PETROPHYSICS, VOL. 56, NO. 3 (JUNE 2015); PAGE 266–275; 7 FIGURES; 2 TABLES
Asphaltenes Explained for the Nonchemist Oliver C. Mullins1, Andrew E. Pomerantz1, A. Ballard Andrews1, Julian Y. Zuo1
ABSTRACT Crude oils consist of dissolved gases, liquids, and dissolved solids—the asphaltenes. The chemical identity and thermodynamic treatment of gas and liquid components of crude oil have long been understood. For example, the cubic equation of state (EoS) is very familiar to the reservoir engineering community. In contrast, in years past, the asphaltenes were viewed as complex, enigmatic and without a thermodynamic foundation. Consequently, oil¿eld observations related to asphaltenes, such as asphaltene gradients in crude oil, heavy-oil gradients, viscosity gradients, tar mat formation, bitumen deposition and asphaltene Àow assurance, were all viewed very much within a phenomenological context without a ¿rst-principles foundation. In the recent past, a simple molecular and nanocolloidal model of asphaltenes, the Yen-Mullins model, has been shown to apply broadly. This model, combined with the Flory-Huggins-Zuo Equation
INTRODUCTION Crude oils consist of dissolved gases, liquids and dissolved solids: the asphaltenes. Figure 1 shows a schematic representing this composition. Gas-liquid equilibria have been treated for decades by petroleum and reservoir engineers using speci¿c cubic equations of state accounting for many reservoir Àuid properties, but only for gas-liquid equilibria.
Fig. 1—Crude oils consist of dissolved gas, liquids and dissolved solids. The cubic EoS treats gas-liquid equilibria. The FHZ EoS treats solutionasphaltene equilibria.
of State (FHZ EoS), accounts for asphaltene gradients in bulk oil and when combined with the Langmuir EoS accounts for oil-water interfacial properties. Such success establishes validation. These new developments in asphaltene science have been closely linked with downhole Àuid analysis (DFA) to address a wide variety of reservoir concerns. Consequently, petrophysicists and other geoscientists traditionally charged with the responsibility of formation evaluation are left with the task of understanding the asphaltenes. Here, we provide an overview of asphaltenes in order to make asphaltenes accessible to technologists who are not expert in petroleum and asphaltene science. The emphasis is on the simplicity of asphaltene chemistry. This discussion naturally leads to basic chemical precepts of solubility especially because asphaltenes are de¿ned by their solubility characteristics.
Within the saturates, aromatics, resins and asphaltenes (SARA) classi¿cation of dead oils (no dissolved gases), the liquids are further divided in to the SAR components (no asphaltenes). The SARA components are de¿ned by solubility and chromatographic properties but chemical identities hold for these fractions. The saturates, meaning no double bonds (all sites are saturated with hydrogen), include all liquid alkanes; the simplest liquid phase alkanes are, pentane, hexane, heptane etc. The aromatics include compounds with carbon=carbon double bonds (so sites of unsaturation)— benzene is the simplest aromatic compound. Compounds are called aromatics even if they have some saturated carbon sites, such as toluene. Resins include compounds with larger aromatic ring systems or polycyclic aromatic hydrocarbons (PAHs), often with peripheral alkane substituents (alkanes bonded around the perimeter of the aromatic ring system). Resin molecules often contain heteroatoms, atoms that are not hydrogen or carbons. For crude oil, the dominant heteroatoms, enriched in asphaltenes, are sulfur, nitrogen and oxygen, and sometimes trace amounts of vanadium
Manuscript received by the Editor May 9, 2015; revised manuscript received June 8, 2015. 1 Schlumberger, 5599 San Felipe St. , Houston, TX, 77056, USA;
[email protected]
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and nickel. Resin molecules approach the size of asphaltene molecules, but are soluble in n-alkanes. These SAR liquid components are treated within the framework of the cubic EoS. In contrast, the asphaltenes have not been incorporated into a thermodynamic treatment until recent years. The challenge had been that all properties of asphaltenes were the subject of intense scienti¿c debate, especially asphaltene molecular weight (Mullins et al., 2008). Without molecular (or nanocollodial) weight, Newton’s second law for gravity (F = mg) remained unresolved. Because gravitational segregation is a signi¿cant factor creating Àuid gradients in reservoirs, the large uncertainty regarding asphaltene mass precluded modeling asphaltene gradients. While many carbonaceous materials contain an asphaltene fraction, it is for crude oils that asphaltenes are fundamental and key. Asphaltenes are de¿ned by a chemical operational de¿nition not a chemical identity because (1) there had been such confusion about their chemical properties, and (2) they are a polydisperse class, that is with many, diverse constituents. Asphaltenes are de¿ned by their solubility characteristics; this fact emphasizes that theoretical treatments of asphaltenes should focus on solubility. Typically asphaltenes are de¿ned as that fraction of crude oil (or carbonaceous material) that is soluble in toluene and insoluble in n-heptane. This solubility de¿nition might seem arbitrary, but it is not. This solubility fraction captures the heaviest aromatic component of crude oil and largely captures that fraction of crude oil that is self-assembled into nanocolloidal particles in solution under both reservoir and lab conditions. Also, this de¿nition largely captures that fraction of crude oil, which is (nonwax) solid. DFA methods have been broadly used to measure distributions of many properties of reservoir Àuids, especially asphaltene distributions. DFA methods provide (relative) asphaltene content in reservoir crude oils using optical absorption. Because color differences can be determined very accurately, DFA measurements of asphaltene gradients are high precision, exactly what is needed for thermodynamic treatment. Such precision generally exceeds that in laboratory phase-separation methods for asphaltene determination. The labs provide overall asphaltene content, so DFA and lab measurements are complementary. Of course, the real-time nature of DFA interpretation carries enormous value. Reservoir compartment identi¿cation is frequently observed in DFA surveys when a higher density of asphaltenes is observed higher in an oil column. Asphaltenes are dense; they don’t Àoat, they sink; thus, observation of higher asphaltene content higher in the column clearly indicates existence of a sealing barrier. Signi¿cantly greater utility of DFA measurements of asphaltene gradients had to
