Yen-Mullins Model Research Articles

Laser-Based Mass Spectrometric Determination of Aggregation Numbers for Petroleum- and Coal-Derived Asphaltenes

Petroleum- and coal-derived asphaltenes have been studied with three laser-based mass spectrometric techniques: laser desorption ionization–mass spectrometry (LDI–MS), in which a single laser desorbs and ionizes solid analytes; surface-assisted laser desorption ionization–mass spectrometry (SALDI–MS), in which a single laser desorbs and ionizes solid analytes from an activated surface; and laser desorption laser ionization mass spectrometry (L2MS), in which desorption and ionization are separated spatially and temporally with independent pulsed laser sources. We find that asphaltene nanoaggregates can be detected in LDI–MS and SALDI–MS under mild conditions of relatively low laser power, whereas L2MS avoids aggregation and fragmentation, detecting asphaltenes as monomeric molecules. A comparison of the L2MS and SALDI–MS results yields an estimate of the distribution of aggregation numbers (number of molecules comprising the nanoaggregate). The most probable aggregation number observed for nanoaggregates of petroleum asphaltenes is approximately 6–8 molecules, consistent with the Yen–Mullins model of asphaltenes and predictions from the island geometry for asphaltene molecules. Additionally, the nanoaggregates are found to be relatively monodisperse, because most aggregates observed have aggregation numbers within one molecule of the most probable. In contrast, the nanoaggregates of coal asphaltenes are found to be smaller and more polydisperse, with aggregation numbers ranging from 3 to 6 molecules. Under higher powers, SALDI–MS measurements show that nanoaggregates decompose to form small multimers and monomers, suggesting that the aggregates are bound noncovalently. Coal asphaltene nanoaggregates decompose at lower laser powers than petroleum asphaltene nanoaggregates, indicating that the coal asphaltene nanoaggregates are bound less strongly than petroleum asphaltene nanoaggregates.

Validation of the Yen–Mullins Model of Athabasca Oil-Sands Asphaltenes using Solution-State 1H NMR Relaxation and 2D HSQC Spectroscopy

The hierarchical Yen–Mullins model of Athabasca oil-sands asphaltene is strongly supported by solution-state 1H NMR relaxation measurements and 2D HSQC-NMR spectroscopy. For the first time, the 1H ...

New thermodynamic modeling of reservoir crude oil

Downhole fluid analysis data from several deepwater oil wells in the Gulf of Mexico are examined. The primary question addressed is whether there is lateral fluid-flow connectivity of the “A” Sand that spans two wells. The predominant fluid gradient observed in the A Sand is the variable dissolved asphaltene content. To perform thermodynamic modeling of the asphaltene gradients, the Flory–Huggins–Zuo Equation of State is used. This formalism relies on using proposed asphaltene colloidal sizes from the Yen–Mullins Model of asphaltenes. In particular, in the reservoir crude oil in Sand A, asphaltenes were presumed to be dispersed as ∼2nm nanoaggregates, which is typical for corresponding black oils. In this case, the Sand A crude oil was shown to be equilibrated thereby indicating reservoir connectivity, which was subsequently proven in oil production. This new analytic methodology is shown to complement a variety of more traditional analyses and is shown to be superior in analyzing the most important reservoir properties. The combination of new petroleum science and new measurement capabilities is yielding many important advances in reservoir evaluation including understanding of fluid-flow connectivity, viscosity gradients, tar mat formation and large gradients associated with fluid disequilibrium.

Singlet–Triplet and Triplet–Triplet Transitions of Asphaltene PAHs by Molecular Orbital Calculations

