas industry. In this sense, this type of nanomaterials has been applied for the viscosity reduction of heavy oils [164,165,166], the optimization of drilling fluids [30], and the inhibition of the formation damage related to the precipitation/deposition of asphaltenes [105,120,123,167]. The process for obtaining the silica nanoparticles through the sol–gel method is particularly simple, and it is summarized below (Figure 5). Typically, the process contains three steps: (I) a hydrolysis reaction, (II) the polymerization of the hydrolyzed silica chains via condensation, and (III) the formation of a gel which is characterized as a colloidal suspension [161]. Moreover, a catalyst favoring the performance of the hydrolysis reaction is usually included, while the gel formation is generally improved by reducing the mixture pH [168,169].

Different silicon precursors can be applied for obtaining any type of desired silica nanoparticles. Among the most used are some alkoxides, such as tetraethyl orthosilicate (TEOS), and sodium silicate [123,160]. The synthesis process has a great sensibility in terms of the nanostructure textural properties such as surface area, size, and roughness regarding the precursor/continuous phase ratio, while the catalyst presence plays a minor role in these properties [160].

3.1.2. Coprecipitation

The coprecipitation method is mainly used for obtaining magnetite nanoparticles which have a great affinity towards asphaltene adsorption and also have a recoverable advantage compared to other types of nanomaterials due to their magnetic behavior [170].

To obtain magnetite nanoparticles, two salts, namely FeCl₂ and FeCl₃, are commonly used; the latter is diluted in HCl (2%), and the former is diluted in deionized water. Both solutions are mixed for 15 min, and then a solid suspension is obtained. The magnetic solid is then separated and further dried at 80 °C for at least 12 h.

An analogous methodology is applied to obtain composite materials. Betancur et al. [171] developed a nanocomposite material based on a core–shell structure, where a magnetite core was used and synthesized using the coprecipitation method, and a silica shell was constructed using the sol–gel method. This composite had the advantage of having a greater affinity towards asphaltene adsorption than the magnetite alone due to the silica inclusion, while its recoverable advantage was maintained.

3.1.3. Nanoparticle Functionalization

The nanoparticle functionalization consists of the assembly of some compounds or smaller nanoparticulated systems on a nanometric support. There are several techniques for achieving a functionalized material; however, the incipient wetness technique, which is a rapid and low-cost methodology, has been used for application in the oil and gas industry in most cases [105].

The method is shown in Figure 6, where it can be observed that from a solution, which is typically formed by hygroscopic salt, the nanoparticulated support is functionalized through continuous and slow dripping. The obtained material is then dried at a temperature of 80–120 °C and further calcined if the functionalizing agent is intended to be a metal oxide [172].

3.2. Phenomenological Approaches to Asphaltene–Nanoparticle Interactions

3.2.1. Behavior of the Crude Oil Fractions in Terms of Their Interaction with Nanoparticles

It should be noticed that asphaltenes are a group of molecules rather than a sole compound even in the same crude oil sample [82,173]. In this sense, it has been considered that those differences in the chemical structure of the asphaltenes produce a distinct behavior in terms of solubility for each molecule due to a different polarity [174]. Therefore, several studies have classified the asphaltenes into two groups depending on their solubility and thus their polarity: A1 and A2, with the former being the most polar group. In this regard, it has been identified that A1 has a typical solubility of 90 mg/L in toluene, while A2 has a much higher solubility of up to 5–12% [175]. The difference in the solubility has been attributed to different factors such as higher aromaticity for the A1, i.e., a larger polyaromatic core and/or a higher aromatic rings/alkyl groups ratio, whereas the A2 fraction has a larger amount of alkyl substituents, which facilitates its stabilization in aromatic solvents [174]. Consequently, the A2 fraction has a great similarity with resins in terms of chemical structure and role in asphaltene aggregation [176].