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await development of a proper theory for asphaltenes. The resolution of asphaltene nanostructures and incorporation of this development into a modi¿ed polymer solution theory has enabled a proper thermodynamic treatment of asphaltene distributions in oil¿eld reservoirs. Asphaltene distributions in oil¿eld reservoirs are now being measured by DFA and analyzed to account for major reservoir concerns. Reservoir connectivity is indicated when asphaltene gradients are in thermodynamic equilibrium; this method of connectivity analysis is now in use around the world (Mullins et al., 2007; Betancourt et al., 2009; Zuo et al., 2013). Other major reservoir concerns are being addressed as well including heavy-oil gradients (Mullins et al., 2013), reservoir Àuid equilibrium (Forsythe et al., in press), and disequilibrium (Zuo et al., 2011), biodegradation (Zuo et al., 2015), bitumen deposition (Dumont et al., 2012), tar mat formation (Seifert et al., 2012) and even fault block migration (Dong et al., 2014). In many reservoirs, the asphaltene gradients are more useful than GOR gradients for formation evaluation because asphaltene gradients can be measured with high accuracy and because these gradients are often large. Moreover, many of the important reservoir concerns (listed above) directly involve asphaltenes. Consequently, it is desirable to make asphaltene chemistry accessible to technologists responsible for formation evaluation. Here, we outline advances in asphaltene science that have enabled broad use of asphaltene measurements for understanding reservoir Àuid processes and associated production concerns. Our goal is to be more descriptive without sacri¿cing accuracy. With greater understanding of asphaltenes, petrophysicists will be enabled to extend existing utility to exploit DFA-measured asphaltene distributions in reservoirs. Comparison of Asphaltenes to Crude Oil Gas and Liquids There is seemingly a paradox in asphaltene science: the asphaltenes constitute a class of compounds with very broad ranges in molecular attributes such as molecular weight, size of polycyclic aromatic hydrocarbon (PAH), heteroatom content, type of substituted alkane, etc., nevertheless, there is substantial convergence of diverse asphaltene studies to the speci¿c nanoscience model, the Yen-Mullins model. The chemical heterogeneity of asphaltenes is expected— the adherence of asphaltenes to a simple model is a welcome development. The overall uniformity of character for unre¿ned crude oil asphaltenes has been shown repeatedly within this overarching construct. The current risk of misunderstanding asphaltenes is to overlook and underappreciate their overall simplicity. Of course, other source materials for asphaltenes such as coal oil or resid foretell different asphaltene properties but still ¿t within the framework presented here.
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To understand this paradox of tremendous heterogeneity yet simplicity of asphaltenes, it is instructive to consider the gas and liquid components of crude oil. These compounds also exhibit huge chemical heterogeneity. Advanced analytical methods such as two-dimensional gas chromatography (GCxGC) elucidate the complexity of the liquid phase of crude oils (Ventura et al., 2011). Nevertheless, many thermodynamic properties of live crude oil (containing the associated gas) are very amenable to cubic EoS treatment. The primary reason that a simple theoretical formalism is effective for a polydisperse chemical mixture, such as live crude oils, is that the corresponding dominant components, i.e., hydrocarbons, have weak intermolecular interaction and thus are nearly ideal. Moreover, there is largely a continuum of chemical composition, which obviates concerns of unusual behavior of speci¿c components. The one component that can be present in abundance is methane, and this is the simplest of all alkanes enhancing applicability of simple models. The intermolecular interactions of alkanes are very weak (per unit carbon) and the chemical population is a continuum thus, simple mixing rules apply and ‘lumping’ approaches work in the application of simple theory for this complex mixture. For asphaltenes, the strength of interaction is certainly greater than for alkanes. Nevertheless, the predominant intermolecular interaction for bulk asphaltenes is via relatively weak dispersion (polarizability) forces, not hydrogen bonding and not polarity. Thus, while the asphaltenes are not the noninteracting ideal (they do interact to form nanocolloidal structures in oil), this fairly weak strength of interaction does not preclude simple formalisms on its own. The important consideration is whether asphaltene molecules have moderate, simple intermolecular interactions on average, and whether they exhibit systematic behavior. In addition, the asphaltenes also contain a chemical continuum similar to the petroleum alkanes. Consequently, a simple nanostructure model can be used for thermodynamic treatment of asphaltenes. In addition, the asphaltenes have a predominant molecular architecture, a relatively well-de¿ned nanoaggregate structure and a well-de¿ned cluster of nanoaggregates. Consequently, the asphaltenes are amenable to a modi¿ed polymer theory, the Flory-Huggins-Zuo (FHZ) EoS to treat asphaltene concentration gradients in reservoir crude oils. ASPHALTENE NANOSTRUCTURES The nanostructures of asphaltenes are now resolved and codi¿ed in the Yen-Mullins model shown in Fig. 2 (Mullins, 2011). Although the composition of asphaltenes from reservoirs worldwide are fairly similar, variations in
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the concentration of asphaltenes and the composition of the rest of the oil result in asphaltenes occurring in different aggregate structures in different reservoirs. Fortunately, the aggregate structures are largely invariant and not dependent on the speci¿c asphaltene.
