UNIVERSITY OF CALGARY. Determining the Onset of Asphaltene Flocculation in Solvent-Diluted Bitumen Systems. Maryam Sattari A THESIS

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1 UNIVERSITY OF CALGARY Determining the Onset of Asphaltene Flocculation in Solvent-Diluted Bitumen Systems by Maryam Sattari A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY GRADUATE PROGRAM IN CHEMICAL AND PETROLEUM ENGINEERING CALGARY, ALBERTA JANUARY, 2016 Maryam Sattari 2016

2 Abstract The past several decades have seen conventional oil resources reaching plateau which triggered a shift in oil exploration towards heavier crudes such as heavy oil and bitumen. Light hydrocarbons, mainly consisting of light paraffins, are widely used as diluents in the production and upgrading of heavy crudes. The addition of a diluent to heavy oil or bitumen alters the chemical forces within the mixture initiating the aggregation of asphaltene particles. Temperature and pressure may also vary during reservoir depletion operations. A crucial and challenging side effect of such thermodynamic changes is solid deposition, which can significantly diminish production, damage the reservoir, or even necessitate early reservoir abandonment. Solid deposition in reservoirs, production wells, and top-side facilities is mainly composed of asphaltenic materials, sand, clay, and corroded metals. Asphaltene is the deposition material of interest in this study. In order to avoid asphaltene precipitation or develop a remedial action plan for the associated problems, it is essential to understand the fundamental variables driving asphaltene precipitation in oil. The first step is to determine the conditions under which asphaltene dispersed in crude oil flocculates out to form an asphaltene-rich solid phase. The current study was designed to locate the onset of asphaltene flocculation with respect to changes in the thermodynamic conditions. Extensive experimental work and mathematical modelling were conducted to predict the incompatibility region of solvent-diluted bitumens. The precipitants in this study, n-pentane, n-hexane, and n- heptane, were chosen from the most common hydrocarbon cuts used in solvent- ii

3 assisted heavy oil recovery methods in Alberta. Two Alberta bitumens were considered in this study. The effects of temperature, pressure, and composition on the onset of flocculation were investigated for various systems of solvent-diluted bitumens. Experimental values for the viscosity and density of the n-alkane-bitumen mixtures produced a pattern from which the onset of asphaltene flocculation could be accurately determined. In the modelling section, a CPA-EOS model was successfully tuned to predict asphaltene precipitation from solvent-diluted bitumens. iii

4 Acknowledgments A distant tour begins with one step This dissertation is a result of a very good chapter of my life. Now that I am closing this chapter, I look back through all those years, and I find out that this would not have been a success without the assistance and support of many good people from whom all I am deeply thankful. I would like to offer my sincere appreciations and gratitude to my supervisor Dr. Jalal Abedi whose insight, support, trust, and encouragement made all this possible. I would like to deeply thank my co-supervisor, Dr. Anil Mehrotra, for his support, knowledge, and constructive feedbacks. During the course of the experimental work, I benefitted a lot from Dr. Brij Maini s immense knowledge, broad vision, and great ideas. I learnt a lot from his kind and comforting personality. I thank him greatly. I sincerely thank Dr. Hassan Hassanzadeh for his helpful discussions, smart ideas, invaluable friendship and kind support. I would like to express my sincerest gratefulness to my company that fully sponsored me through the whole course of this study. Statoil ASA Norway and Statoil Canada Ltd. generously provided me with all kinds of support. I am greatly thankful to all the wonderful colleagues whose support and understanding made this undertaking possible. In particular, I would like to express my deepest thankfulness to Erik Skjetne, Lars I. Berge, Grethe M. Ledsaak, Karl J. Hersvik and Rolf H.Utseth. iv

5 I wish to express my appreciation for the financial support of Natural Sciences and Engineering Research Council of Canada Industrial Research Chair (NSERC-IRC) program and all member companies of SHARP Research Consortium: BP Canada Energy Group ULC, Brion Energy, Cenovus Energy, Computer Modelling Group Ltd., ConocoPhillips Canada, Devon Canada Co, Foundation CMG, Husky Energy, Imperial Oil Limited, Japan Canada Oil Sands Limited, Nexen Energy ULC, N-Solv Co., PennWest Energy, Statoil Canada Ltd., Suncor Energy, and Total E&P Canada. The support of the Department of Chemical and Petroleum Engineering, Schulich School of Engineering, and the machine shop at the University of Calgary is also acknowledged. I would like to deeply thank Mr. Dan F. Marentette for that I learnt a lot from his technical skills and lab experience. I would like to thank my dear friends and colleagues in our research group at the University of Calgary who contributed to my research by providing their valuable comments, critiques, questions, discussions and ideas over official meetings and unofficial friendly tea times. In particular, I would like to sincerely thank Mohsen Zirrahi for his excellent technical skills, genuine ideas and friendly support, and Amir Ahmad Shirazimanesh for the helpful scientific discussions we had. Last but not least, I would like to offer my deepest love, appreciations and gratitude to my beloved family; to my lovely mother, the strong woman who has always been my role model, who lovingly taught me the very first words to talk, and to my lovely father who aspiringly taught me how to read, write and keep eyes high. The continuous love and moral support of Hoda, my dearest sister, has always been v

6 extremely precious to me. I would also like to sincerely thank my great uncle Dr. Mohsen Kiani, who lit this road for me. I would like to dearly thank my lovely Farshad, my better half and friend in crime, for his consistent love, true friendship, encouragements, support, kindness and understanding. vi

7 To My Beloved Parents, Sister & Husband vii

8 Table of Contents Abstract... ii Acknowledgments... iv Table of Contents... viii List of Tables... x List of Figures... xi Nomenclature... xiv CHAPTER 1 : INTRODUCTION Objectives of the Present Study Organization of Thesis... 5 CHAPTER 2 : LITERATURE REVIEW Oil Characterization and Chemistry (SARA) Asphaltenes Asphaltene Self-Association Asphaltene Phase Formation Precipitation Flocculation Deposition The Onset of Asphaltene Precipitation and Flocculation Detection Summary CHAPTER 3 : EXPERIMENTAL METHODS AND PREPARATIONS Material Experimental Apparatus Temperature-Controlled Oven Viscometer Densitometer Quizix Pump Calibration Densitometer Calibration Viscometer Calibration Leak Test Validation Tests (Quality Control tests) n-decane-methanol System n-butanol-water System CHAPTER 4 : EXPERIMENTAL RESULTS viii

9 4.1 Bitumen Characterization Asphaltene Fraction Measurement SIMDIST Analysis ( Compositional Analysis) Density Measurements of the Dead Bitumens Viscosity Measurements of the Dead Bitumens Asphaltene Flocculation Onset Experimental studies Bitumen Sample Preparation Experimental Procedure n-pentane- Bitumen Systems n-hexane- Bitumen Systems n-heptane- Bitumen Systems Effect of Temperature Analogy between Viscosity and Density Data Trends CHAPTER 5 MODELLING Literature Review on Modelling Cubic Plus Association Equations of State (CPA-EOS) Thermodynamic Approach Model Implementation: Results and Discussion Conclusions CHAPTER 6 :CONCLUSIONS AND RECOMMENDATIONS Dissertation Conclusions Recommendations for Future Work ix

10 List of Tables Table 3-1-Bitumen properties Table 3-2- Solvent properties at atmospheric pressure (NIST) Table 3-3-Viscosity range of the viscometer Table 3-4-Quizix QX 6000 pump specifications Table 3-5-Coefficients calculated for the densitometer calibration equation Table 3-6-Density measurements and reference data(nist) Table 3-7-Viscosity measurements and reference data Table 3-8 Leak test conditions Table 3-9-Measured and literature density data of n-decane-methanol system Table 3-10-Measured and literature density data for the n-butanol-water system Table 4-1-Oil sample properties Table 4-2-Measured asphaltene and maltene contents of bitumen samples used in this study Table 4-3-Compositional analysis of bitumen samples Table 4-4-Measured density data for ATB and MacKay River bitumen samples Table 4-5-Measured viscosity data for ATB and MacKay River bitumen samples Table 4-6-Experimental results for nc5-atb bitumen systems at different temperatures and pressures Table 4-7-Experimental results for nc6-atb bitumen systems at different temperatures and pressures Table 4-8-Experimental results for nc7-atb bitumen systems at different temperatures and pressures Table 5-1-Riazi and Al-Sahhaf equation constants(riazi & Al-Sahhaf, 1996) Table 5-2-Physical properties and model parameters Table 5-3-Adjusted ε/k B for the n-pentane, n-hexane and n-heptane systems Table 5-4-Model outputs for different Alberta bitumen types and their properties. 131 x