A generic representation of both fractions is presented in Figure 7. M1 (A1 fraction) has a larger number of aromatic rings, while M2 (A2 fraction) has an additional alkyl substituent [176]. The higher aromaticity of the A1 fraction hinders its solubility. This lack of solubility facilitates the interaction between A1 species through acid–base and H-bonding mechanisms because of their high polarity. This behavior leads to the formation of nuclei and further small aggregates with which the A2 species start to interact, mainly by π–π stacking positioned on the aggregate borders [174,176]. Then, the aggregate formation is slowed down until the interaction with resins, which leads to the final formation of the supramolecular assembly and stabilization in the oil matrix [177].

It is worth mentioning that the shapes of the nanoaggregates are continuously changing (size, molecule arrangement) as the nanoaggregates are susceptible to shearing forces, such as those exerted by the fluids in the porous medium and in the production systems, and to environmental conditions (pressure and temperature) [120,178,179]. However, before the addition of the nanoparticles, the crude oil microstructure is defined as being at equilibrium for at least an interval of time. In this way, the nanoparticles interact with the asphaltenes as their addition disrupts the equilibrium of the crude oil microstructure [120].

The asphaltene–nanoparticle interactions are guided by the same asphaltene aggregation mechanisms; nonetheless, the asphaltenes’ attractive forces towards the surface of the nanoparticles are greater than the asphaltene–asphaltene ones [164]. This is explained by two principles: (I) the presence of active sites on the surface of the nanoparticles, where those active sites are generally chemical species with a high dipole moment [180], and (II) the low amount of surface-active groups in the asphaltene structure compared to the surface of the nanoparticulated sorbents [181]. Hence, when the nanoparticles are added to the crude oil, their inclusion leads to a perturbation in the microstructure equilibrium [164,182]. In this regard, several studies have proven that the mentioned disruption is intrinsically fomented by an unconstrained difference in potential [84,183,184]. Thereby, the asphaltene adsorption on the surface of the nanoparticles is spontaneous, and in general, it is exothermic [185,186].

As with the asphaltene aggregation where nuclei are formed by the most polar species, the asphaltene–nanoparticle interaction is firstly driven by the A1 fraction [176,187]. Hence, the A1 fraction interacts with and adsorbs on the adsorbent surface until its active sites are inaccessible either due to being saturated or due to steric effects [176]. At this point, it is considered that the adsorbed asphaltenes have formed a monolayer on the nanomaterial’s surface [123,171]. Moreover, the adsorbed asphaltenes can interact with the free asphaltenes in the crude oil depending on the same steric effects mentioned before; that is, the asphaltenes aggregating on the nanoparticle’s surface is the reason why the asphaltene adsorption is ensembled in a multilayer [188]. This multilayer is composed mainly of the A2 fraction due to its higher availability compared to the A1. Thus, the interaction between the asphaltenes in the monolayer and the free asphaltenes is driven by π–π stacking [176,181]. A representation of this phenomenon is shown below (Figure 8). It must be clarified that there are different types of active sites that depend on the chemical nature of the nanoparticles, while their availability depends on the synthesis method and on the textural properties of the nanomaterial (roughness, surface area, size) [180,181].

Even though the resins play a major role in the formation and stabilization of the aggregate by surrounding the A1–A2 assembly, it has been demonstrated that their interaction with the surface of the nanoparticles is almost negligible, and thus, a similar trend is observed for the adsorption [189], and this is explained by the resins’ dipole moment. As for the A2 fraction, the resins have a minor dipole moment compared to the A1 molecules due to their chemical structure which includes a larger amount of alkyl substituents. In this way, the small fraction of resins adsorbed onto nanoparticles could be considered as part of the multilayer assembly alongside the A2 fraction [190].

3.2.2. Adsorption Isotherms: Construction and Modeling

Several methodologies have been proposed to experimentally evaluate the asphaltene adsorption over the nanoparticle surface. Among these techniques are the thermogravimetric decomposition of the adsorbed fraction and colorimetry through UV–visible spectrophotometry [84,85,189,191,192]. The most used method is spectrophotometry which includes the development of batch adsorption tests in which model solutions of asphaltenes in toluene are generated from a stock solution (typically of 5000 mg/L). Then, the nanoparticles are added to the different model solutions and further stirred and centrifuged. The adsorbed asphaltenes and the nanoparticulated nanomaterial are deposited, while a supernatant with the free asphaltenes is obtained. It is worth mentioning that the model solutions could be made while varying the asphaltene or adsorbent concentration, and that this differentiated procedure affects the adsorption phenomena. The amount of asphaltenes adsorbed relative to the mass of nanoparticles $Q_{ads}$ (mg/g) is then estimated using the following mass balance:

$$Q_{ads}=\frac{(C_i-C_E)}{M}$$

(1)

where $C_i$ (mg·L⁻¹) and $C_E$ (mg·L⁻¹) are the initial concentration of asphaltenes in solution and the equilibrium concentration of asphaltenes (i.e., the asphaltene concentration in the obtained supernatant at the time $t$ (min)), respectively, and $M$ (g·L⁻¹) is the mass ratio of the nanoparticles and solution volume.

The adsorption isotherms can exhibit different shapes, with types I and III according to the IUPAC classification being common [193]. These distinct behaviors are mainly related to the availability of active sites on the nanoparticles and also to the amount of A1 asphaltenes that form the monolayer on the nanoparticle surface. In this way, the type I isotherm is observed when the asphaltene concentration is varied in the model solutions, while type III is exhibited when the nanoparticle concentration is varied [188].

To describe an adsorptive process, different models have been developed, involving either (I) a phenomenological approach to the adsorbate–adsorbent interactions or (II) a mere fitting of the experimental data. Different authors have attempted to describe the phenomena related to the asphaltene adsorption on the nanoparticles using the well-known Langmuir and Freundlich models [85,183]. A more specific asphaltene adsorption model was developed by Montoya et al. [194] and named the solid–liquid equilibrium (SLE) model; it accounts for the asphaltene–nanoparticle interactions and the assembly of asphaltenes onto the nanoparticle surface in the form of the mentioned monolayer–multilayer phenomena. The model is described by the following equations:

$$C_E=\frac{\psi H}{1+K\psi }exp\left(\frac{\psi }{q_m\cdot A}\right)$$

(2)

$$\psi =\left(-1+\sqrt{1+4K\cdot \xi }\right)$$

(3)

$$\xi =\left[q_m\cdot q/\left(q_m-q\right)\right]A$$

(4)

where $C_E$ (mg·g⁻¹) is the equilibrium concentration of the adsorbate in the solution; $q$ (mg·m⁻²) and $q_m$ (mg·m⁻²) are the adsorbed amount and the maximum adsorption capacity, respectively; $A$ (m²·mg⁻¹) is the surface area measured through the BET method. $K$ (g·g⁻¹) is a constant related to the self-assembly of the asphaltenes on the surface of the nanoparticle, and $H$ (mg·g⁻¹) is the Henry constant related to the nanoparticle–asphaltene affinity. Thus, the SLE model can predict the adsorption behavior in terms of the asphaltene–nanoparticle interactions.

An example of the above is shown in Figure 9. For this case, two different methodologies are used for constructing adsorption isotherms: (I) the use of the experimental data of the adsorption with a fixed mass of asphaltenes (adsorbate), which is represented by the black symbols, and (II) the use of variation in the adsorbent concentration compared to the model solutions, which is represented by the white symbols. Moreover, the M letter for the black symbols represents the nanoparticle dosage that was utilized for obtaining the supernatants. On the other hand, the Ci is the asphaltene concentration in the model solution when varying the nanomaterial.

It would be expected that with an increase in the amount of the nanoparticles, the asphaltene adsorption would be enhanced; however, the contrary was obtained for both isotherm construction methods. For method 1, it was observed that by increasing the number of nanoparticles, both the affinity (slope at low adsorbed amount) and the total amount of adsorbed asphaltenes are reduced. Moreover, even though method 2 exhibits a lower affinity towards asphaltene adsorption, the adsorbed amount of asphaltenes is higher than that in method 1 for several $C_E$ values, and the increase in nanoparticle concentration leads to lower adsorption. Both phenomena are derived from the asphaltene aggregation behavior. However, with a higher adsorbent availability, there would be larger spaces for the interaction of asphaltenes with the A1 fraction of the asphaltenes which forms the monolayer and has a limited amount on the crude oil compared to other compounds [176]. Thus, a larger number of nanoparticles would allow the adsorption of individual and separated molecules, rather than the formation of a monolayer which plays a major role in the attraction of the A2 fraction for its assembly in a multilayer ensemble. Moreover, with an increase in the adsorbent concentration, the nanoparticle–nanoparticle interactions are also promoted, and their aggregation would decrease the active site availability for asphaltene adsorption [188,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209].

