Flory Huggins Zuo Research Articles

Gravitational Gradient of Asphaltene Molecules in an Oilfield Reservoir with Light Oil

In toluene, asphaltenes are dispersed as molecules at low concentrations, as nanoaggregates at moderate concentrations, and as clusters of nanoaggregates at high concentrations. These three asphaltene species are codified in the Yen–Mullins model. For reservoir crude oils, equilibrated asphaltene gradients can be modeled with the Flory–Huggins–Zuo equation of state (EoS). The gravity term and other terms depend on the particle sizes of the asphaltenes which are given in the Yen–Mullins model; these different asphaltene species (molecular and two nanocolloidal species) have been identified in gravity gradients in various reservoir studies. Here, the asphaltene gradient in a large reservoir is examined and found to be consistent with a molecular dispersion of asphaltenes in the crude oil. A variety of fluid and reservoir properties are evaluated to ensure validity of the analysis, particularly of thermodynamic equilibrium of the reservoir fluid. For crude oil samples throughout the reservoir, downhole fluid analysis (DFA), gas chromatography (GC), and two-dimensional gas chromatography (GC×GC) with cubic EoS and geochemical interpretation are consistent with fluid equilibration. Pressure measurement and production results are also consistent with fluid equilibration. This analysis is applicable to other reservoirs; molecular dispersions of asphaltenes are expected for other light oil reservoirs.

Asphaltene Densities and Solubility Parameter Distributions: Impact on Asphaltene Gradients

Analysis of spatial gradients of the concentration of asphaltenes in reservoir crude oils has become an important tool for oilfield reservoir characterization. Modeling these gradients requires knowledge of asphaltene properties. For example, the commonly employed Flory–Huggins–Zuo (FHZ) equation requires the asphaltene particle size, solubility parameter, and density. Asphaltenes from various sources, particularly including sources beyond conventional crude oil, can have widely varying compositions such as the H/C ratio. Here, we measured the solubility parameters and densities of five asphaltenes that span a large range of H/C ratios, including asphaltenes from coal, petroleum, and shale, and then performed a sensitivity analysis to examine how that range of compositions impacts the magnitude of asphaltene gradients as predicted by the FHZ equation. Two case studies are included in the sensitivity analysis, one in which the asphaltene gradient is driven primarily by gravity, and another in which the asp...

A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

The heavy oil rim of a large Saudi Arabian oilfield has been shown to be in vertical and lateral equilibrium, matching predictions of the gravity term from the Flory–Huggins–Zuo equation of state for asphaltenes in the form of 5.2 nm clusters of the Yen–Mullins model. The large (10×) vertical gradient of asphaltene concentration over a very large perimeter (≫10 km) of the oilfield provided a stringent test of this equation-of-state fit. Two-dimensional gas chromatography (GC × GC) and stable isotope analysis (δ D and δ 13C) were used to determine the consistency of the liquid-phase components with equilibration and the effects of biodegradation or thermal maturity on the observed asphaltene gradient. These analyses confirm homogeneity of equilibrated liquid-phase components of similar chemical character and equilibrated asphaltene isotopes. Biodegradation is minimal and there is no maturity variation among the samples. Thus, the large asphaltene gradient did not result from these secondary processes and is not a remnant from how the reservoir was charged with crude oil. The results are consistent with original findings that the oil column is equilibrated. Thermodynamic equilibration over such large distances (>10 km) requires convective currents and provides constraints on fluid dynamic processes in reservoirs. A simple one-dimensional (1-D) three-component single-phase model is introduced to account for asphaltene accumulation by way of convective currents established from a diffusive gas front at the top of the oil column.

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

A very large monotonic variation of asphaltenes and viscosity has been measured by downhole fluid analysis (DFA) in crude oils in five-stacked sandstone reservoirs in the northern part of the Barmer Basin, northwest India, undergoing active biodegradation; each of the five-layered sand bodies shows overlaying fluid gradients with depth providing replicate validation of the measurements. Fluid data from four wells across the field shows that the gradients are uniform across the formation. The crude oil in the upper half of the oil column exhibits an equilibrium distribution of asphaltenes matching predictions of the Flory–Huggins–Zuo Equation of state (FHZ EoS) with the gravity term only using asphaltene nanoaggregates of the Yen–Mullins model. However, the bottom half of the reservoir reveals a large asphaltene gradient approximately three times larger than the equilibrium predictions from the FHZ EoS. This increase in asphaltenes creates a very large (8×) viscosity gradient and is a major production conc...