11 List of Figures Figure 2-1-Molecular structure of lighter paraffin hydrocarbons in the saturates fraction of oil (generated with ChemSketch software) Figure 2-2- Molecular structure of lighter aromatic hydrocarbons in the aromatics fraction of oil... 9 Figure 2-3-Resins molecular structure models of (A) Athabasca heavy oil resin and (B) Athabasca tar sand resin (Suzuki et al., 1982) Figure 2-4-Asphaltene molecular structure, adopted from (Carbognani, 1992) Figure 2-5-The molecular weight distribution of asphaltene extracted from a petroleum sample by three normal alkanes measured by Gel Permeation Chromatography (adopted from Mansoori et al. 2007b (Mansoori et al., 2007)) Figure 2-6- Aggregation and flocculation of asphaltene molecule (sketched based on concepts from (Carbognani, 1992; Mansoori, 2009; Suzuki et al., 1982)) Figure 3-1-The schematic diagram of the modified apparatus designed in-house to detect the asphaltene flocculation onset Figure 3-2-Temperature-controlled conventional oven Figure 3-3-Inside the asphaltene onset experimental apparatus Figure 3-4-Cambridge ViscoPro 2000 Viscometer Figure 3-5-Cutaway view of the viscometer's sensor tip installed in a pipeline Figure 3-6-AntonPaar Density measuring cell (mpds2000 V3) and read-out unit Figure 3-7- Schematics of Quizix pump QX Figure 3-8-Reference vs. measured density data for Nitrogen and Water systems Figure 3-9-n-Decane-Methanol phase change Figure n-butanol-water phase change Figure 4-1-Measured density data for ATB bitumen at two lower and upper bands of experimental pressures Figure 4-2-Measured density data for MacKay River bitumen at two lower and upper bands of experimental pressures Figure 4-3-Measured Viscosity data for ATB bitumen at two lower and upper bands of experimental pressures Figure 4-4-Measured viscosity data for MacKay River bitumen at two lower and upper bands of experimental pressures Figure 4-5-Viscosity versus n-pentane concentration for different n-pentane-atb bitumen mixtures at 40 C and 2.76 MPa xi

12 Figure 4-6-Viscosity versus n-pentane concentration for different n-pentane-atb bitumen mixtures at 25, 40, 60 and 80 C and 2.76 MPa Figure 4-7-Viscosity deviation versus n-pentane mass fraction at 25, 40, 60 and 80 C and 2.76 MPa Figure 4-8-Density versus n-pentane concentration in n-pentane-atb bitumen mixtures at 40 C and 2.76 MPa Figure 4-9-Density versus n-pentane concentration in n-pentane-atb bitumen mixtures at 25, 40, 60 and 80 C and 2.76MPa Figure 4-10-Density deviation versus n-pentane mass fraction at 25, 40, 60 and 80 C and 2.76 MPa Figure 4-11-Viscosity versus n-pentane concentration for different n-hexane-atb bitumen mixtures at 40 C and 2.76 MPa Figure 4-12-Viscosity versus n-pentane concentration for different n-hexane-atb bitumen mixtures at 40, 60, and 80 C and 2.76 MPa Figure 4-13-Viscosity deviation versus n-hexane mass fraction at 40, 60, and 80 C and 2.76 MPa Figure 4-14-Density versus n-hexane concentration in n-hexane-atb bitumen mixtures at 40 C and 2.76 MPa Figure 4-15-Density versus n- hexane concentration in n-hexane-atb bitumen mixtures at 40, 60, and 80 C and 2.76 MPa Figure 4-16-Density deviation versus n-hexane mass fraction at 40, 60 and 80 C and 2.76 MPa Figure 4-17-Viscosity versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40 C and 2.76 MPa Figure 4-18-Viscosity versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure Viscosity deviation versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure 4-20-Density versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40 C and 2.76 MPa Figure 4-21-Density versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure 4-22-Density deviation versus n-hexane concentration in n-heptane-atb bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure 4-23-Effect of temperature on the viscosity of n-pentane-bitumen mixtures at 2.76 MPa per n-pentane mass percent Figure 4-24-Effect of temperature on the viscosity of n-pentane-bitumen mixtures at 2.76 MPa per n-pentane mass percent xii

13 Figure 4-25-Effect of temperature on the viscosity of n-heptane-bitumen mixtures at 2.76 MPa per n-heptane mass percent Figure 4-26-Effect of temperature on the density of n-heptane-bitumen mixtures at 2.76 MPa per n-heptane mass percent Figure 4-27-Analogy between the viscosity and density trends in n-pentane-bitumen mixtures at 25, 40, 60 and 80 C and 2.76 MPa Figure 4-28-Analogy between the viscosity and density trends in n-hexane-bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure 4-29-Analogy between the viscosity and density trends in n-heptane-bitumen mixtures at 40, 60 and 80 C and 2.76 MPa Figure 5-1-Asphaltene-maltene cross-association energy over Boltzmann constant versus temperature in n-pentane-bitumen systems at 2.76 MPa Figure 5-2-Asphaltene-maltene cross-association energy over Boltzmann constant versus temperature in n-alkane-bitumen systems at 2.76 MPa Figure 5-3-Adjusted cross-association energy over Boltzmann constant versus the molecular weight of the n-alkane for a range of temperatures at the pressure of 2.76 MPa Figure 5-4-Solvent concentration at the asphaltene flocculation onset in n-pentanebitumen systems in a range of temperature at 2.76MPa Figure 5-5-Solvent concentration at the asphaltene flocculation onset in n-hexanebitumen systems in a range of temperature at 2.76MPa Figure 5-6-Solvent concentration at the asphaltene flocculation onset in n-heptanebitumen systems in a range of temperature at 2.76MPa Figure 5-7-Effect of solvent carbon number on the n-alkane-bitumen mixture composition at onset in a range of temperature at 2.76 MPa Figure 5-8-Adjusted cross-association energy over Boltzmann constant versus the molecular weight of different bitumen types from Alberta at T=23 C and 0.1 MPa Figure 5-9-Predicted and experimental onset points for different bitumen types in mixtures with n-heptane at 23 C and 0.1 MPa plotted versus the bitumen molecular weight xiii

14 Nomenclature Symbol C p f H k B k ij mass% N P P c R S T T b T c V x Z Definition Heat capacity Fugacity Enthalpy Boltzmann constant Binary interaction coefficient Mass percent Number of association sites on a molecule Pressure Critical pressure Universal gas constant Sulphur atom Temperature Normal boiling point Critical temperature Volume Mole fraction Compressibility factor Units Definition C centigrade mpa.s MPa psi milli-pascal second mega pascals pound force per cubic inches xiv

15 Scripts a assoc. Ai Bj c fus. Definition Asphaltene Association Association sites of molecule i Association sites of molecule j Critical Fusion i, j i th and j th component of a mixture l m n phys. s v Liquid Melting point Normal Physical Solid Vapour Greek symbols β Δ ε θ γ μ ɸ ω Definition Association volume Association strength Association energy General property of pseudo-component Activity coefficient Viscosity Density Fugacity coefficient Acentric factor xv

16 Abbreviation ARD% CMC CPA EOS MW PR PVT SAGD SARA SIMDIST SRK VPO Definition Absolute Relative Deviation Percent Critical Micelle Concentration Cubic Plus Association Equation of State Molecular Weight Peng-Robinson Pressure-Volume-Temperature Steam Assisted Gravity Drainage Saturate-Aromatics-Resins-Asphaltene Simulated Distillation Soave-Redlich-Kwong Vapor Pressure Osmometry xvi