Fig. 2—The Yen-Mullins model of asphaltenes consisting of (left) the dominant molecular structure, (center) the nanoaggregate structure, and (right) a cluster of nanoaggregates.
At low concentrations, as in light oils and even condensates, asphaltenes occur as isolated molecules (Fig. 2, left). At moderate concentrations, as in black oils, asphaltenes are aggregated as nanoaggregates consisting of 6 to 8 molecules (Fig, 2, center). At high concentrations, as in heavy oils, asphaltenes are aggregated as a cluster consisting of ~8 nanoaggregates (Fig. 2, right). Various review articles have delineated the many studies that have con¿rmed the model shown in Fig. 2 (Mullins, 2011). Many more recent studies continue to strongly con¿rm this model including direct molecular imaging (Schuler et al., in press), mass spectrometry (Pomerantz et al., 2015), NMR spectroscopy and relaxometry (Dutta Majumdar et al., 2013), centrifugation and DC-conductivity (Goual et al., 2014) and interfacial studies (Rane et al., 2013; Andrews et al., 2011; Ruiz-Morales and Mullins, 2015). These studies validate the approach of using the model of Fig. 2 as a foundation for theoretical modeling of reservoir crude oils. Figure 2 represents the dominant asphaltene structures, but not the width of the distribution of the structures. For example, the molecular structure depicted in Fig. 2, has a single molecular weight (~769 g/mole) whereas asphatlene molecular weight, and thus molecular size, spans a range of ~500 to 1,000 g/mole and with a high mass tail extending to 1,500 g/mole (Pomerantz et al., 2015). Figure 2 does not represent this width of the distribution. Nevertheless, the mean structure is very useful for thermodynamic characterization of asphaltenes in crude oil. This is similar to lumping methods that are used with the cubic EoS. Figure 2 shows details of the asphaltene molecular structure. Asphaltene molecules are predominantly carbon and hydrogen, and are shaped ‘like your hand’. The key structure features include a single, central PAH core
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dominated by aromatic carbon, with peripheral substituents including saturated rings, straight chains and branched chains. Attractive Molecular Forces At this point, it is instructive to consider the ordering and scale of different types of intermolecular interactions. Attractive forces between molecules in crude oil result mostly from the familiar attractive force between positive and negative electric charges (the Coulomb force). The strongest such attraction involves fully charged molecules (ions), where a positively charged compound with a missing electron is strongly attracted to a negatively charged compound with an extra electron. Ions are common in water but rare in oil. Within a molecule, electrons that are shared in a chemical bond may be shared unevenly if the two bonded elements have different electron attraction strengths (electronegativity). The result is a polar molecule containing a pair of partial charges: a partial negative charge on the more electronegative atom and a partial positive charge on the less electronegative atom. The polar interactions are weaker than ionic interactions. Because hydrogen and carbon have similar electronegativities, bonds between these elements are nonpolar. Polar molecules in oil and asphaltenes contain a heteroatom (generally nitrogen, sulfur, or oxygen) with differing electronegativities. For nonpolar molecules in which electrons are shared evenly, the electron distribution may become distorted or ‘polarized’ when two molecules approach one another. For example, loosely bound electrons in one (the ¿rst) molecule may temporarily move towards a positive charge in another molecule inducing a dipole moment in the ¿rst molecule; this is known as polarizability. Polarizability refers to the ease of formation of a temporary (electric) dipole moment in a molecule. ‘Fluffy’ electron clouds are polarizable thus chemically ‘sticky’. The most common polarizable chemical group found in oil is the aromatic ring. Alkanes are even less polarizable and thus more weakly interacting. If a liquid contains no molecules with permanent dipoles, then the attractive intermolecular interactions are induced dipoleinduced dipole in two adjacent molecules; this attraction is very weak. A simple gauge of the strength of attractive forces is the boiling point. Boiling occurs when the thermal energy to vaporize exceeds the attraction energy between molecules, so a larger boiling point temperature indicates stronger attraction. Larger molecules also typically have higher boiling points (they contain more sites for attraction), so a simple comparison of molecular attraction is obtained by examining the boiling points of molecules with similar molecular weight but different molecular interactions.
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Figure 3 presents such a comparison. Potassium chloride (KCl) salt has ionic attraction, resulting in a high boiling point—it is dif¿cult to boil salt. Attractions are signi¿cantly weaker for polar components. Dimethylsulfoxide, shown in Fig. 3, has very high polarity (about double that of water) but a much lower boiling point than KCl salt. Polarizability is an even weaker attraction than polarity, but the difference is small relative to the contrast with ions, resulting in a small decrease in boiling point for the strongly polarizable (aromatic) compound, benzene, relative to the polar compound. Alkanes such as n-pentane have the weakest interactions found in crude oil and thus the lowest boiling point in this example.