Heuristics for Equilibrium Distributions of Asphaltenes in the Presence of GOR Gradients

Production of crude oil from subsurface reservoirs is greatly impacted by many complexities such as reservoir fluid flow, connectivity, viscosity gradients, and tar mat formation. In situ fluid analysis in oil wells has enabled facile measurement of fluid gradients of dissolved gases and dissolved solids in reservoir crude oils; these gradients have proven very useful for analysis of reservoir complexities. The analysis of solution gas generally uses the cubic equation of state. However, until recently, there had been no predictive equation of state to model asphaltene gradients. Recently, the different nanostructures of asphaltenes in crude oil have largely been resolved and codified in the Yen–Mullins model. In turn, this has enabled equation of state development for asphaltene gradients in crude oils. For example, the Flory–Huggins–Zuo EoS is now ubiquitously utilized in modeling asphaltene gradients. Here, the magnitude and dependencies of the three terms of this EoS, gravity, solubility, and entropy, are considered in detail. Simple expressions for ratios of these terms are obtained as a function of the gas/oil ratio of the crude oils. In particular, the transition from gravity dominance to solubility dominance is examined. A variety of heuristics are developed to guide interpretation of asphaltene gradients that are so routinely measured. Expressions are given that could be used for real-time interpretation upon measurement of these gradients. The utility of EoS modeling of asphaltene gradients is significantly enhanced when incorporating these heuristics.

Downhole Fluid Analysis and Asphaltene Science for Petroleum Reservoir Evaluation

Petroleum reservoirs are enshrouded in mysteries associated with all manner of geologic and fluid complexities that Mother Nature can inspire. Efficient exploitation of petroleum reservoirs mandates elucidation of these complexities; downhole fluid analysis (DFA) has proven to be indispensable for understanding both fluids and reservoir architecture. Crude oil consists of dissolved gases, liquids, and dissolved solids, known as the asphaltenes. These different fluid components exhibit fluid gradients vertically and laterally, which are best revealed by DFA, with its excellent precision and accuracy. Compositional gradient analysis falls within the purview of thermodynamics. Gas-liquid equilibria can be treated with a cubic equation of state (EoS), such as the Peng-Robinson EoS, a modified van der Waals EoS. In contrast, the first EoS for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS, was developed only recently. The resolution of the asphaltene molecular and nanocolloidal species in crude oil, which is codified in the Yen-Mullins model of asphaltenes, enabled the development of this EoS. The combination of DFA characterization of gradients of reservoir crude oil with the cubic EoS and FHZ EoS analyses brings into view wide-ranging reservoir concerns, such as reservoir connectivity, fault-block migration, heavy oil gradients, tar mat formation, huge disequilibrium fluid gradients, and even stochastic variations of reservoir fluids. New petroleum science and DFA technology are helping to offset the increasing costs and technical difficulties of exploiting ever-more-remote petroleum reservoirs.

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.

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.

Asphaltene gradients and tar mat formation in reservoirs under active gas charging

Based on downhole fluid analysis (DFA) measurements and advanced asphaltene science, a new modified Flory–Huggins regular solution model that has been referred to as the Flory–Huggins–Zuo equation of state (FHZ EOS) has successfully been developed and used to delineate reservoir connectivity recently. It provides the industry's first simple explicit EOS for predicting asphaltene grading and has proven reliable to predict connectivity in equilibrated oil columns. In this paper, the methodology for equilibrated reservoirs is extended to a nonequilibrium oil column, especially for a reservoir under active charging of biogenic gas. Based on the properties computed by the Peng–Robinson EOS with methane influx, the FHZ EOS for asphaltenes – originally formulated for equilibrium columns – is used to model the asphaltene (optical density) gradient in the nonequilibrium oil column. In addition, the FHZ EOS with the same parameters is employed to check whether the predicted asphaltenes can be stably dispersed or suspended in the oil. The predicted results show that the asphaltenes at the base of the oil column are unstable because late gas charging (diffusing downward) results in a reduction of oil solvation power for asphaltenes and makes asphaltenes concentrated near the base of the oil column. The concentrated asphaltenes are too much for the oil and they tend to be flocculated out to form an asphaltene-rich phase and a tar mat could form if asphaltene flocculation is severe over geological time.