17 CHAPTER 1 : INTRODUCTION Asphaltene precipitation and deposition is a common problem in the oil industry. During the past several decades, conventional oil resources have begun to plateau and oil exploration has shifted towards heavier crudes such as heavy oil and bitumen. Production and upgrading of heavy crudes usually involves the use of lighter hydrocarbons as diluents to reduce the viscosity of the crude. This viscosity reduction is crucial both in reservoirs and at surface facilities. In addition to changes to the composition of the production fluid, the operational temperature and pressure vary during depletion operations. All of these thermodynamic changes can result in operational challenges including solid deposition (Carbognani et al., 1999; Cosultchi et al., 2002; Leontaritis, 1996). Solid deposition is primarily composed of waxy organic and asphaltenic materials with sand, mud, clay and corroded metals. Over the course of reservoir production, such depositions can significantly reduce production and damage flow paths within the reservoir and wells. The resulting reduction in production may eventually lead to early abandonment of a reservoir and huge benefit loss. Asphaltene is the deposition material of interest in this study. Asphaltene is believed to exist in petroleum in partly dissolved and partly colloid and/or micelle forms (Wu & Prausnitz, 1998). Several definitions of asphaltene exist in the literature; one definition states that they are a fraction of petroleum that is insoluble in n-heptane but soluble in toluene. In general, asphaltenes are defined as the solid material precipitating from crude oil, 1

18 asphalt, or bitumen with the addition of a low-molecular-weight paraffin solvent (Andersen, 1994; Hammami et al., 1995). In this work, the heavy bitumen components that precipitate with the addition of n-heptane are considered asphaltenes. In order to avoid asphaltene precipitation or develop a remedial action plan for the associated problems, it is important to understand the fundamental variables of asphaltene precipitation in oil. The first step is to determine the conditions under which asphaltene dispersed in crude oil flocculates out to form an asphaltene-rich dense-liquid or solid phase. The point that asphaltene begins to separate from the crude, is called the onset of asphaltene. Knowledge of the onset conditions is extremely valuable, especially for bitumens containing large amounts of heavier hydrocarbons. Asphaltene macromolecules undergo a solid-liquid or a liquid-liquid phase change with changes in temperature, pressure, or oil composition. The presence of other fractions including aromatics and resins influences the shape of the asphaltene aggregates. Random asphaltene aggregates can form steric-colloids in the presence of excess amounts of resin (Mansoori, 1997). If the concentration of these micelles exceeds a threshold known as the critical micelle concentration, they may selfassemble into geometric shapes and form disk-like, cylindrical, and spherical aggregates (Priyanto et al., 2001b). The existence of various states of asphaltenes in crude oil is discussed in several publications (Mansoori, 1996b; Vazquez & Mansoori, 2000). 2

19 Detecting the onset of asphaltene flocculation is of particular interest because it represents the beginning of a potentially problematic region that could lead to many field problems caused by asphaltene deposition. Several experiments have been conducted to study the nature of asphaltenes and the effective parameters of asphaltene deposition. However, most of the reported data is limited to ambient temperature and pressure conditions and light oils. There is inadequate information on asphaltene flocculation and onset detection at elevated temperatures and pressures for heavy oils and bitumens. Different methods of detecting the onset of asphaltene flocculation are outlined in the literature. This study focuses on the viscometric method of onset detection because it is accurate and practical for darker crudes such as heavy oils and bitumens. A modification to the original viscometric method (Escobedo & Mansoori, 1995) was made in this research in order to confirm the location of the onset points. The experimental part of this study used the density and viscosity of solvent-dilutedbitumen to detect and confirm the concentration of the onset points. It was found that the asphaltene flocculation onset points for n-alkane-diluted bitumens could be detected by monitoring both the density and viscosity of diluted bitumen. The viscometric method, with the densitometry modification, is an accurate, quick, and inexpensive method. Another merit of this method is that additional valuable data such as density, viscosity and phase volume of the mixtures is obtained during the experiment. 3

20 1.1 Objectives of the Present Study The four main objectives of the present study are: 1- To experimentally examine the ability of the viscometric method to detect the onset of asphaltene flocculation in heavy crudes such as bitumens. 2- To test the hypothesis concerning the capability and accuracy of density measurements as a tool for locating the onset of asphaltene flocculation in bitumens. 3- To experimentally locate the onset of asphaltene flocculation in a variety of solvent-diluted bitumen mixtures. A number of experimental apparatuses were designed, assembled, and tested until the results from a number of dry-run experiments became repeatable. 4- To predict the formation of the asphaltene-rich phase in solvent-bitumen mixtures upon changes in the thermodynamic condition (temperature, pressure, and composition). In this section, a cubic-plus-association equation of state (CPA-EOS) was employed to model asphaltene precipitation from Alberta bitumen samples upon the addition of n-alkanes. The physical interactions were described by the Soave-Redlich-Kwong equation of state. In the association term, the interactions between bitumen pseudo-components were described based on perturbation theory. The model parameters were tuned based on the results of experiments in this research. Our modelling attempt showed that a tuned CPA-SRK combined with the solid solubility model is capable of predicting the onset of asphaltene flocculation. 4

21 1.2 Organization of Thesis This dissertation is organized into six chapters as described below: Chapter two presents a short review of petroleum chemistry and characterization. The physical structure of the four main fractions of oil is briefly reviewed with respect to SARA analysis. The chapter focuses on the physical structure and properties of asphaltene and the mechanisms of asphaltene precipitation, flocculation, and deposition from lessons learned in the literature. Asphaltene association and various experimental methods for detecting asphaltene precipitation in the literature are also reviewed. Chapter three describes the experimental procedure in the preparation phase including the materials and their properties, the designed apparatus, calibration of the measuring devices, and validation of the apparatus. In chapter four, the procedure and results of the asphaltene flocculation onset tests are described in detail. The physical interpretation behind the behaviour of the mixtures is discussed and the detected onset points are reported. Chapter five reviews the literature on current thermodynamic approaches to model asphaltene precipitation and phase behaviour. The implementation of thermodynamic modelling in this study is described in detail. The specifics of the bitumen characterization, pseudo-component property calculations, and model parameter tuning are also discussed. Chapter six summarizes the most notable findings of this research and provides recommendations for further work. 5

22 CHAPTER 2 : LITERATURE REVIEW This chapter presents a short review on petroleum chemistry and characterization. The physical structure of the four main fractions of oil is briefly reviewed with respect to SARA analysis. The main focus of the chapter is to understand the physical structure and properties of the asphaltene fraction and the mechanisms of asphaltene precipitation, flocculation, and deposition from the literature. Asphaltene association and the various experimental methods of detecting asphaltene precipitation in the literature are also reviewed. 2.1 Oil Characterization and Chemistry (SARA) One of the most common methods of characterizing heavy oils and bitumens is SARA analysis. In this solubility-based method, heavy oil or bitumen is divided into four fractions, saturates, aromatic, resins, and asphaltenes. An n-alkane precipitant such as n-heptane or n-pentane is added and the asphaltene fraction precipitates out of the crude (ASTM D6560 and ASTM D2007M). The asphaltene fraction is generally classified as a part of the heavy fraction of crude which is normally assumed to have little effect on the liquid-vapor phase behaviour of petroleum fluids. Asphaltenes, along with other heavy fraction components (e.g. waxes, diamondoids, and resins) are typically considered in solid-liquid phase behaviour, solid separation, and deposition. The general closed chemical formulae of 6

23 petroleum asphaltene is assumed to be C m H n N i O j S k,where C is Carbon, H is Hydrogen, N is Nitrogen, O is Oxygen and S is Sulphur (Mansoori, 2009). After separating the asphaltene fraction, the remaining materials, known as maltenes, undergo further fractionation. The maltenes pass through a separation column and are flushed with different solvents to separate the saturates, aromatics, and resins. The following is a brief review of the molecular structure and properties of each of the four fractions recovered during SARA. Saturates: The molecular structure of saturates is similar to alkanes in that it only contains single-bonds between carbon and hydrogen atoms (e.g. no double or triple bonds). All of the alkane content of crude is classified under the saturate fraction. Figure 2-1 depicts a number of lighter components in the saturate fraction. As the crude becomes heavier, the lighter saturate (alkanes) content normally decreases (Mansoori, 2009). 7

