Asphaltene Deposition Control by Chemical Inhibitors: Theoretical and Practical Prospects

By Ali Ghamartale, Shokufe Afzali, Nima Rezaei and Sohrab Zendehboudi

Asphaltene Deposition Control by Chemical Inhibitors: Theoretical and Practical Prospects is the most advanced reference focused on chemical dispersants and inhibitors from both an experimental and modeling viewpoint. Adequate knowledge of the effective parameters in each treatment method, interactions, mechanisms and economic viewpoints involved in asphaltene treatment are crucial for future development, recovery forecast, and reserve prediction, hence this reference delivers on all these aspects. Sections cover the environmental impacts of asphaltene deposition, prevention methods, and experimental methods, both static and dynamic, to test the effectiveness of inhibitors on restricting asphaltene deposition.

Rounding out with modeling methods used to simulate asphaltene-inhibitor interactions and a workflow to select suitable inhibitors by technical, economic and environmental considerations, this book will give today’s engineers and researchers the right tool to mitigate formation damage in a sustainably responsible way.

• Focuses on inhibitors, mitigators and the interplay between the asphaltene-inhibitors
• Helps readers learn from experimental models and replicate treatments with screening workflows
• Includes case studies that help readers make sustainable and economically-sound decisions on treatments

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 21, 2021
• ISBN: 9780323910453

Ali Ghamartale

Ali Ghamartale is currently a Ph.D. Candidate at the Department of Process Engineering, Memorial University of Newfoundland and Labrador, Canada. He earned a PhD in oil and gas engineering from the Memorial University of Newfoundland, a M.Sc. and B.Sc., both in reservoir engineering from Shiraz University. Ali was previously a technician at PetroAzma Company and programmer in MATLAB and has earned one patent.

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Chapter One: Asphaltene and asphaltene precipitation/deposition

Abstract

Asphaltenes are polyaromatic and heavy compounds that are soluble and stable in the original conditions of oil reservoirs. Any condition disturbance can result in asphaltene aggregation, precipitation and deposition. This chapter provides general information and knowledge about asphaltene, causes for asphaltene aggregation, and the consequences of asphaltene precipitation and deposition. The typical tools and tests for evaluation of asphaltene behavior as well as asphaltene aggregation mechanisms are briefly described. Then, the field conditions of asphaltene depositions, and corresponding technical and economic difficulties are discussed. A part of this chapter is also dedicated to introduction of the modelling methods for simulation of asphaltene precipitation and deposition.

Keywords

Asphaltene characteristics; Asphaltene structure; Asphaltene aggregation; Precipitation and deposition modeling; Technical challenges; Economical and environmental concerns; Benefits and drawbacks of asphaltene

1.1: Introduction

Asphaltenes are the heaviest, and the most polar among the crude oil constituents [1]. They are defined by the solubility criteria such as species that are soluble in aromatic solvents (e.g., toluene, benzene, or pyridine) but insoluble in alkanes (e.g., n-pentane or n-heptane) [2, 3]. The asphaltenes have over 10⁵ different types of molecular structures [4]. Even if asphaltenes of different oils have a comparable solubility, their chemical properties can be completely different [5]. Reservoirs with a lighter crude oil are being depleted faster than heavy oil and bitumen hydrocarbon resources, and the use of heavy oil and bitumen are becoming more common in refineries. Accordingly, the asphaltene problems have become more pronounced as the mentioned unconventional resources provide feedstock to the industrial refining systems [6, 7]. The precipitation and deposition of asphaltene in the production and transportation of crude oil cause two main problems: formation damage, and plugging of wellbore and production equipment [8, 9]. The complications from asphaltene stability and the necessity to support oil banks have remarkable harmful side effects on the whole production chain, such as:

•Production impairment by altering the reservoir wettability and pore plugging [10–13].

•Depositions of the asphaltene in the well [14] and refining facilities [15].

•Arterial clogging of the destabilized asphaltene particles in the pipelines and wellbores [16, 17], leading to the formation of water and oil emulsions [2].

•Contamination and corrosion of production facilities and catalysts [7, 15, 18, 19].

•Deactivation of catalyst in hydrocarbon processing [20].

•Creation of coke [15, 21–23].

Asphaltene conservation and asphaltene deposition rejection are considered as approaches that overcome the economic, environmental, and technical problems due to asphaltene flow-assurance issues [2]. In the conservation approach, the asphaltene content of the crude oil is inhibited from precipitating by the addition of solubilization agents [24–29], a dilutant [16, 30], or stabilized oils [31]. In the asphaltene rejection approach, the inhibition of asphaltene deposition is conducted by solvent deasphalting [7, 26, 32]; solvent deasphalting is performed by emulsion extraction [33, 34], ultrafiltration [35], fluid carbon rejection [36], and selective oxidation of heteroatom-containing species followed by their selective separation [37]. Various experimental methods are used to detect the asphaltene deposition onset at reservoir conditions. These methods include a gravimetric technique, acoustic resonance technology (ART), light scattering technique (LST), high-pressure microscopy (HPM), high-pressure and high-temperature (HPHT) filtration, and quartz crystal resonance (QCR). Some of these techniques, such as LST and HPM, become industry standards. Nevertheless, considering the high costs associated with these methods, most of the current research studies are conducted at ambient pressure, using dead oil, and thermodynamic models are implemented to evaluate the asphaltene behavior at reservoir conditions [38].