Fig. 3—Molecular weights (MW) and boiling points (BP) of molecules with different intermolecular interactions. These molecules have similar molecular weights, so their boiling points are indicative of the strength of the intermolecular interactions. Ionic interactions are strongest, followed by polar interactions and then polarizability interactions, which are weaker for alkanes than aromatics.
Hydrogen bonding is another kind of intermolecular interaction that is quite strong where a hydrogen atom, bonded to one molecule (in a polar bond), is in part shared with an adjacent molecule. Hydrogen bonding is largely responsible for the relatively high boiling point of water, but hydrogen bonding is rare in crude oil, relevant for only a subset of the heteroatom-containing molecules. Indeed, the inability of hydrocarbons to form hydrogen bonds is a major reason why oil and water are immiscible. Overall, the strongest attractions result from permanent full charges (ions, rare in oil), then weaker attractions result from permanent partial charges (polar molecules), and the weakest attractions result from induced partial charges (polarizable molecules). Asphaltene Molecules With this understanding, Fig. 4 reviews the dominant molecular structure and forces of asphaltenes. The central PAH (the shaded rings in Fig. 4) is polarizable and dominates intermolecular attraction. The peripheral alkanes interfere with close approach of the PAHs of different molecules yielding “steric repulsion”. There is a little heteroatom (N, S, O, V, and Ni for asphaltenes) substitution in asphaltenes. Sulfur is usually the dominant heteroatom and is generally in nonpolar groups in asphaltenes, so contributes little to intermolecular
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interactions. Nitrogen appears in polar groups (pyridinc nitrogen is depicted in Fig. 4), which can play a role in intermolecular interaction. Typically, there is about one (or fewer) nitrogen atom in an asphaltene molecule; the dipole interactions are very limited. In addition, all asphaltene nitrogen is contained in the PAHs and so is somewhat sequestered.
Fig. 4—Speci¿cs of the dominant asphaltene molecular structure with a single PAH core, peripheral alkanes, and the occasional heteroatom.
Asphaltene Nanoaggregates The aggregation number of asphaltene nanoaggregates is very small. The attractive forces of the PAHs cause formation of a ‘stack of pancakes’. After association of several molecules, as depicted in Fig. 5, the PAH stack becomes enshrouded in peripheral alkanes which preclude growth of the PAH stack. Conceptually, the peripheral alkanes of the nanoaggregate look like a porcupine so additional asphaltene molecules would rather start a new nanoaggregate instead of adding onto an existing nanoaggregate. In chemistry, many other aggregation systems are known. For example, soap forms micelles which are conceptually related to nanoaggregates. For example, soap micelles are generally small spherical structures with oil inside, water outside, and with an interfacial layer one molecule thick of soap molecules. Typically, soap micelles also have well-de¿ned aggregation numbers, but generally soap micelles have larger aggregation numbers, of 60 to 100 molecules depending on the speci¿c soap compound. The asphaltene nanoaggregate has aromatic carbon on the inside and alkane carbon on the outside, but it is not a micelle as it is not separating two phases. The interaction of asphaltene PAHs in a nanoaggregate is largely the interaction of two planar regions of the molecules. Similarly, the planar PAH has been shown to spatially align at the (planar) interface of oil and water. (Andrews et al., 2011; Rane et al., 2013; Ruiz-Morales and Mullins, 2015) The understanding of oil-water interfacial
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properties has been greatly extended by application of the Langmuir EoS along with the Yen-Mullins model.(Rane et al., 2013)
Fig. 5—Nanoaggregate with six molecules, the typical aggregation number. The PAHs form a disordered ‘stack of pancakes’ (viewed transverse to the PAH plane). The PAHs act as sites of attraction while the alkanes tend to disrupt stacking thereby limiting the aggregation number.
An everyday observation is related to these principles of asphaltene chemistry. Charcoal, which is used for various purifying ¿lters, such as water puri¿ers, has large surface area and PAH ‘stickiness’ to remove various chemical impurities. Of course, wax (or any alkane) is never employed for these ¿lters—wax is not chemically sticky. Clusters of Asphaltene Nanoaggregates At much higher asphaltene concentrations, the nanoaggregates can associate in part by virtue of the weakly attractive force of the peripheral alkanes. The cluster of Fig. 6 consists of eight nanoaggregates and is an open, fractal structure. High asphaltene concentration is needed to overcome the very weak polarizability forces of alkanes to form the nanoaggregate.
Fig. 6—Cluster of nanoaggregates. The eight nanoaggregates are distributed in an open 3D (not 2D) structure.
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There are no larger stable asphaltene colloidal particles; with suf¿cient instability, slightly larger asphaltene particles than clusters can undergo phase separation forming a solid mass (Hoepfner and Fogler, 2013). Over time, these smaller Àocs can aggregate to form large Àocs that settle. ASPHALTENE SOLUBILITY General Considerations In solution chemistry, lack of solubility can be caused by adding too much solute or by making the solvent worse. Increasing solution gas makes a crude oil less able to dissolve asphaltenes. This can lead to convective currents that ‘pump’ asphaltenes to the base of the oil column (Mullins et al., 2013; Forsythe et al., in press). Such accumulation of asphaltenes at the base of an oil column can lead to heavy oil and, with suf¿cient asphaltene, tar mats (Mullins et al., 2013). In addition, a pressure drop during production of oil causes the oil to expand, especially if high in solution gas. This increased expansion decreases the solubility parameter of the oil (cf Eq. 1), making the oil a less good solvent and can cause asphaltene precipitation, a Àow assurance problem. The asphaltene onset pressure is above the bubblepoint; once gas comes out of solution, the oil becomes a better solvent for asphaltenes. The exact asphaltene concentration of these phase instabilities and of nanoaggregate and cluster formation depends on the speci¿cs of the solvent or crude oil. The energetics of solubility, that is, the intermolecular interaction of solubility is a signi¿cant component of understanding asphaltene chemistry. There is a well-developed formalism treating solubility that is reasonably understandable. Solubility is intimately involved with intermolecular interactions and provides a means to understand an overview of the chemistry of crude oils and asphaltenes.