Two important features of the molecular structure of asphaltenes remain unresolved; the size distribution of the asphaltene polycyclic aromatic hydrocarbons (PAHs) and the number of PAHs per asphaltene molecule. The relatively small molecular weight of asphaltenes restricts the PAH size; if there are several PAHs per asphaltene molecule, they must be rather small. Optical spectroscopy especially when coupled with molecular orbital (MO) calculations is an excellent probe of asphaltene PAH populations and thus asphaltene molecular architecture. Previously, singlet–singlet transitions for asphaltenes were analyzed using both experiment and MO theory. Here, we describe MO calculations performed to treat triplet–triplet transitions from the ground triplet state for 103 PAHs. Qualitative comparisons with corresponding triplet–triplet transition measurements for asphaltenes are discussed. In addition, spin-forbidden transitions between the singlet and the triplet states, corresponding to phosphorescence, are calculated and discussed in terms of the probability of intersystem crossing of PAHs in asphaltenes and crude oils. Conclusions obtained here are consistent with the corresponding study of singlet–singlet transitions and support the model of a single, relatively large PAH per asphaltene molecule as the predominant asphaltene molecular architecture: the island model. This is consistent with a most probable asphaltene PAH of seven fused aromatic rings (7FAR) with a width of four to ten fused aromatic rings (4FAR-10FAR). This molecular architecture is a central feature of the Yen–Mullins model of asphaltene nanoscience.

Interfacial Rheology of Asphaltenes at Oil–Water Interfaces and Interpretation of the Equation of State

In an earlier study, oil-water interfacial tension was measured by the pendant drop technique for a range of oil-phase asphaltene concentrations and viscosities. The interfacial tension was found to be related to the relative surface coverage during droplet expansion. The relationship was independent of aging time and bulk asphaltenes concentration, suggesting that cross-linking did not occur at the interface and that only asphaltene monomers were adsorbed. The present study extends this work to measurements of interfacial rheology with the same fluids. Dilatation moduli have been measured using the pulsating droplet technique at different frequencies, different concentrations (below and above CNAC), and different aging times. Care was taken to apply the technique in conditions where viscous and inertial effects are small. The elastic modulus increases with frequency and then plateaus to an asymptotic value. The asymptotic or instantaneous elasticity has been plotted against the interfacial tension, indicating the existence of a unique relationship, between them, independent of adsorption conditions. The relationship between interfacial tension and surface coverage is analyzed with a Langmuir equation of state. The equation of state also enabled the prediction of the observed relationship between the instantaneous elasticity and interfacial tension. The fit by a simple Langmuir equation of state (EOS) suggests minimal effects of aging and of nanoaggregates or gel formation at the interface. Only one parameter is involved in the fit, which is the surface excess coverage Γ∞ = 3.2 molecules/nm(2) (31.25 Å(2)/molecule). This value appears to agree with flat-on adsorption of monomeric asphaltene structures consisting of aromatic cores composed of an average of six fused rings and supports the hypothesis that nanoaggregates do not adsorb on the interface. The observed interfacial effects of the adsorbed asphaltenes, correlated by the Langmuir EOS, are consistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen-Mullins model.

Clusters of Asphaltene Nanoaggregates Observed in Oilfield Reservoirs

Many studies have indicated that asphaltenes have two hierarchical nanocolloidal species, the nanoaggregate and the cluster of nanoaggregates. These two species, along with the dominant molecular architecture of asphaltenes comprise the Yen–Mullins model of asphaltenes. Delineating different nanocolloidal species is a challenge, and moreover, elucidating their corresponding practical importance is a necessity in this applied science. Moreover, it is necessary to continue testing this asphaltene nanoscience model especially for crude oils, as opposed to simply asphaltene solutions in laboratory solvents. Both the validity and applicability of this model are addressed by observing gravitational gradients of asphaltenes in black oil and mobile heavy oil columns in oilfield reservoirs. In this paper, we examine stacked oil reservoirs in an oilfield in Saudi Arabia using simple predictions from the Flory–Huggins–Zuo equation of state (FHZ EoS), with its foundation in the Yen–Mullins model. The extraordinary fi...

Advances in the Flory–Huggins–Zuo Equation of State for Asphaltene Gradients and Formation Evaluation

Recent advances in the understanding of the molecular and colloidal structure of asphaltenes in crude oils are codified in the Yen–Mullins model of asphaltenes. The Yen–Mullins model has enabled the development of the industry’s first asphaltene equation of state for predicting asphaltene concentration gradients in oil reservoirs, the Flory–Huggins–Zuo equation of state (FHZ EOS). The FHZ EOS is built by adding gravitational forces onto the existing Flory–Huggins regular solution model that has been used widely to model the phase behavior of asphaltene precipitation in the oil and gas industry. For reservoir crude oils with a low gas/oil ratio (GOR), the FHZ EOS reduces predominantly to a simple form, the gravity term only, and for mobile heavy oil, the gravity term simply uses asphaltene clusters. The FHZ EOS has successfully been employed to estimate the concentration gradients of asphaltenes and/or heavy ends in different crude oil columns around the world, thus evaluating the reservoir connectivity, w...