Hence, the asphaltene aggregation behavior plays a major role in the adsorption phenomena over the nanoparticles, as the asphaltene–nanoparticle interactions and affinity plays a major role just in the monolayer formation [90,101,120,210], while further adsorbed molecules depend on the self-assembly of asphaltenes [84,85,109,191]. In this sense, planning a stimulation process including nanoparticles for inhibition of asphaltene precipitation/deposition requires rigorous experimentation in order to select the best material in terms of chemical nature and concentrations [205,208].

3.3. Inhibition of Asphaltene Precipitation/Deposition: Upscaling through Coreflooding Tests and Field Trial Application

Following the advances in the asphaltene precipitation/deposition inhibition technique using nanoparticles [101,113,118,124,190], different upscaling processes have been carried out through coreflooding tests, and real field tests have been developed in which superb results have been achieved.

Betancur et al. [123] evaluated different silica nanoparticles for the asphaltene adsorption and disaggregation and synthesized a nanofluid with the best nanomaterial to determine the injectivity enhancement through coreflooding tests after inducing asphaltene-associated formation damage. The authors determined that the improved flow conditions for the nanofluid injection would generate an additional recovery factor of 11%. Similar tests were carried out by Franco et al. [172] with nanoparticulated alumina functionalized with nickel oxide. The authors obtained a great increase in crude oil mobility as the S_wr (residual water saturation) increased by 23% upon nanoparticle injection. The formation damage remotion also caused an increase in the recovery factor of 9%.

It is also well known that several recovery methods such as CO₂ could affect asphaltene stability in crude oil [211]. In this regard, Hashemi et al. evaluated the effect of NiO nanoparticles in the removal of asphaltenes from a carbonate porous medium under CO₂ injection [122]. In this case, the authors observed that the remaining asphaltene concentration in the core was reduced by 88%. Azizkhani et al. also evaluated the effect of nanoparticles as asphaltene inhibitors during CO₂ injection processes [116]. For the experiments, the authors employed Fe₃O₄ and Al₂O₃ nanoparticles, finding a lower degree of asphaltene aggregation when using the latter. The asphaltene precipitation was reduced by approximately 83% [122,209].

Accounting for the mentioned advances in the nanoparticle-based treatments for the inhibition of asphaltene precipitation, an approximation towards technique massification was carried out by applying an Al₂O₃-based nanofluid for the inhibition of asphaltene-associated damage in the Cupiagua Sur field [212]. After the nanofluid injection, daily production monitoring was carried out (Figure 10), where it was observed that the process not only promoted formation damage removal, but also encouraged reserve incorporation by affecting the production curve declination. The treatment had a perdurability of more than 1 year, and an increment of 150,000 bbl in the cumulative oil production was observed during the first 8 months after the intervention.

4. Outlook and New Technologies

As explored extensively in this article, asphaltenes are complex organic compounds found in crude oil that can form insoluble aggregates and cause production problems, such as clogging reservoir pores, reducing reservoir permeability, and decreasing oil flow. The hydrocarbon industry urgently needs to control this class of compounds. Therefore, the four nanotechnology solutions presented are considered new trends for asphaltene control: dispersed nanoparticles, nanosensors, nanostructured coatings, and nanostructured systems for solvent release control (See Figure 11). The characteristics of each of these technologies are explained below.

Dispersant nanoparticles: Nanoparticles are being developed to disperse asphaltene aggregates and prevent their precipitation. These nanoparticles can be chemically modified to selectively interact with asphaltenes and prevent their deposition on pipelines and production equipment. These nanoparticles are designed to have nanometer-sized dimensions and are intended to interact with asphaltenes, inhibiting their aggregation and deposition. They can be synthesized using various techniques, and their performance is based on specific surface and chemical properties [214,215].