compounds (excluding ion interactions); polarizability (known as the dispersion force), įD, polarity, įP, and hydrogen bonding, įH. The PAH core of asphaltenes is Àuffy and polarizable. Alkanes have lower polarizability. TeÀon is even lower polarizability, and thus is not at all chemically sticky. Helium atoms have almost no polarizability, which is why helium does not liquefy until the temperature is 4oK. Polarity refers to molecules that have a permanent dipole moment. Carbon and hydrogen have nearly equal electronegativity, or electron attraction, so compounds made of these two elements are nonpolar. Alkanes and aromatic compounds have no dipole moment. In contrast, oxygen and hydrogen have very different electronegativity so bonds made of these two elements are quite polar (e.g. water). Asphaltenes are dominantly carbon and hydrogen and the dominant heteroatom is sulfur, which is typically in nonpolar groups. Asphaltenes might contain a few percent nitrogen and oxygen, which give asphaltenes a bit of polarity. Hydrogen-bonding (H-bonding) compounds refer to compounds that are involved with this special, strong bond where a proton is shared between two molecules. Water molecules form strong hydrogen bonds and consequently water remains liquid at unusually high temperatures for compounds of such small mass. All compounds that form H-bonds are polar but the converse is much less true. (2) The projection of the Hildebrand solubility parameter into three orthogonal Hansen solubility parameters is shown in Eq. 2 and Fig. 7.
Hildebrand Solubility Parameter and the Hansen Solubility Parameters The Hildebrand solubility parameter į is a numerical value that indicates the relative solvency behavior of a speci¿c solvent (or solute). It is derived from the cohesive energy density of the solvent, which in turn is obtained from the heat of vaporization ǻE per unit molar volume Ȟ (similar ideas are expressed in Fig. 3). į
ǻE
(1)
The Hildebrand solubility parameter can be decomposed into three constituent solubility parameters accounting for the three dominant intermolecular interactions of organic
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Fig. 7—Graphical representation of the Hildebrand solubility parameter, S, for a given compound projected into the three Hansen components, dispersion (or polarizability), D, polarity, P, and hydrogen bonding, H. The chemistry axiom “like dissolves like” dictates that only compounds within the sphere of radius R shown centered on S will be soluble with the given compound S.
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In chemistry “like dissolves like”—the speci¿c condition of two compounds being mutually soluble is that their Hildebrand solubility parameters should be similar. Moreover, each component of their respective Hansen solubility parameters should also be similar. In Fig. 7, this condition of similar solubility parameters is graphically represented by a sphere centered on the solubility parameter, S, of a given compound. Points inside the sphere centered on S represent similar solubility parameters to the given compound, thus are soluble. Open green circles in Fig. 7 represent compounds soluble in the given compound (inside the sphere) with similar Hansen components. Points outside the sphere are insoluble in/with the given compound. Solid red points in Fig. 7 represent compounds with very different Hansen components, thus they are insoluble with the given compound. A table of Hildebrand and Hansen solubility parameters is instructive. TeÀon (and other perÀuorocarbons) are known to have very low intermolecular interactions, which is why TeÀon is a preferred coating in cookware; nothing sticks to it. A value of 12.4 MPa1/2 for TeÀon in Table 1 is seen to correspond to such a low intermolecular interaction. In contrast, Table 1 shows that water, with its strong intermolecular interaction, has a Hildebrand solubility parameter of 47.8 MPa1/2. TeÀon and water establish the range of Hildebrand parameters from very weak 12 MPa1/2 to very strong 48 MPa1/2. Moreover, water is dominated by hydrogen bonding, a strong and spatially directional bond. Alkanes, the dominant component of crude oil are seen to have Hildebrand parameters 16 MPa1/2, somewhat above TeÀon, and those of aromatics are somewhat above the alkanes. Moreover, Table 1 shows that the alkanes and the aromatics are dominated by the simple, weak polarizability forces. The solubility parameters of the asphaltenes are very similar to the aromatics. Indeed, asphaltenes are de¿ned to be toluene soluble and thus must have solubility parameters similar to toluene. Note that the difference between the solubility parameters of asphaltenes and alkanes are suf¿cient to prevent solubility. The solubility parameters of toluene and asphaltenes are dominated by the weak and simple polarizability component. The asphaltenes do have a component of polarity and H-bonding, but these contributions are small especially considering it is the square of each component that contributes to the Hildebrand solubility parameter (Eq. 2). Methane has low intermolecular interaction, but is not listed in Table 1 because the molar volume is very dependent on temperature and pressure making the numeric value of solubility parameter somewhat arbitrary (cf. Eq. 1). Nevertheless, methane added to crude oil causes a decrease
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in solubility parameter because both the density decreases and methane has low intermolecular interaction. Table 1—Hildebrand and Hansen Solubility Parameters (Hansen, 2007)
THE FLORY-HUGGINS-ZUO EOS With this understanding, we can now examine the FHZ EoS. The FHZ is generally written as a ratio of ODs, optical densities as measured by DFA tools (Eq. 3). The ODs correspond to the strength of oil ‘color’ or electronic absorption at a speci¿c wavelength. The color measurement is linear in asphaltene content. However, we do not assume a constant of proportionality, so the absolute asphaltene content is not determined. For the OD ratio, the proportionality constant cancels; thus the FHZ EoS is written in terms of the OD ratio. The FHZ provides a thermodynamic expression for the DFA measured asphaltene gradients. (3) The FHZ EoS takes into account three factors that result in asphaltene gradients: gravity, entropy, and solubility. The variables k, , v, į, T, g, ȡ, and h are Boltzmann’s constant, volume fraction, molar volume, solubility parameter, temperature, earth’s gravitational acceleration, density and depth, respectively. Subscript a, denotes the properties of asphaltenes; subscripts h1 and h2, represent two depths. Gravitational gradients of asphaltenes result from the density difference ǻȡ, or negative buoyancy between asphaltenes and the remainder of the oil, with relatively