A comprehensive thermodynamic knowledge of asphaltene precipitation and deposition is needed as the first step in planning processes to reduce the asphaltene problems. Thermodynamic principles govern the equilibrium distribution of the acting components in different phases. For crude oil systems, the thermodynamic studies can predict if the asphaltenes are stable or instable (including a second solid phase that can be potentially deposited onto surface facilities) [38]. The deposition of asphaltene in pipeline or wellbore can substantially reduce the productivity of an oilfield; in extreme cases, the pipeline or well can be completely plugged. Currently, huge investments are annually carried out to prevent, mitigate, or dissolve the asphaltene precipitates and deposits. Extensive research studies are also conducted to develop cost-effective inhibition or mitigation approaches of asphaltene deposition. Various strategies are devised to prevent asphaltene deposition in the wellbore, including asphaltene chemical inhibitors, controlling the operating conditions (pressure, temperature, and flow rate), and providing an internal surface coating. There are different asphaltene inhibitors, designed to stabilize asphaltenes by reducing the aggregate size, but their performances are related to oils and asphaltene characteristics. Despite all efforts to understand the asphaltenes and control asphaltenes’ adverse effects, asphaltene deposition remains a challenge for the industry and more research and engineering activities are needed to find comprehensive and cost-effective treatment ways.

1.2: Asphaltenes: Properties, structures, and thermodynamic behaviors

The term Asphaltene was initially used by J.B. Boussingault in 1837 for the residue compounds of the bitumen distillations, which are insoluble in alcohol and soluble in turpentine [39]. This initial definition is similar in some aspects to the current asphaltene description, which argues that asphaltene has insoluble components in n-alkanes, such as n-heptane, but is soluble in toluene [16]. Asphaltenes are a branch of hydrocarbon, which have a dark color, containing heavy compounds with an average density of 1.2 g/cm³, and they are fragile and infusible solids [40]. They do not feature a unique specific melting point and leave a carbonaceous residue while being decomposed on heating. Asphaltenes significantly increase the viscosity of crude oil due to self-association [41]; the high viscosity impedes oil transportation and reduces the yield of petroleum distillates due to high resistance against cracking [42]. Asphaltenes are undesirable from an oil waste management perspective due to having heavy metals and consequently the lack of biodegradability [43]. Nevertheless, asphaltenes have applications in industry [44, 45], such as surface protective coatings (in road construction) because of their adhesion properties, bio-inertness, and rheological toughness [42]. They also incorporate in drilling fluids to control fluid loss.

Elemental analysis of asphaltenes shows that they are mostly composed of carbon and hydrogen with an average C/H ratio of 1–1.2 [16]. In addition to carbon and hydrogen, asphaltenes contain heteroatoms such as nickel, sulfur, nitrogen, oxygen, and vanadium. In the literature, two general types of chemical structures are proposed for the asphaltene molecules. The first one is referred to continental or island form [46, 47] that is formed by a single polycyclic aromatic hydrocarbon (PAH), being attached to pendant alkyl groups. The second structure is archipelago form, wherein multiple PAHs (usually two) are linked together by alkyl groups [48, 49] (Fig. 1.1). These two structures are distinguishable unless the continental structure asphaltenes aggregate and form a structure similar to the archipelago structure; this might be a reason for the prevalence of the archipelago molecular structure idea for asphaltene structure in the open sources [50,51]. Recently, the results of asphaltene disaggregation along with the laser adsorption and atomic force microscopic tests have confirmed that the continental structure is the dominant structure for asphaltene [52,53].

Fig. 1.1

Fig. 1.1 Common chemical structure types of asphaltenes: (A) continental/island model [47] and (B) archipelago model [49].

Scanning tunneling microscopy (STM) provides a unique analytical methodology in imaging single adsorbates at an atomic scale. The asphaltene molecular structure can be characterized using STM [54,55]. In recent developments, atomic force microscopy (AFM) enables the visualization of the atomic structure of individual molecules on real surfaces [56]. This technique is also applied to analyze the bond order [57], obtain the molecular structure of natural compounds [58,59] and graphene nanoribbons [60], and to identify chemical synthesis products [61] and surface reactions [62,63]. In addition, the molecular orbitals can be mapped using STM and ultra-thin insulating films as a substrate [64,65].