Advances in Asphaltene Science and the Yen–Mullins Model

The Yen–Mullins model, also known as the modified Yen model, specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents and consists of the following: The most probable asphaltene molecular weight is ∼750 g/mol, with the island molecular architecture dominant. At sufficient concentration, asphaltene molecules form nanoaggregates with an aggregation number less than 10. At higher concentrations, nanoaggregates form clusters again with small aggregation numbers. The Yen–Mullins model is consistent with numerous molecular and colloidal studies employing a broad array of methodologies. Moreover, the Yen–Mullins model provides a foundation for the development of the first asphaltene equation of state for predicting asphaltene gradients in oil reservoirs, the Flory–Huggins–Zuo equation of state (FHZ EoS). In turn, the FHZ EoS has proven applicability in oil reservoirs containing condensates, black oils, and heavy oils. While the development of the Yen–Mullin...

Asphaltene Grading and Tar Mats in Oil Reservoirs

Advances in asphaltene science and a new generation of downhole fluid analysis (DFA) technology have been combined to yield powerful new insights to reservoir tar mats. The asphaltene nanoscience model, the modified Yen model, also known as the Yen–Mullins model, has enabled development of the industry’s first predictive equation of state for asphaltene concentration gradients. This equation of state (EOS) is a modified Flory–Huggins regular solution model for the asphaltene part that has been referred to as the Flory–Huggins–Zuo (FHZ) EOS for asphaltene concentration gradients in oil reservoirs. Measurement of these gradients using “downhole fluid analysis” coupled with analysis using the FHZ EOS has successfully addressed a variety of reservoir concerns including reservoir connectivity, viscosity gradients, and fluid disequilibrium. The EOS model shows that asphaltene concentration gradients can be large owing to both the gravity term and gas/oil ratio (GOR) gradients. The FHZ EOS is reduced to a very simple form—the gravity term only for low GOR black oils and heavy oils—and heavy oils are shown to exhibit enormous asphaltene concentration gradients in contrast to predictions from conventional models. In this paper, the FHZ EOS has been applied not only to calculate asphaltene concentration gradients but also to predict asphaltene phase instability in oil reservoirs. Two types of tar mats are discussed: one with a large discontinuous increase in asphaltene concentration versus depth typically at the base of an oil column (corresponding to asphaltene phase transition); the second with a continuous increase in asphaltene content at the base of a heavy oil column due to an exponential increase in viscosity with asphaltene content. Both types of tar mats are consistent with the Yen–Mullins model of asphaltenes within the FHZ EOS analysis discussed herein. The predictions are in good agreement with the laboratory and field observations, and the mechanisms of forming these two kinds of tar mats are also discussed. This methodology establishes a powerful new approach for conducting the analyses of asphaltene concentration grading and tar mat formation in oil reservoirs by integrating the Yen–Mullins model and the FHZ EOS with DFA technology.

The Asphaltenes

Asphaltenes, the most aromatic of the heaviest components of crude oil, are critical to all aspects of petroleum utilization, including reservoir characterization, production, transportation, refining, upgrading, paving, and coating materials. The asphaltenes, which are solid, have or impart crucial and often deleterious attributes in fluids such as high viscosity, emulsion stability, low distillate yields, and inopportune phase separation. Nevertheless, fundamental uncertainties had precluded a first-principles approach to asphaltenes until now. Recently, asphaltene science has undergone a renaissance; many basic molecular and nanocolloidal properties have been resolved and codified in the modified Yen model (also known as the Yen-Mullins model), thereby enabling predictive asphaltene science. Advances in analytical chemistry, especially mass spectrometry, enable the identification of tens of thousands of distinct chemical species in crude oils and asphaltenes. These and other powerful advances in asphaltene science fall under the banner of petroleomics, which incorporates predictive petroleum science and provides a framework for future developments.