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dense asphaltenes falling towards the bottom of the reservoir. The FHZ EoS includes the Boltzmann distribution, which describes the population distribution of ground and excited states at temperature T. For gravity, this population distribution is given by exp(-mgh/kT) where m is the mass of the particle, g is earth’s gravitational acceleration, and h is height. This equation describes the distribution of air molecules (m is the mass of an air molecule) thus the pressure gradient of planet earth and is called the barometric equation. For asphaltenes in crude oil, it is Archimedes buoyancy (multiplied by h) of the asphaltene particles of volume Ȟa in oil that is used—Ȟaǻȡgh—this gives the energy required to lift the asphaltene particle to height h from the bottom of the reservoir and thermal energy kT is doing the lifting; k is Boltzmann’s constant and T is temperature. The solubility term uses the Hildebrand solubility parameter į, discussed above, and depends on the difference in solubility parameters of the oil and asphaltene at different heights in the column. GOR can change for crude oils at different heights in the column, and this yields a difference in liquid phase solubility parameter, which produces large changes in asphaltene concentration. Thus, for oils with high solution gas, asphaltene concentration will be low. For oils with large GOR gradients, such as equilibrated condensates and oil columns undergoing gas entry by diffusion, there will also be a large gradient in asphaltenes (or heavy ends). For crude oils with low GOR, the solubility parameter of the oil does not change much with height. Consequently, the asphaltene gradients for these oils are dominated by the gravity term. In addition, gas injection into oil at high pressure, thereby increasing solution gas, can cause asphaltenes to precipitate out. For example, gas lift for heavy oil at signi¿cant depths requires a Àow assurance solution such as chemical injection. The entropy term accounts for randomness that naturally occurs. For example, a newly opened bottle of ammonia in a room can be smelled everywhere after a while as a consequence of entropy. Likewise, the entropy term in the FHZ EoS attempts to randomize (homogenize) the asphaltenes placing them everywhere, but this term is small and so loses to the larger terms of gravity and solubility. The entropy term in the FHZ employs ratios of the molar volumes of crude oil and asphaltene at different heights. This term tends to be small and so is often neglected. Essentially the entropy term is related to the statistical permutations of arranging solute and solvent. CONCLUSIONS Asphaltenes have been considered complex and
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intractable from a modeling standpoint. Consequently, they have been ignored even though they are central actors in heavy oils, tar mats, bitumen deposits, and of course asphaltene and viscosity gradients. Nevertheless, a now ubiquitous practice is employed to understand reservoir connectivity by analysis of DFA data to establish asphaltene thermodynamic equilibration. This success has led to the study of in-reservoir Àuid processes in geologic time, reservoir Àuid geodynamics, with its dependence on understanding asphaltene thermodynamics and DFA data sets. Our objective here is to illustrate the understandable, basic nature of asphaltenes while retaining chemical accuracy. Greater conceptual understanding of asphaltenes will hopefully enhance their utility in the quest to understand reservoirs and their complexities. Chemical modeling is simplest for molecules that interact weakly. The well-known ideal gas law (PV = nRT) assumes that molecules do not interact at all. Alkanes, the dominant fraction of crude oils, interact very weakly and therefore can be modeled successfully by the cubic EoS, which is based on the ideal gas law and includes corrections for very weak intermolecular interactions. Asphaltenes interact more strongly that alkanes, but asphaltene intermolecular interactions are still weak compared to other molecules such as water. In addition, asphaltene molecules have a characteristic structure with the attractive part, the polycyclic aromatic hydrocarbon (PAH), in the molecular interior and the repulsive part, the alkane substitution, in the molecular periphery. This structure dictates that nanoaggregates with just several molecules can form. With much weaker binding, a second nanocolloidal particle can form at high asphaltene concentration, the cluster of nanoaggregates. These nanostructures species comprise the Yen-Mullins model. Weak asphaltene intermolecular interactions coupled with a simple nanostructure model are consistent with a basic polymer solution theory for describing asphaltene solubility in crude oils. The Flory-Huggins (FH) theory represents each chemical compound with a single solubility parameter, which is the cohesive energy density (intermolecular interaction energy per unit volume) of the substance. Two compounds dissolve one another if their solubility parameters are similar, which is a quantitative representation of the familiar chemical axiom where similar chemical species dissolve one another: “like-dissolves-like”. The dark brown solid asphaltenes are not chemically like hydrocarbon gas; asphaltene solubility in oil decreases with increasing solution gas. FH theory also includes a small entropy term. By adding a gravity term that accounts for the negative buoyancy of speci¿c asphaltene species, the FHZ EoS results and applies broadly to reservoir crude oils.