The characterization of asphaltenes based on their solubility is more challenging than their chemical classes compared to lighter components of hydrocarbons. The lighter fractions of hydrocarbon, such as saturates and aromatics, are easily categorized according to their chemical structures while providing such distinctions for the heavier fraction of crude oil is difficult. In other words, the lighter fractions of crude oil have much simpler chemical structures and better economic perspectives in the industry; also, they are easier to measure and analyze than the heavy components [16]. For instance, gas chromatography characterizes light hydrocarbon components (< C36), while more complicated methods are required to extract detailed data about the structures of heavy components. Some techniques are extensively applied in the examination of asphaltene and other heavy compounds, such as mass spectrometry, electron microscopy, nuclear magnetic resonance, small-angle neutron, X-ray scattering, ultrasonic spectroscopy, dynamic light scattering, fluorescence depolarization, vapor pressure osmometry, and gel permeation chromatography. Table 1.1 summarizes the recent works on asphaltene characterization [20,66].

Table 1.1

a OST: Optical scattering technique. FD: Fluorescence depolarization. FES: Fluorescence emission spectroscopy. ¹³C NMR: 13-Carbon Nuclear magnetic resonance. IR: Infrared. ST: Surface tension. EC: Elemental composition. VPO: Vapor pressure osmometry. GPC: Gel permeation chromatography. IT: Interfacial tension. ESR: Electron spin resonance. GC/MS: Gas chromatography/mass spectrometry. FTIR: Fourier transformed infrared. SEC: Size-exclusion chromatography. HTGC: High-temperature gas chromatography. XRD: X-ray diffraction. py/GC/MS: Pyrolysis gas chromatography/mass spectrometry. MALDI TOF: Matrix-assisted laser desorption/ionization time of flight. PDMS: Plasma desorption mass spectrometry. LDMS: Laser desorption mass spectrometry. HRTEM: High-resolution transmission electron microscopy. SANS: Small-angle neutron scattering. HRMS: High-resolution mass spectrometry. TLC: Thin-layer chromatography. FS: Fluorescence spectroscopy. HPLC: High-performance liquid chromatography.

Asphaltenes have the highest fraction of metals among other constituents of crude oils [89]. The metals existing in the asphaltene structure are in the form of metalloporphyrins, and heterocyclic macrocycle organic compounds that interconnect with internal ligands and form complex structures. The metal content also exists at the defect center of the aromatic structures [89]. Two conventional methods for analyzing the metal content of the asphaltenes are atomic adsorption and mass spectroscopy. In both methods, the asphaltene sample should be free from organic components. To eliminate the organic part, a heat treatment technique such as calcination is implemented, following an acidic digestion process, using concentrated (70 wt%) nitric acid [38]. Fourier transform infrared (FTIR) spectroscopy is also used to determine the asphaltene chemical structure; this analytical method identifies the organic and inorganic compounds according to the absorption of the mid-IR radiation in the wavelength range of 2.5 μm (4000 cm− 1) and 25 μm (400 cm− 1) of the light spectrum [38]. As the asphaltene sample is exposed to the IR radiation, different bonds of molecules absorb a specific radiation wavelength, shifting their dipole moment and also the state of excited vibration [38]. The absorbed light wavelength of each bond is controlled by the energy difference of the base and excited vibrational state, which links the absorbed light wavelength to the molecular structure of the compound. The output of the FTIR test is a plot of radiation absorbance (or transmittance) against the light wavenumber (cm− 1) that is the reciprocal of the wavelength [90]. Table 1.2 provides the main adsorption peaks for the common components in crude oil.

Table 1.2

Nuclear magnetic resonance (NMR) spectroscopy also characterizes the chemical structure of the asphaltene based on the atoms’ nuclear spin. A magnetic field is created as a result of the nuclear spin in the presence of an external magnetic field [38]. The energy applied for the magnetic field is related to the energy difference between the aligned and misaligned nucleuses, and electromagnetic signals are emitted at specific frequencies according to the energy difference. The emitted signals are collected and reported on a graph of intensity versus frequency (or chemical shift, in ppm). The most conventionally analyzed nuclei using NMR include 1H, 13C, 15 N, and 31P; among these tests, 1H and 13C are commonly used to characterize the asphaltenes. The oil industry employs the NMR method to compute the aliphatic and aromatic carbon fractions as well as the alkyl-substituted and unsubstituted aromatic carbons in a given asphaltene sample [29,90,91].

1.3: Asphaltene precipitation/deposition: Description, mechanisms, and field conditions

Asphaltene compounds are initially stable in crude oil, but they flocculate, precipitate, and deposit due to temperature, pressure, and composition changes in the system [92]. Flocculation is the asphaltene molecules’ agglomeration. Precipitation is a solid-phase formation in the liquid phase (while Hirschberg et al. [93] defined the asphaltene precipitation as a liquid-liquid separation), and deposition occurs when the solid particulates are placed onto a surface