24 Figure 2-1-Molecular structure of lighter paraffin hydrocarbons in the saturates fraction of oil (generated with ChemSketch software). Aromatics: The aromatics fraction of oil generally consists of single or multiple benzyl rings (C 6 H 6 ) where the carbon atoms are connected to each other by aromatic double bonds. The simplest aromatic compound is benzene with a single aromatic ring. Double-ring and triple-ring aromatics also exist in oil. Depending on the source of 8

25 the crude, tetra-aromatics and penta-aromatics (consisting of 4 and 5 benzyl rings) may also be present. Aromatics are generally more polar than saturates with molecular weights ranging from 450 to 550 kg/kmol and densities ranging from 960 to 1003 kg/m 3 (Akbarzadeh, Rahimi, et al., 2005). Excess amounts of aromatics in a crude in contact with asphaltene may lead to micelle formation (Mansoori, 1996a). Figure 2-2 illustrates the molecular structure of some of the simpler aromatic materials in petroleum crudes. Benzene Toluene CH3 O-Xylene CH3 CH3 Naphtalene Anthracene Figure 2-2- Molecular structure of lighter aromatic hydrocarbons in the aromatics fraction of oil. 9

26 Resins: Resins consist primarily of non-volatile polar materials which unlike asphaltenes are soluble in n-alkanes but insoluble in liquid propane. The general closed chemical formulae for petroleum resins is shown as C m H n N i O j S k (i,j,k =0 or 1.0) where C is Carbon, H is Hydrogen, N is Nitrogen, O is Oxygen, and S is Sulphur (Mansoori, 2009). The molecular weight of resins ranges from 859 to 1240 kg/kmol which is lower than that of asphaltenes (Akbarzadeh, Alboudwarej, et al., 2005; K. Akbarzadeh, S. Ayatollahi, et al., 2004) but their molecular structures are similar (Acevedo et al., 1995; Bunger & Li, 1981; Suzuki et al., 1982). The molecular structures of resins and asphaltenes vary between reservoirs. Resins and asphaltenes have different physiochemical properties. Despite asphaltenes, resins are totally miscible with lighter fractions of oil. They are the main reason that asphaltenes remain dispersed in the oil phase by peptizing their surfaces and preventing them from self-associating. This factor is more important in less-aromatic crudes. However, if the concentration of the saturate fraction in the oil increases, asphaltene will eventually precipitate out of the solution. In this case, resins may only delay the asphaltene deposition process (Mansoori, 1996a, 2009; Suzuki et al., 1982). Similar to asphaltenes, petroleum resins have a range of molecular weights. The density of resins ranges from 1007 to 1066 kg/m 3 (Akbarzadeh, Alboudwarej, et al., 2005). The polarities of the molecules in the resin fraction differ from one reservoir to another. Figure 2-3 shows two molecular structure models of resin fractions from 10

27 Athabasca heavy oil and Athabasca tar sands. The aromatic rings are hexagonal with a yellow circle inside. The plain hexagonal structure represents the cyclohexane rings. The resin sample from Athabasca heavy oil has heteroatoms of oxygen and sulphur. (A) OH S (B) Figure 2-3-Resins molecular structure models of (A) Athabasca heavy oil resin and (B) Athabasca tar sand resin (Suzuki et al., 1982). 2.2 Asphaltenes Asphaltenes are a solubility class of petroleum fluids which are soluble in benzene or toluene and insoluble in excess amounts of low-molecular weight alkanes such as n- pentane and n-heptane(mansoori, 2009). They consist of high molecular weight polycyclic carbon-hydrogen rings with oxygen, nitrogen, and sulphur in their molecular structure (Becker, 1997; Mullins & Sheu, 1998; Mullins et al., 2007). 11

28 At temperatures above C, they generally decompose into carbon and volatile products (Mansoori, 2009). Asphaltenes and resins in crude strongly adsorb to each other such that even when asphaltenes precipitate, some attached resin molecules precipitate with them, too. It is too difficult to quantitatively separate asphaltenes from other molecular attachments (e.g. resins) to obtain purified components (Priyanto et al., 2001a, 2001b). Asphaltene forms various spatial structures with other polar molecules in a crude medium depending on their polarity and molecular size. Asphaltenes are soluble in aromatics and form scattered reverse-micelle solutions in contact with them (Chilingarian & T.F.Yen, 1994; Priyanto et al., 2001a, 2001b). When dissolved in aromatics (e.g. toluene), their solutions vary in colour from yellow to dark red depending on the concentration. Asphaltenes and resins also influence the colour of crude. When asphaltenes are mixed with low-molecular weight alkanes, they do not tend to dissolve and instead form scattered flocs. If resins are present, steric colloids may be formed (Chilingarian & T.F.Yen, 1994; Mansoori, 1996a). In crude oils with high amounts of light paraffins, only small asphaltene particles may exist in the liquid phase while larger asphaltene aggregates may flocculate out of the solution to form a solid phase deposit. In the presence of appropriate aromatics and resins, mid-sized aggregates may still stay in the liquid phase in the form of colloids and/or micelles. 12

29 Figure 2-4 represents three sample asphaltene structures but there are many other possible structures. Here, the aromatic rings are the hexagonal with a yellow circle inside. The plain hexagon stands for cyclohexane rings. The structural similarities between these highly dispersed molecules are that they are composed of grouped aromatic rings (shown in yellow) and non-aromatic rings with alkane chains. Heteroatoms such as oxygen, nitrogen, and sulphur may also be attached to their aromatic or non-aromatic rings. The right diagram in Figure 2-4 illustrates a sample where a sulphur heteroatom is attached to the asphaltene molecule. Figure 2-4-Asphaltene molecular structure, adopted from (Carbognani, 1992). As mentioned, asphaltenes are more of a solubility class separated from the crude by the addition of a precipitant rather than a pure material. However, over the last few years a number of researchers have tried to further characterize asphaltenes based on their molecular structure. The physical methods used to characterize asphaltenes include but not limited to: X-rays, light scattering spectroscopy, VPO, NMR, small 13

30 angle neutron scattering, HPLC, ESR, mass spectrometry, and IR (Mansoori, 2009). There have also been attempts using chemical methods such as hydrogenation and oxidation (Chilingarian & T.F.Yen, 1994). As a result of these studies, we know that asphaltene molecules generally contain carbon, hydrogen, oxygen, nitrogen, and sulphur atoms with poly-nuclear aromaticity. Depending on the source of the crude, different polar and nonpolar chemical groups may also be attached to the asphaltene molecules. For example, asphaltenes extracted from the Athabasca region generally have a longer straight-chain paraffinic arm attached to their aromatic rings (Mansoori, 2009). In general, asphaltenes are poly-dispersed materials consisting of molecules with a wide range of molecular weights. Depending on the measurement method, their average molecular weight ranges from 1,000 to 2,000,000 kg/kmol (Mansoori, 2009). However, due to the association tendency between asphaltene and asphalteneresin molecules, different numbers are reported in the literature for the mean molecular weight. In some studies, the average molecular weight of the asphaltene monomer is reported instead of that of the asphaltene fraction (Arya et al., 2015; Li & Firoozabadi, 2010; Zhang et al., 2012). 14

31 Figure 2-5-The molecular weight distribution of asphaltene extracted from a petroleum sample by three normal alkanes measured by Gel Permeation Chromatography (adopted from Mansoori et al. 2007b (Mansoori et al., 2007)). In general it seems true to conclude that the solubility of asphaltenes in crude oil is controlled by the ratio of polar components to nonpolar components and the ratio of high-molecular-weight components to low-molecular-weight components in the oil. 2.3 Asphaltene Self-Association Asphaltene molecules self-associate to form larger asphaltene structures, which vary in size and shape. Asphaltene association happens even in dilute solutions and makes it challenging to report an accurate molecular weight for asphaltene (Alboudwarej et al., 2003). 15