For the optimal production of these nanoparticles, metal oxides such as silica (SiO₂) and titanium oxide (TiO₂) or metal nanoparticles such as gold (Au) and silver (Ag) are utilized. Subsequently, synthesis techniques such as chemical precipitation, thermal decomposition, chemical reduction, or vapor phase synthesis are employed. Each method has advantages and is selected based on the material and desired nanoparticle characteristics. After the synthesis of the nanoparticles, surface modification can be performed to enhance their affinity and selectivity towards asphaltenes. This involves introducing specific functional groups onto the nanoparticle surface, enabling chemical interactions with the asphaltenes [216,217].

The performance of dispersant nanoparticles is attributed to various active correlations, particularly the electrostatic interaction between the nanoparticles and asphaltenes through different mechanisms. One possibility is through electrostatic interaction, where the charged nanoparticles attract and disperse oppositely charged asphaltene aggregates. Another mechanism is steric interaction, where the nanoparticles adsorb onto the surface of the asphaltenes, preventing their aggregation and deposition. Additionally, the stabilization of dispersant nanoparticles involves controlling the surface charge of the nanoparticles, selecting appropriate functional groups, and optimizing dispersion conditions. These factors contribute to enhancing the dispersing properties and effectiveness of the nanoparticles [218,219].

On the other hand, nanosensors offer the potential for real-time monitoring of asphaltene concentration. These devices provide accurate information about the presence and quantity of asphaltenes in crude oil, enabling proactive measures to be taken before production issues arise. Nanosensors are nanostructured devices with dimensions on the nanometer scale (1 nanometer = 10⁻⁹ m). They can be constructed using various structures such as nanowires, functionalized nanoparticles, carbon nanotubes, or quantum dots. These sensors are designed with selective receptors that exhibit an affinity for asphaltenes. These receptors can be specific molecules or functional groups that interact with asphaltenes, producing a detectable signal for quantification and analysis [220,221].

In line with the above, the interaction with asphaltenes occurs as follows: When nanosensors come into contact with asphaltenes present in crude oil, a specific interaction takes place between the receptors of the nanosensor and the asphaltenes. This interaction can be of a chemical, electrical, or magnetic nature, depending on the design of the nanosensor. Initially, the nanosensor generates a signal. The interaction between the nanosensor and the asphaltenes leads to a change in the physical or chemical properties of the nanosensor, resulting in the generation of a detectable signal. This signal can be electrical, optical, or magnetic and is utilized for quantifying the concentration of asphaltenes [222,223].

Subsequently, the signal generated by the nanosensor is detected and analyzed using specific techniques such as atomic force microscopy, fluorescence spectroscopy, magnetic resonance spectroscopy, or electrochemical techniques. These techniques enable the measurement and quantification of the signal generated by the nanosensor, providing information about the presence and concentration of asphaltenes in real time. Finally, nanosensors can be integrated into online monitoring systems, facilitating continuous tracking of asphaltenes during oil production. The data collected by the nanosensors are utilized for making operational decisions and implementing preventive strategies, such as the addition of dispersants or the implementation of cleanup and maintenance measures [224,225].

Among other emerging technologies, nanostructured coatings are applied to the internal surfaces of production equipment to mitigate the adhesion and accumulation of asphaltenes. These coatings possess unique surface properties that impede the adhesion of asphaltenes and facilitate their removal during maintenance operations. Various materials can be utilized for these coatings, including polymers, ceramics, metal oxides, and nanocomposites. These materials are designed to have a nanoscale structure, which can consist of nanometer-thin layers, embedded nanoparticles, or porous structures. These nanostructural features enhance the effectiveness of the coatings in preventing the adhesion and build-up of asphaltenes [226,227].

