Asphaltene Gravitational Gradient in a Deepwater Reservoir as Determined by Downhole Fluid Analysis

Mullins, Oliver C.; Cribbs, Myrt Eugene; Betancourt, Soraya S.; Dubost, Francois Xavier

doi:10.2118/106375-ms

Cited by 22 publications

(4 citation statements)

References 30 publications

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“…10 shows the Downhole Fluid Analysis (DFA) optical spectra acquired in situ in wells of crude oils at various locations in the Tahiti filed, deepwater Gulf of Mexico. DFA is a relatively new technology that has importance in sample acquisition (Mullins and Schroer 2000), characterization of reservoir compositional variation of equilibrated light ends (Fujisawa et al 2004;Dubost et al 2007), of equilibrated heavy ends (Mullins et al 2007a), of disequilibrium Elshahawi et al 2007), and for compartments . Fig.…”

Section: Asphaltene Nanoaggregate Clusteringmentioning

confidence: 99%

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Mullins

2008

SPE Journal

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116

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Summary Tremendous strides have been made recently in asphaltene science. Many advanced analytical techniques have been applied recently to asphaltenes, elucidating many asphaltene properties. The inability of certain techniques to provide correct asphaltene parameters has also been clarified. Longstanding controversies have been resolved. For example, molecular structural issues of asphaltenes have been resolved; in particular, asphaltene molecular weight is now known. The primary aggregation threshold has recently been established by a variety of techniques. Characterization of asphaltene interfacial activity has advanced considerably. The hierarchy of asphaltene aggregation has emerged into a fairly comprehensive picture, essentially in accord with the Yen model with the additional inclusion of certain constraints. Crude oil and asphaltene science is now poised to develop proper structure-function relations that are the defining objective of the new field: petroleomics. The purpose of this paper is to review these developments in order to present a more clear and accessible picture of asphaltenes, especially considering that the asphaltene literature is a bit opaque. Introduction The asphaltenes are a very important class of compounds in crude oils (Chilingarian and Yen 1978; Bunger and Li 1981; Sheu and Mullins 1995; Mullins and Sheu 1998; Mullins et al. 2007c). The asphaltenes represent a complex mixture of compounds and are defined by their solubility characteristics, not by a specific chemical classification. A common (laboratory) definition of asphaltenes is that they are toluene soluble, n-heptane insoluble. Other light alkanes are sometimes used to isolate asphaltenes. This solubility classification is very useful for crude oils because it captures the most aromatic portion of crude oil. As we will see, this solubility defintion also captures those molecular components of asphaltene that aggregate. Other carbonaceous materials such as coal do possess an asphaltene fraction, but that often will not correspond to the most aromatic fraction. Petroleum asphaltenes, the subject of this paper, can undergo phase transitions that are an impediment in the production of crude oil. Fig. 1 shows a picture of an asphaltene deposit in a pipeline; obviously, asphaltene deposition is detrimental to the production of oil. Immediately it becomes evident that different operational definitions apply for the term asphaltene in the field vs. the lab. Indeed, the field deposit is very enriched in n-heptane-insoluble, toluene-soluble materials, but this field asphaltene deposit is not identically the standard laboratory solubility class. It is common knowledge that a pressure drop on certain live crude oils (containing dissolved gas) can cause asphaltene flocculation, the first step in creating deposits that are seen in Fig. 1. Highly compressible, very undersaturated crude oils are most susceptible to asphaltene deposition problems with a pressure drop (de Boer et al. 1995). In depressurization flocculation, the character of the asphaltene flocs is dependent on the extent of pressure drop, suggesting some variations in the corresponding chemical composition (Hammami et al. 2000; Joshi et al. 2001). Comingling different oils can result in asphaltene precipitation that can resemble solvent precipitation. Asphaltenes are hydrogen-deficient compared to alkanes; thus, either hydrogen must be added or coke removed in crude oil refining to generate transportation fuels. Thus, asphaltene content lowers the economic value of crude oil. Increasing asphaltene content is associated with dramatically increasing viscosity, especially at room temperature; again, this is of operational concern. The strong temperature dependence of viscosity of asphaltic materials is one of their important properties that make them useful for paving and coating; application of asphaltic materials is facile at moderately high temperatures, while desired rheological properties are obtained at ambient temperatures.

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Section: Asphaltene Nanoaggregate Clusteringmentioning

confidence: 99%

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Mullins

2008

SPE Journal

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116

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“…Oil Compositional Gradients. Mullins et al [3][4][5][6] suggested oil compositional gradients (i.e. asphaltene gravitational gradients) should be taken into account in the case of fields that have large structural depth.…”