32 Many factors affect the shape, size, quality, and quantity of asphaltene aggregates including temperature, pressure, solvent type, the average polarity of the asphaltene particles, and the presence of other materials such as aromatics, resins, and saturates. Several studies have attempted to understand the nature of asphaltene aggregation and its properties. Ravey et al. studied asphaltene aggregation in aromatic solvents using a small angle X-ray scattering method (SAXS) and Xu et al. investigated asphaltene aggregation with a small angle neutron scattering method (SANS) (Ravey et al., 1988; Xu et al., 1995). They reported aggregates in the range of 3 to 15 nm. Sheu, et al., Mohamed et al., and Rogel et al. employed interfacial tension and surface tension methods to study asphaltene aggregates (Mohamed et al., 1999; Estrella Rogel, 2000; Sheu, 1996). Interfacial (or surface) tension decreased with increasing asphaltene concentration in solution until a limiting value was reached at a certain concentration. Beyond this concentration, referred to as the critical micelle concentration, asphaltene aggregates were formed. Several studies have attempted to describe asphaltene self-association. It is believed that self-association occurs as a result of the intermolecular forces between asphaltene particles. Depending on the type of asphaltene (from different sources) and the heteroatoms and functional groups in its molecular structure, various intermolecular forces such as van der Waals, hydrogen bonding, and acid-base interactions may be present. It is believed that all types of these intermolecular forces simultaneously contribute to asphaltene self-association. Depending on the spatial configuration and the 16

33 confrontation angle, one of these forces may dominate. For example, when asphaltene molecules are very close together, the van der Waals interactions become important but when their aromatic rings are in contact with each other π-π bonding becomes the dominant factor(barbour & Petersen, 1974; Dickie & Yen, 1967; Murgich, 2002; E. Rogel, 2002; Yen, 1974). In some studies, asphaltene particles were stacked on each other to form a larger structure. In such structures, the aromatic rings of two or more asphaltene particles are held together by π-π bonding (Dickie & Yen, 1967). Sometimes resins could sit on the asphaltene stacks and keep them dispersed in the solution (Dickie & Yen, 1967). Another intermolecular force that causes asphaltenes to aggregate is hydrogen bonding of the heteroatoms (Barbour & Petersen, 1974; Murgich, 2002). Barbour and Petersen conducted methylation experiments on asphaltene and reported the role of the hydrogen bonding in asphaltene self-association (Barbour & Petersen, 1974). Van der Waals forces between the hydrogen and carbon atoms in the molecular structure of asphaltene must also be considered. Van der Waals interaction forces are much weaker than hydrogen bonding but they do affect self-association in large, tightly packed asphaltene stacks (Murgich, 2002; E. Rogel, 2002, 2004). Several studies proposed a colloidal model for asphaltene aggregates in which asphaltenes were assumed to be colloidal particles peptized by resins on their surface (Dickie & Yen, 1967; Pfeiffer & Saal, 1940; Yen, 1974). X-ray crystallography experiments and later on, small angle x-ray scattering (SAXS), and small angle x-ray 17

34 neutron scattering (SANS) measurements suggested the colloidal model for asphaltene aggregates. These studies also found stacked-sheet, disk-shaped, and spherical aggregates in oil and asphaltene-toluene mixtures. The average diameter of the dispersed aggregates was around 8 nm (Priyanto et al., 2001a; Ravey et al., 1988; Xu et al., 1995). In colloidal solutions, the surface area-to-volume ratio is large making the surface forces more significant than the gravity forces. The magnitude of the surface forces amongst the colloids depends on factors such as colloid size, shape, and distance from each other. Some colloids tend to act as a separate phase, while others, such as lyophilic colloids can be assumed to exist as part of a continuous phase. Storm and Sheu proposed a micellar (or reverse micellar) model (Storm & Sheu, 1995). Micelles are surfactant aggregates with a specific configuration such that their hydrophilic parts stand on the surface of the aggregate while their hydrophobic part stays in the center. This way, in aqueous phases, their hydrophilic parts stay in contact with water and their hydrophobic part abstains from water. It is believed that micelle formation occurs above a certain concentration called the critical micelle concentration (CMC). Reverse micelles are micelles with reverse alignment. This alignment happens in oil phases such that the hydrophobic parts stand on the surface of the aggregate and the hydrophobic parts stay in the center making reverse micelles stable in the oil phase. 18

35 Another modelling approach considered asphaltene aggregates as macromolecules in non-ideal solution (Ade Hirschberg et al., 1984; Strausz et al., 2002; I. Wiehe & Liang, 1996; Yokota et al., 1986). In this model, it is assumed that a number of the aromatic rings of the asphaltene molecules attach to each other by aliphatic chains to form a macromolecule. Agrawala et al. suggested an oligomer model in which asphaltene aggregation was assumed to be similar to polymerization. The driving forces in polymerization are a result of chemical bonding between monomers while in the asphaltene oligomer model, aggregation is caused by van der Waals forces between asphaltene particles (monomers). In this model, asphaltene aggregates are treated as macromolecules (like polymers) rather than colloids (Agrawala & Yarranton, 2001). 19

36 2.4 Asphaltene Phase Formation The literature includes three expressions of asphaltene phase formation and separation from a crude sample that are sometimes referred to interchangeably. These stages are called precipitation, flocculation, and deposition, that are, in fact, different in physical and/or chemical aspects. In the following section theses three stages are described in more details Precipitation When the thermodynamic condition of oil sample changes and affects the thermodynamic equilibrium state, an asphaltene solid, or dense liquid phase may be formed. The formation of a solid or liquid asphaltene phase is called asphaltene precipitation. Some studies have considered this process as a reversible process while other have described it as irreversible. It is believed that precipitation occurs when a change in the system condition forces resins to desorb. This could be a change in the temperature, pressure, and/or composition of the system. Resins and aromatics keep the asphaltene particles soluble in oil by peptizing the particle and preventing selfaggregation. When these peptizing covers (resin and /or aromatics) are removed from the surface of the colloids, the small asphaltene colloids tend to aggregate and form larger aggregates that could eventually precipitate out of the oil phase. The reversibility of asphaltene precipitation has been a controversial issue. Several factors affect precipitation each in a different way. Due to the lack of wide range experimental data and the different mechanisms of each of the influential parameters, 20

37 it is still unresolved. If asphaltene precipitation is caused by a change in pressure, it is usually assumed to be reversible. However, when asphaltene precipitation is caused by a change in composition such as by the addition of a precipitant, it is generally assumed to be an irreversible process. (Rassamdana et al., 1996) Flocculation In an oil sample that has formed solid (or dense liquid) precipitated asphaltene, these precipitated particles tend to agglomerate together. This agglomeration is called flocculation. The formed flocs vary in size and shape. Their size depends on many factors among which the type of the precipitant is very important. For example, the carbon number of normal alkanes (as precipitant) has a reverse effect on the size of flocs meaning that as the carbon number increases the mean floc size decreases (Ferworn, Mehrotra, et al., 1993). This size difference primarily comes from the number of resin molecules attached to asphaltene particles. Moschopedis also showed that the dipole moment and dielectric constant of the solvent affects the quality and quantity of precipitation (Moschopedis et al., 1976). Several studies have determined that the most important factors in the size distribution of flocs are temperature, pressure, and hydrodynamic condition (Bouts et al., 1995; Ferworn, Svrcek, et al., 1993; Nielsen et al., 1994). Depending on the operational conditions, any of these parameters could have a dominant effect on precipitation. 21

38 Figure 2-6- Aggregation and flocculation of asphaltene molecule (sketched based on concepts from (Carbognani, 1992; Mansoori, 2009; Suzuki et al., 1982)). 22