It is important to note that nanostructured coatings exhibit unique surface properties, including low surface energy, hydrophobicity or hydrophilicity, and non-stick characteristics. These properties effectively hinder the adhesion of asphaltenes to the coated surfaces and facilitate their subsequent removal. Nanostructured coatings prevent asphaltene adhesion by creating a physical or chemical barrier on the surface. These barriers prevent direct contact between the asphaltenes and the substrate, thereby reducing their ability to adhere and form deposits. Moreover, they aid in the easy removal of asphaltenes during maintenance activities. The special surface properties of these coatings enable easier dislodging or dissolution of the asphaltenes, minimizing the need for extensive mechanical or chemical cleaning procedures. Additionally, these coatings are designed to be durable and stable under the harsh conditions of oil production, withstanding corrosive fluids, high pressures, and elevated temperatures commonly encountered in production systems [228,229,230,231,232].

Finally, the controlled solvent release is another area of investigation in the field of nanotechnology for asphaltene control. Nanostructured systems are being developed that can release specific solvents at the appropriate time and location to effectively dissolve asphaltenes. This approach aims to prevent the formation of asphaltene deposits and maintain the permeability of reservoirs and production systems [233,234].

These nanostructured systems can take various forms, such as nanoparticles, nanocapsules, or nanostructured matrices. They are designed to contain a specific solvent with the ability to effectively dissolve asphaltenes. The selection of the solvent depends on the chemical composition of the asphaltenes and the solvent’s properties of interaction with them [235,236].

These systems can release solvents in different ways. For instance, they may undergo controlled breakdown of the nanostructures, allowing the solvent to be released gradually. Alternatively, they can release solvents through pores in the nanostructured systems or respond to external stimuli, such as changes in pH, temperature, or concentration. The design ensures that solvents are released at the right place and time [237,238].

For example, these systems can be employed in oil wells to dissolve asphaltenes present in reservoirs, or in production and refining equipment to prevent the formation of deposits. By delivering solvents directly to the problematic areas, these nanostructured systems offer targeted and efficient asphaltene control [237,238,239].

5. Final Considerations

Asphaltene precipitation is widely studied in the oil industry. Nonetheless, its prediction and control are still challenging. From the experiences acquired observing the fluid in porous media and production lines, it can be concluded that the influence of pressure is the most important factor for precipitation and deposition of the asphalt fraction. However, recent studies show that studying the effect of pressure on asphaltene destabilization by experimental means is challenging, mainly because it requires more time and larger sample amounts. On the other hand, non-pressure tests are relatively simple, although less representative. Whereas several techniques have improved the asphaltene precipitation prediction under pressurized conditions, most of them lack in coupling this sensitivity improvement to other variables.

Although the uncertainty in the prediction of the behavior of asphaltenes hinders their control, several techniques have emerged that provide more technical and economical alternatives for avoiding the problems related to their precipitation/deposition. Nanotechnology is presented in the petroleum industry as one of these novel alternatives for improving existing production mechanisms including the formation damage process associated with asphaltene destabilization. Due to the characteristics of these heavy fractions such as their high dipole moment, nanoparticulated systems can capture them through an adsorption phenomenon. The asphaltene adsorption principle has been widely exploited for inhibiting asphaltene precipitation in processes such as primary and secondary recovery as well as in CO₂ injection and deasphalting scenarios.

Nanoparticles have also been tested in real field conditions in which their application reliability was validated through parameters associated with well productivity enhancement such as skin decrease via formation damage inhibition, recovery increment, and production declination diminishing.

As recommendations for future research, we consider it appropriate to orient future research in nanotechnology for the control of asphaltenes on four fundamental axes: intelligent and responsive nanoparticles, self-assembling nanoparticles, nanoparticles with self-healing capacity, and nanoparticles with detection and monitoring capacity. The first group should be based on the study of multifunctional properties that combine different mechanisms of action for the control of asphaltenes. For example, nanoparticles could be developed that act as dispersants and as solvent-controlled release agents to dissolve asphaltenes.

Similarly, smart and responsive nanoparticles would respond to changes in pH, temperature, or asphaltene concentration. These nanoparticles could be activated under specific conditions to release solvents, dispersants, or other agents that control the formation of asphaltene deposits. In turn, self-assembling nanoparticles whose structures are ordered in the presence of asphaltenes could have specific surface properties to prevent the adhesion of asphaltenes and facilitate their removal.

Finally, nanoparticles with sensing and monitoring capabilities could act as sensors to measure asphaltene concentration and provide valuable information for operational decision making.