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This theoretical development coupled with DFA has led to elucidation of many reservoir properties and reservoir Àuid geodynamics. ACKNOWLEDGEMENTS The authors are thankful for the critical reviews and improvements of the manuscript by Eric Soza, BP, Chengli Dong, Shell, and Hadrien Dumont, Schlumberger. NOMENCLATURE a ǻE g h h1 h2 k T į įD įP įH Ȟ ȡ
= properties of asphaltenes = heat of vaporization = earth’s gravitational acceleration = height (or depth) = depth 1 = depth 2 = Boltzmann’s constant = temperature = Hildebrand solubility parameter = Hansen Dispersion (or polarizability) solubility parameter = Hansen Polarity solubility parameter = Hansen Hydrogen bonding solubility parameter = molar volume = density = volume fraction
REFERENCES Acevedo, S., Castro, A., Vasquez, E., Marcano, F., and Ranaudo, M.A., 2010, Investigation of the Physical Chemistry Properties of Asphaltenes Using Solubility Parameters of Asphaltenes and their Fractions A1 and A2, Energy & Fuels, 24, 5921í5933. Andrews, A.B., McClelland, A., Korkeila, O., Krummel, A., Mullins, O.C., Demidov, A., and Chen, Z., 2011, Molecular Orientation of Asphaltenes and PAH Model Compounds in LangmuiríBlodgett Films Using Sum Frequency Generation Spectroscopy, Langmuir, 27(10), 6049í6058. Betancourt, S.S., Ventura, G.T., Pomerantz, A.E., Viloria, O., Dubost, F.X., Zuo, J.Y., Monson, G., Bustamante, D., Purcell, J.M., Nelson, R.K., Rodgers, R.P., Reddy, C.M., Marshall, A.G., abd Mullins, O.C., 2009, Nanoaggregates of Asphaltenes in a Reservoir Crude Oil, Energy & Fuels, 23(3), 1178–1188. Dong, C., Hows, M.P., Cornelisse, P.M.W., and Elshahawi, H., 2014, Fault Block Migrations Inferred from Asphaltene Gradients, Petrophysics, 55(2), 113í123. Dumont, H., Mishra, V., Zuo, J.Y., and Mullins, O.C., 2012, Permeable Tar Mat Formation Within the Context of Novel Asphaltene Science, Paper SPE-163292 presented at the
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SPE Kuwait International Petroleum Conference and Exhibition, Kuwait City, Kuwait, 10í12 December. Dutta Majumdar, R.D., Gerken, M., Mikula, R., and Hazendonk, P., 2013, Validation of the YeníMullins Model of Athabasca Oil-Sands Asphaltenes using Solution-State 1H NMR Relaxation and 2D HSQC Spectroscopy, Energy & Fuels, 27(11), 6528–6537. Forsythe, J., Pomerantz, A.E., Seifert, D.J., Wang, K., Chen, Y., Zyo, J.Y., Nelson, R.K., Christopher M. Reddy, C.M., Schimmelmann, A., Sauer, P., Peters, K.E., and Mullins, O.C., in press, Con¿rmation of Equilibration of a Heavy Crude Rim of a Large Saudi Arabian Oil¿eld using GCxGC and Isotope Analysis, Energy & Fuels. Goual, L., Sedghi, M., Mostow¿, F., McFarlane, R., Pomerantz, A.E., Saraji, S., and Mullins, O.C., 2014, Cluster of Asphaltene Nanoaggregates by DC-Conductivity and Centrifugation, Energy & Fuels, 28(8), 5002–5013. Hansen, C.M., 2007, Hansen Solubility Parameters, A Users Handbook, 2nd edition, CRC Press. ISBN 978-0849372483. Hoepfner, M.P., and Fogler, H.S., 2013, Multiscale Scattering Investigations of Asphaltene Cluster Breakup, Nanoaggregate Dissociation, and Molecular Ordering, Langmuir, 29(49), 15423í15432. Mullins, O.C., 2011, The Asphaltenes, in Annual Review of Analytical Chemistry, 4, 393–418. Mullins, O.C., Betancourt, S.S., Cribbs, Dubost, F., Creek, J.L., Andrews, A.B., and Venkataramanan, L., 2007, The Colloidal Structure of Crude Oil and the Structure of Reservoirs, Energy & Fuels, 21(5), 2785-2794. Mullins, O.C., B. Martinez-Haya, and Marshall, A.G., 2008, Contrasting Perspective on Asphaltene Molecular Weight, this Comment vs. the Overview of Herod, A.A., Bartle, K.D., and Kandiyoti, R., Energy & Fuels, 22(3), 1765-1773. Mullins, O.C., Zuo, J.Y., Seifert, D., and Zeybek, M., 2013, Clusters of Asphaltene Nanoaggregates Observed in Oil¿eld Reservoirs, Energy & Fuels, 27(4), 1752–1761. Pomerantz, A.E., Wu, Q., Mullins, O.C., and Zare, R.N., 2015, Laser-Based Mass Spectrometric Assessment of Asphaltene Molecular Weight, Molecular Architecture and Nanoaggregate Number, Energy & Fuels, 29(5), 2833–2842. Rane, J.P., Pauchard, V., Couzis, A., and Banerjee, S., 2013, Interfacial Rheology of Asphaltenes at Oil-Water Interfaces and Interpretation of the Equation of State, Langmuir, 29(15), 4750í4759. Ruiz-Morales, Y., and Mullins, O.C., 2015, Coarse-Grained Molecular Simulations to Investigate Asphaltenes at the Oil– Water Interface, Energy & Fuels, 29(3), 1597–1609. Schuler, B., Meyer, G., Pena, D., Mullins, O.C., and Gross, L., in press, Unraveling the Molecular Structures of Asphaltenes by Atomic Force Microscopy, Journal of the American Chemical Society. Seifert, D.J., Qureshi, A., Zeybek, M., Pomerantz, A.E., Zuo, J.Y., and Mullins, O.C., 2012, Heavy Oil and Tar Mat Characterization Within a Single Oil Column Utilizing Novel Asphaltene Science, Paper SPE-163291, presented at the SPE Kuwait International Petroleum Conference and Exhibition, Kuwait City, Kuwait, 10í12 December.