39 2.4.3 Deposition When previously precipitated and flocculated asphaltene moves toward and attaches to the surface of the system (e.g. reservoir rock, pipes, process facilities), we encounter the asphaltene deposition process inside the system. During this process, random solid particles inside the system, such as soil or metals, may co-deposit with the asphaltenes. During deposition process, depending on the operational conditions and surface properties, some of the precipitated asphaltene may stay suspended inside the oil phase and never deposit. Asphaltene deposition is one of the major flow assurance problems in the oil industry. Several field problems such as wellbore and reservoir blockage have been reported as a result of asphaltene deposition (Juyal et al., 2013; Kurup et al., 2012; Leontaritis & Mansoori, 1988; Rondon et al., 2014; Thawer et al., 1990). For example, in a water-wet oil reservoir, asphaltene deposition on the rock may reduce oil production by changing the wettability to oil-wet (Yan et al., 1997). Moreover, if asphaltene deposition occurs in process facilities, it could cause a variety of process problems from clogging the line to poisoning catalysts (Gray, 2015) The Onset of Asphaltene Precipitation and Flocculation Detection The onset of asphaltene precipitation is normally reported as the threshold concentration of a precipitant agent (usually a light normal alkane) at which the asphaltene content of an oil sample (or model solution such as asphaltene + toluene solution) begins to precipitate out of the solution. 23

40 The onset of asphaltene flocculation occurs when the asphaltene particles present in the solution in the form of colloids, aggregate and form flocs. This stage happens after the precipitation onset; however, the two expressions are often used interchangeably in the literature. Detecting the onset point is crucial to study the phase behavior of petroleum crude. In industrial applications, determining the asphaltene onset point helps to design processes in a way to minimize the risk of field problems such as reservoir damage due to reduced porosity and permeability, clogged wellbores and piping, and facility damage due to deposited asphaltene. A number of experimental studies have been conducted on the detection of asphaltene onset point in mixtures of crude oil and solvents using different methods. However, the studies are limited when it comes to bituminous mixtures. Different approaches for detecting the onset of asphaltene flocculation include but are not limited to, the light scattering method, the gravimetric method, interfacial tension measurements between water and oil upon dilution with a flocculating agent, visual methods, and viscometric methods. The most popular laboratory method for determining the onset of asphaltene flocculation is the light scattering method (Turta et al., 1999). The dark and thick nature of extra heavy oils makes many of the visual methods inappropriate. Hotier and Robin reported increases in the viscosity of solvent-oil mixtures with increased solvent concentration. The solvent they used was a mixture of heptane and benzene (Hotier & Robin, 1983). Hirschberg et al. measured pressure at the onset of 24

41 asphaltene precipitation for a propane and crude oil system (Ade Hirschberg et al., 1984). The solubility of propane in the oil increased with pressure at a given temperature, eventually leading to asphaltene precipitation. Fotland et al. introduced a novel technique to detect the onset of asphaltene precipitation (Per Fotland et al., 1993). The technique was based on measuring the electrical conductivity of the oil at different solvent-oil ratios. The main solvent used in their studies was n-pentane, and the oil was North Sea crude oil. The conductivity versus weight fraction of the pentane curve showed a maximum point that has been interpreted as the onset point. After this maximum, the conductivity decreased with increasing solvent. Nielsen et al. observed the effect of temperature and pressure in the range of C and MPa respectively, on the size distribution of asphaltene agglomerates upon adding n-pentane to four different oil samples including a heavy oil sand bitumen (Nielsen et al., 1994). They used a Brinkmann particle size analyzer which measured particle size by laser beam. They concluded that in most cases, the particle size distribution was log-normal but in two cases the distribution showed a bimodal shape. For most cases, an increase in test pressure increased the mean asphaltene particle size. As the oil became heavier (e.g. Cold Lake bitumen) super agglomeration was observed at and above 75 degrees resulting in particle adhesion. Escobedo and Mansoori introduced a novel method of viscometric determination for the onset of asphaltene flocculation (Escobedo & Mansoori, 1995). Their tests were conducted on two Mexican crude oils with n-pentane, n-heptane, and n-nonane. The 25

42 experiments were conducted at room temperature (24-27 C) and atmospheric pressure. Over a wide range of solvent concentrations, the viscosity curves showed a change in the slope of the viscosity versus solvent concentration in comparison with those of toluene-oil and THFS-oil mixtures. THFS was a 64 volume percent solution of Tetrahydrofuran in toluene with approximately the same kinematic viscosity as n- heptane. Their method is very effective, particularly for darker and heavier crudes such as heavy oil and bitumen as no visual detection is required for the experiments. Later on this technique, the theory of viscosity change at the onset point, was used to find the onset of asphaltene aggregation in different solvent-oil systems (Escobedo & Mansoori, 1997). Hammami et al. observed that the concentration of the n-alkane, as titrant, at the asphaltene onset decreased as the molecular weight of the titrant decreased in an approximately linear trend (Hammami et al., 1995). They conducted experiments at 100 C and 29.9 MPa with carbon dioxide (CO 2 ) and C1, C2, C3, n-c4, n-c5, and n- C6 as titrants in live North Sea stock tank oil. Fotland et al. investigated the effect of oil composition on the onset pressure of asphaltene precipitation for North Sea oil mixtures with various ratios of separator oil and separator gas (P Fotland et al., 1997). Chenguang et al. determined the onset of asphaltene flocculation for six different crudes with heptane using the viscometric method (Meixia et al., 1998). They verified their results with the electrical conductance method. Their tests were completed at 50 C and atmospheric pressure. It was seen that the viscosity of the toluene-oil 26

43 mixtures decreased smoothly with the addition of toluene, whereas the viscosity of the heptane-oil mixtures initially decreased with the addition of heptane until the solvent to oil ratio reached a definite value. At this point, called the knee point, an upward trend was observed, after which the viscosity again started to decrease. This knee point could be an indication of a change in dispersion within the solvent-oil mixture and was reported as the onset of asphaltene flocculation for the solventmixture system. As a comparison between the viscometric and electrical conductance methods, their results showed good agreement. However, the onset point measured by the electrical conductance method usually occurred earlier than that measured by the viscometric method. This indicates that the former method is more sensitive to the formation of the first asphaltene follicles. Turta et al. tested propane with Rainbow Keg crude (Turta et al., 1999). The densities of the mixtures showed quasi-linear decreases with increases in propane concentration. This study showed that, up to a volume percentage of 54.5%, the oil viscosity decreased continuously with increases in the propane portion of the mixture; whereas, the viscosity increased for propane volume percentages between 54.5 and 62.6%. The operational pressure was 17 MPa at 87 C. Above 62.6% volume percent, the viscosity of the mixture again decreased, but with a smaller slope. The increase in the viscosity of the mixture could be interpreted in two ways: it is either the result of asphaltene flocculation due to the agglomeration of particles or the deposition of agglomerated asphaltene particles on the wall of the capillary tube 27

44 (Turta et al., 1999). The starting point of the region where the viscosity started to increase was considered the onset of asphaltene flocculation at a given temperature and pressure. Sivaraman et al. employed acoustic resonance technology to obtain high-quality data on asphaltene onset in very dark live or dead oil at reservoir conditions (Sivaraman et al., 1999). Their tests were performed at temperatures of C and pressures of psi. They claimed that the color of the fluid did not have any impact on the ability of the technique to detect the phase transition. Wang et al. observed the onset of asphaltene for seven crude oil samples containing propane at elevated temperatures and pressures (Qin et al., 2000). They used an optical cell to detect the onset point. Jianzhong Wu and Prausnitz considered more than twenty different solvents with crude and bitumen (Wu et al., 2000). They found that the amount of asphaltene precipitation was, indeed, determined by the solvent density. They verified their correlation using the experimental data by Mitchell and Speight (Mitchell & Speight, 1973) and found that, as the solvent density increased at a given temperature and mixing ratio, the weight percent of asphaltene precipitated from the bitumen oil decreased. They also predicted that there would not be any asphaltene precipitation when an aromatic diluent was added because the density of aromatics is higher than normal alkanes, olefins, and cycloparaffins. Peramanu et al. investigated the reversibility of asphaltene precipitation by adding and removing n-heptane solvent for two types of Athabasca and Cold Lake bitumen 28

45 (Peramanu et al., 2001). They continuously circulated diluted bitumen through a flow-loop apparatus. The formation and dissolution of asphaltene in the mixture was measured by measuring the pressure drop across an in-line filter. Temperature and pressure controls were installed in the system to allow experiments at different temperatures (40 C and 60 C) and pressures. In their apparatus the pore size of the stainless steel filter was 60 μm. Akbarzadeh et al. measured asphaltene precipitation from bitumen diluted with C3, n- C4, n-c5, and n-c7 and found the onset point for each system (K Akbarzadeh et al., 2004). The amount of precipitation decreased significantly with increases in temperature and increases in the carbon number of the n-alkane solvent and decreased slightly with increased pressure. However, the mass ratio of the solvent to bitumen at the onset point was almost the same for each solvent over the studied range of temperatures and pressures. Marugan et al. tried to characterize the near onset region in crude oil-heptane blends using a focused-beam-laser reflectance technique (FBRMs)(Maruga n et al., 2008). They concluded that the aggregation kinetics and particle size distribution are not functions of total asphaltene content in the oil. While most asphaltene precipitation experiments neglected the effect of time on the onset, Maqbool et al. considered the kinetics of asphaltene flocculation (Maqbool et al., 2009). Using microscopic measurements, they concluded that the onset time of asphaltene precipitation may vary up to several months. However, few modelling approaches consider the effect of time. 29

46 Ekulu et al. studied the reliability of the densitometry technique for detecting asphaltene flocculation onset (Ekulu et al., 2010). They added n-heptane to the oil and measured the density. Since asphaltene flocculation changes the volumetric mass of the mixture, measuring the density change led them to investigate asphaltene flocculation. They observed a break point in the density versus solvent concentration curve which was translated as the onset of asphaltene flocculation. An Anton Paar DMA 60 was used to measure the densities. They compared the volumetric mass dm of a mixture of crude oil + cyclohexane + n-heptane to a standard mixture of de-asphalted oil + toluene + n-heptane and interpreted the break point in the dm versus concentration ratio of heptane-to-crude oil as the asphaltene flocculation threshold. They eventually concluded that the densitometry measurements could reliably predict the asphaltene flocculation onset. Badamchizadeh et al. studied the asphaltene precipitation in the reservoir during the injection of n-alkanes along with the steam in ES-SAGD process(badamchizadeh et al., 2011). They characterized Athabasca bitumen based on the experimental SIMDIST data and gamma distribution function, temperature-dependent volume shifts were calculated to accurately predict the bitumen density and Pederson correlation was used to model the bitumen viscosity. They have used experimental data in order to tune PR-EOS model for the asphaltene precipitation. They tuned solid solubility model in order to develop an asphaltene precipitation model. The novelty of their method is to develop a model for ES-SAGD which covers the phase behavior of the solvent-bitumen mixtures while being able to predict and model the asphaltene deposition. Their model shows the onset of asphaltene precipitation as well as the 30

47 maximum amount of deposition (or yield), however, in between these two amounts, the solid solubility model showed some limitations. The developed model covers a wide range of temperature ( C), pressure ( MPa) and solvent mole fraction (0.8-1). Agrawal et al. used the microscopic method to detect asphaltene flocculation onset (Agrawal et al., 2012). They used a HPM cell (High Pressure Microscope) consisting of two Pyrex windows with a micron gap in between to allow them to see through the dark fluids. A camera connected to the set-up captured photos and videos when necessary. The HPM operates at temperatures up to 200 C and pressures up to 20000psi (138 MPa). The whole apparatus was placed inside an air bath to control the temperature. Bitumen was diluted with solvent and mixed by reciprocated interchanges between two cylinders each placed at one side of the HPM. During this back and forth fluid displacement, the bitumen and solvent were mixed to form a uniform mixture. The HPM continuously monitored the mixture to observe any asphaltene precipitation. Based on the initial amount of bitumen and the total amount of the solvent injected before the onset, the researchers could measure the concentration of the solvent at which the onset of asphaltene flocculation occurred. NAE de Silva et al. developed a new method for detecting the onset of asphaltene precipitation (da Silva et al., 2014). They measured the difference between the solubility of asphaltene and a solvent in non-asphaltenic-co 2 blends. They tested the method for various oil samples with different compositions and found it working efficiently regardless of the asphaltene and methane content of the oil. They used an equation of state and activity coefficient to model their results. 31

48 Mendoza et al. studied the onset of asphaltene flocculation by analyzing the viscosity and density of oil-heptane and oil-heptane-toluene blends (Mendoza de la Cruz et al., 2015). Their study focused on one concentration point as the flocculation onset and also on a range of incompatibility. The incompatibility region of an oil sample mixed with light hydrocarbons such as n-heptane was determined based on the viscosity and density of the blends. In addition to the experimental methods applied to determine the asphaltene flocculation and precipitation onset points and the amount of precipitation, various modelling approaches are found in the literature. Some of the most well-known modelling approaches will be referred to in the modelling chapter of this thesis. 2.5 Summary The structure and phase behaviour of asphaltene has been the subject of many studies over the past several decades. It has been in particular interest of many researchers which tried to find remedies for asphaltene deposition in the upstream and downstream oil and gas industry. There is still significant uncertainty surrounding the properties of asphaltenic materials and their thermodynamic behaviour mainly due to the fact that asphaltenes are not pure components with well-understood molecular structures. Instead, they consist of thousands of complicated hydrocarbon molecules interacting with each other via various types and intensities of intermolecular forces. Many factors affect the way that asphaltene particles interact with each other. The composition and properties of different asphaltene samples also vary from one 32

49 reservoir to another. The methods of extracting asphaltene from crude, as well as the washing extent, also influence the properties of this solubility class of material. However, with all the knowledge accrued up to day about the nature of asphaltenes, this solubility class of material is believed to exist in crudes in three main formats: colloids, micelles (or reverse micelles), and macromolecules. Based on these assumptions, several mathematical models have been tuned and applied to predict asphaltene behaviour. These models try to predict when asphaltene particles will form an asphaltene-rich phase in either the solid or liquid state and how much asphaltene will be precipitated. The ultimate goal is to avoid asphaltene deposition problems in the oil and gas industry. It is crucial to know the onset of asphaltene precipitation in order to design remedies and avoid flow assurance problems. 33

50 CHAPTER 3 : EXPERIMENTAL METHODS AND PREPARATIONS This chapter describes the experimental procedures used in this study including the materials and their properties, the designed apparatus, calibration tests on the measuring devices and validation tests on the apparatus. The procedure and results for the asphaltene flocculation onset tests are described. 3.1 Material Bitumens: Two samples of chemical-free, water-free, and sand-free bitumen from Athabasca region were used in this study. They will be referred to as ATB and MacKay River bitumen. Table 3-1 shows the properties of these samples measured in the lab. Although composition of bitumen samples from the Athabasca region are not exactly the same, some data for the Athabasca bitumen are also gathered from the literature shown in the last column of the table, only for a qualitative comparison (A. Mehrotra & W. Svrcek, 1985; Peramanu et al., 1999). 34

51 Table 3-1-Bitumen properties. Properties ATB bitumen MacKay River bitumen Athabasca reference Density (at 40 C), kg/m (at 51 C) Viscosity(at 45 C), mpa.s (at 45.4 C) MW, kg/kmol Maltenes (mass%) Asphaltenes (mass%) The bitumen samples were gas-free and chemical-free. The water content of the bitumen samples was negligible (below 0.5 mass %). In order to minimize changes in the oil properties over time, the bitumen samples were kept in a cool, dry place in opaque containers. Although the bitumen samples were gas-free and the lighter cuts such as C10 and C11 were too small to measure, the bitumen storage vessels were sealed tightly to minimize any possible evaporation of the lighter components. Solvents: The precipitant agents, referred to as solvents in this study were normal-pentane, normal-hexane, and normal-heptane. The solvents were provided by OmniSolv (Merck) with 99.9% purity. The properties of theses solvents are listed in the Table