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Asphaltenes Explained for the Nonchemist
Ventura, G.T., Hall, G.J., Nelson, R.L., Frysinger, G.S., Raghuraman, B., Pomerantz, A.E., Mullins, O.C., and Reddy, C.M., 2011, Analysis of Petroleum Compositional Similarity Using Multiway Principal Components Analysis (MPCA) with Comprehensive Two-Dimensional Gas Chromatographic Data, Journal of Chromatography A, 1218(18), 2548í2592. Zuo, J.Y., Elshahawi, H., Dong, C., Latifzai, A.S., Zhang, D., and Mullins, O.C., 2011, DFA Asphaltene Gradients for Assessing Connectivity in Reservoirs Under Active Gas Charging, Paper SPE-145438 presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, 30 Octoberí2 November. Zuo, J.Y., Mullins, O.C., Freed, D.E., Elshahawi, H., Dong, C., and Seifert, D.J., 2013, Advances in the Flory-Huggins-Zuo Equation of State for Asphaltene Gradients and Formation Evaluation, Energy & Fuels, 27(4), 1722–1735. Zuo, J.Y., Jackson, R., Agarwal, A., Herold, B., Kumar, S., De Santo, I., Dumont, H., Beardsell, M., and Mullins, O.C., 2015, A Diffusion Model Coupled with the Flory-HugginsZuo Equation of State and Yen-Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation, Energy & Fuels, 29(3), 1447–1460.
infrared spectroscopy. That molecular information is used to understand fundamental physical and chemical processes in petroleum such as asphaltene compositional grading and storage and transport in shales. He graduated from Stanford University with a PhD in chemistry in 2005 and has coauthored 50 peer-reviewed publications.
ABOUT THE AUTHORS
Julian Youxiang Zuo is currently a Scienti¿c Advisor at Schlumberger Houston Pressure & Sampling Center. He has been working in the oil and gas industry since 1989. Recently, he has been leading the effort to develop new answer products for new formation testing tools and to apply the Flory-HugginsZuo EOS for advanced formation evaluation, such as asphaltene gradients, reservoir connectivity, disequilibrium with gas charges, tar mat formation, and Àow assurance. He has coauthored more than 180 technical papers in peerreviewed journals, conferences, and workshops and is coinventor on 50+ granted US patents and patent applications. Zuo holds a PhD degree in chemical engineering from the China University of Petroleum in Beijing.
Oliver C. Mullins is a PhD chemist and Scienti¿c Advisor in Schlumberger. He is the primary originator of Downhole Fluid Analysis and authored the award winning book “The Physics of Reservoir Fluids; Discovery through Downhole Fluid Analysis.” He has received the SPWLA Distinguished Technical Achievement Award and the SPE Distinguished Membership Award. Dr. Mullins also leads an active research group in culminating in the Yen-Mullins model of asphaltenes. He is now leading the new discipline Reservoir Fluid Geodynamics. He has co-edited thee books and coauthored 13 chapters related to asphaltenes. He has coauthored 225 publications with 9000 citations on Google Scholar. He is co-inventor on 95 allowed US patents. He is Adjunct Professor of Petroleum Engineering at Texas A&M University.
Ballard Andrews is a principal scientist in the Sensor Physics Dept. at Schlumberger Doll Research in Cambridge MA, USA, specializing in optics, photonics, lasers, infrared thermography, Raman spectroscopy and asphaltene science. Prior to joining Schlumberger he did his postdoctoral research at Los Alamos National Lab and worked in the computing division at Brookhaven National Lab. He has published more than 60 articles in physics, chemistry, optics and energy journals and coauthored a book chapter in the Handbook of Physics and Chemistry of the Rare Earths. He graduated from the University of Texas at Austin with a PhD in condensed matter physics. He has given over 38 conference presentations and has over 22 patents granted or ¿led.
Andrew E. Pomerantz is the Geochemistry Program Manager at SchlumbergerDoll Research. His research focuses on the development of novel techniques to characterize the chemical composition of kerogen and asphaltenes, including methods in mass spectrometry, X-ray spectroscopy, and
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