52 Table 3-2- Solvent properties at atmospheric pressure (NIST). Properties n-pentane n-hexane n-heptane 25 C, kg/m C, mpa.s MW, kg/kmol Critical temperature, C Critical pressure, bar Acentric factor Normal boiling point, C Toluene and nitrogen were also used in this study. Toluene was provided by VWR International with the grade of 99.7% and pure nitrogen gas with % purity (argon-free basis) was provided by Praxair in Calgary. 3.2 Experimental Apparatus The experimental apparatus was designed, fabricated, and assembled in the lab (CCIT015) and the vessels were fabricated in the in-house machine shop at the Schulich School of Engineering. Several rigs were designed for this study. The first apparatus was designed with a continuous flow of bitumen and solvent injected into the main stream at the desired proportions. The stream then passed through an in-line mixer to be mixed and the mixed stream of bitumen and solvent passed through the 36

53 viscometer and densitometer to record the viscosity and density. The pressure of the line was controlled by a back pressure regulator mounted at the very end of the line. With each design, validation tests were conducted and repeated to assess the reproducibility of the results. The original fully continuous design was modified to a batch design and the final apparatus is a batch system with a circulation loop which circulates the material inside the loop system until the viscosity and density readings become constant. The experiments on asphaltene flocculation onset detection in solvent-bitumen systems indicated that effective mixing and accurate property measurements are keys to successfully reproducing the data. The main challenges in this study are: 1- Moving very high viscosity bitumen through the narrow tubing of the rig. This is especially challenging at lower operational temperatures and solvent concentrations. 2- Effective mixing of solvent and bitumen to make the stream completely homogeneous and enable accurate viscosity and density readings. 3- In order to avoid the risk of high local concentrations of solvent into bitumen, the solvent must be injected into the bituminous mixture at a very low rate. This makes the experiments very time consuming, especially since low temperature and high viscosity increase the risk of local concentration. Experiments at lower temperatures and lower molecular weight solvents have even longer run times. 37

54 The final modified apparatus was designed to acquire viscosity, density, and phase volume data for various solvent-bitumen systems at a wide range of temperatures and pressures. This apparatus has a creative mechanism which was completely designed and modified in-house. The schematic diagram of the apparatus is shown in Figure 3-1. The apparatus consisted of a temperature-controlled conventional oven equipped with thermocouples, a mixture containing vessel, an asphaltene filter set-up, a Haldex- Barnes gear pump, a solvent feeding cell, an in-line viscometer, an in-line densitometer, a nitrogen tank for keeping the pressure constant, and a Quizix pump. All of the tubing used inside the oven was stainless steel 1/8 OD ( cm). The Quizix pump fluid was water which was incompressible with similar motion properties as the solvents. The solvent cell had a piston which was perfectly sealed with Viton O-rings and Teflon back-up rings to ensure accurate volume measurements and prevent any solvent- water contamination. 38

55 Figure 3-1-The schematic diagram of the modified apparatus designed in-house to detect the asphaltene flocculation onset. 39

56 The following sections describe each part of the apparatus Temperature-Controlled Oven The entire system loop including a 2500 ml vessel, filter, gear pump, viscometer, and densitometer were placed inside Blue M 146 series convection oven in order to maintain constant temperature during each test. Thermocouples controlled the temperature inside the oven from ambient to 350 C with an error of ±1%. Figure 3-2 shows a schematic of the oven used in this study. (Picture from ) Figure 3-2-Temperature-controlled conventional oven. Figure 3-3 is a photo taken from the inside of the oven. The bigger vessel shown in the picture contained the fluid mixture. The oven kept the temperature constant 40

57 during each test. The pressure was controlled with nitrogen gas from the vessel s topside. The vessel was connected to a filter with three different mesh-screen layers allowing only particles below 30 microns pass through. Following the filter, a small gear pump assembly was attached to mix the fluids. The gear pump was a Haldex-Barnes small pump with rotation speeds between rpm. There was a vessel placed inside the oven connected to the gear pump by a three-way valve, which provided the solvent s make-up stream. All of the valves inside the system are HiP valves capable of withstanding high operational pressures and temperatures. Figure 3-3-Inside the asphaltene onset experimental apparatus. 41

58 Once the fluids, bitumen, and the precipitant solvent are well mixed inside the gear pump, they are transferred to an in-line viscometer and densitometer to record their viscosity and density Viscometer The viscometer is a high quality Cambridge ViscoPro 2000 series, which offers menu-driven electronics to drive and interpret the data. It provides true viscosity, accurate stream-temperature measurement, and temperature-compensated viscosity. The viscometer is capable of accurately and continuously measuring the viscosity of the stream. Figure 3-4 shows a schematic of the viscometer and its read-out unit. Figure 3-4-Cambridge ViscoPro 2000 Viscometer. 42

59 The viscometer is factory calibrated to detect viscosities in a range of 0.25 to mpa.s with an accuracy of ± 1.0% of full scale. The viscosity range and its corresponding piston size are summarized in Table 3-3. Table 3-3-Viscosity range of the viscometer. Viscosity Range (mpa.s) Piston size (inches) This model is a piston-style viscometer with two magnetic coils within a 316 stainless steel sensor. A magnetic piston is placed inside the main cylindrical chamber surrounded by the fluid sample inside the measurement chamber. Two coils inside the sensor body magnetically force the piston back and force in the cylindrical chamber at a fixed distance of 0.2 inches. Measuring the piston s travel time over the fixed distance with constant magnetic force allows the viscometer to measure the viscosity of the medium. An increase in the viscosity of the medium results in a longer travel time of the piston. The travel time for the piston is calculated based on a two way direction so that variations due to flow forces and gravity are annulled. Moreover, as the piston has very little mass and is moving in a magnetic field, any vibration disturbance is negligible. Figure 3-5 shows a cut-away profile of the viscosity sensor (pictures from ). 43

60 Figure 3-5-Cutaway view of the viscometer's sensor tip installed in a pipeline. After the fluid passes through the viscometer, it flows to an in-line densitometer Densitometer We used an Anton Paar vibrating-tube densitometer with a DMA HPM measuring cell. This type of densitometer is capable of measuring the density in the range of 0 to 3 gr/cc with an error of ± gr/cc, and temperature and pressure ranges of -10 to 200 C and 0 to psi (0-70 MPa). 44

61 The fluid passes through a U-shaped tube inside the measuring cell, which vibrates electronically while the fluid is being transferred. The frequency of the fluid vibration changes depending on the density of the passing-by fluid. The densitometer s processing unit translates this unique frequency into a density measurement using a mathematical correlation defined during the densitometer s calibration. Figure 3-6 shows a schematic of the density measuring cell and its read-out unit (from ). Figure 3-6-AntonPaar Density measuring cell (mpds2000 V3) and read-out unit. 45

62 3.2.4 Quizix Pump The Quizix pump is a compact dual-cylinder pump able to correctly measure the amount of pumped fluid. This pump is capable of accurately delivering flowrates in increments as small as cc/min. The pump charges and discharges water to the solvent cell to control the pressure and displace the desired amount of solvent into the bituminous mixture at each solvent-to-oil ratio stage. The accuracy of the injection is ±0.1% of the set-flowrate regardless of the pressure, temperature, and/or fluid properties. This technology also allows long-term stability in delivering very small flowrates. This pump runs at constant pressures up to 6000PSI (41.4 MPa) without compromising the volume measurement. Table 3-4 summarizes the properties of the pump and Figure 3-7 shows schematics of the Quizix QX pump used in this study ( ). Table 3-4-Quizix QX 6000 pump specifications. Model Max Pressure Max Flowrate Min Flowrate QX psi (41 MPa) 50 cc/min cc/min 46

63 Figure 3-7- Schematics of Quizix pump QX Calibration The results of this study depend on the accuracy of the measurement elements of the system, the viscometer and the densitometer. In order to assure the accuracy of the data, the devices were extensively calibrated and several validation tests were conducted Densitometer Calibration The densitometer used in our experiments is an Anton Paar vibrating-tube densitometer with a DMA HPM measuring cell. It is capable of measuring the density in the range of 0 to 3 gr/cc with an error of ± gr/cc and the temperature and pressure ranging from -10 to 200 C and 0 to psi (0-70 MPa). 47