Section: Conventional Explanation For Anomalous Aop Resultsmentioning

confidence: 99%

Geology and Geohistory Contribute to Flow Assurance

Yonebayashi

Tosic

O'Brien

2012

SPE Europec/Eage Annual Conference

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Kashagan is a super giant offshore carbonate field which was discovered in 2000 by a consortium of oil companies (currently, affiliates of): ExxonMobil, ENI, Shell, TOTAL, Conoco-Phillips, INPEX and KazMunaiGaz. The field is located in an environmentally sensitive area of the North Caspian Sea. The field is a deep, large structural relief, over pressured, isolated, carbonate build-up with a high-permeability, karstified and fractured rim and relatively low-permeability platform interior. The field contains a sour, undersaturated light oil with a large gas content. High pressure miscible gas injection is planned for oil recovery enhancement, as well as sulfur management.No-one doubts the importance of flow assurance in offshore projects in particular. Moreover, it is now well known that gas injection operations require the evaluation of asphaltene deposition risk. The consortium has undertaken extensive evaluations to ascertain the likelihood of any flow assurance risks from subsurface to surface. During the asphaltene risk evaluation, many bottomhole samples have been collected and analyzed for asphaltene content, asphaltene onset pressure (AOP), and SARA (saturates, aromatics, resins and asphaltenes). These continuous analysis efforts have revealed some anomalous results such as AOP being detected from some fluid samples while not being detected from others.The apparently inconsistent AOP results are critical to understand how to guide flow assurance measures. Therefore, all available asphaltene data were re-assessed in all their aspects to attempt to clarify asphaltene risk. This paper presents a multidisciplinary approach where a synergy between reservoir engineering and geoscience (geology and geohistory) has been developed to explain AOP results for this complex fluid. The results should help flow assurance specialists to better define the asphaltene operating envelope, which will be used for reservoir and production operations optimization. In addition, these results should be useful for optimizing data-surveillance, flow assurance, and for defining new sample acquisition plans. These findings may also be helpful to minimize future sampling and fluids analysis while achieving reliable flow assurance. The paper will show examples of the related flow assurance analyses, and the geological information which were incorporated in the study, resulting in a detailed asphaltene matrix risk profile for this reservoir.

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“…Downhole fluid analysis (DFA) has been successfully used to delineate reservoir attributes such as vertical and lateral connectivity and properties of the produced fluids. − DFA not only measures bulk fluid properties such as the gas−oil ratio (GOR), density, and light-end compositions of CO 2 , CH 4 , C 2 H 6 , C 3 H 8 −C 5 H 12 fraction, and hexane and heavier (hexane +) fractions more accurately but also color (optical density, OD) that is linearly related to the heavy ends (asphaltenes and heavy resins) in real time at downhole conditions. In addition, the color measurement is one of the most robust measurements in DFA.…”

Section: Introductionmentioning

confidence: 99%

“…Although asphaltene gradients do not explicitly prove reservoir connectivity, the lack of an asphaltene gradient indicates a lack of connectivity. The recent advances in asphaltene science from the laboratory and field data have demonstrated that asphaltenes are dispersed and/or suspended in crude oils and/or solvents in three forms: molecules, nanoaggregates, and clusters of nanoaggregates. − This provides us with a new foundation for methods of determining flow connectivity in the reservoir. In particular, in situ measurements of color (thus asphaltene content) and GOR (composition) by DFA in the reservoir enable coupling data with new thermodynamic models to determine the state of the asphaltene distribution.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Downhole Asphaltene Gradients in Oil Reservoirs with a New Bimodal Asphaltene Distribution Function

et al. 2011

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Downhole fluid analysis (DFA) has been successfully used to describe reservoir connectivity and fluid properties. DFA not only measures bulk fluid properties such as the gas-oil ratio (GOR), density, and light-end compositions of CO 2 , CH 4 , C 2 H 6 , C 3 H 8 -C 5 H 12 fraction, and hexane and heavier (hexane þ) fractions but also color (optical density) that is related to the heavy ends (asphaltenes and resins) in real time at downhole conditions. Therefore, color gradient analysis in oil columns becomes essential to determine reservoir complexities. In this paper, a new bimodal Γ-distribution function (asphaltene molecules þ nanoaggregates and clusters) was proposed to characterize asphaltene components. A thermodynamic asphaltene-grading model was also developed to describe equilibrium distributions of heavy ends (heavy resins and asphaltenes) in oil columns using the multicomponent Flory-Huggins regular solution model coupled with a gravitational contribution. The variations of oil properties (such as molar volume, molar mass, solubility parameter, and density) with depth were calculated by the equation of state (EOS). The primary factors governing asphaltene distribution in reservoirs are the gravitational term, which is determined in part by the size of the asphaltene colloidal particle, and the solubility term, which is determined in large part by the GOR (composition). Consequently, it is critical to accurately measure both the fluid coloration and the GOR (composition) to understand the asphaltene gradient in oil columns. The results obtained in this work are in accordance with the Yen-Mullins model in asphaltene science. In particular, if asphaltenes are equilibrated in an oil reservoir, then a massive fluid flow through the reservoir had to have taken place, and permeable rocks are required, thereby implying connectivity, because asphaltenes necessarily enter the reservoir out of their ultimate equilibrium at the beginning of the reservoir charging. Therefore, a new powerful approach is established for conducting DFA color and GOR gradient analysis by coupling advanced asphaltene science with DFA technology (profiling of fluids) to address reservoir connectivity.

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Asphaltene Gravitational Gradient in a Deepwater Reservoir as Determined by Downhole Fluid Analysis

Cited by 22 publications

References 30 publications

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Geology and Geohistory Contribute to Flow Assurance

Analysis of Downhole Asphaltene Gradients in Oil Reservoirs with a New Bimodal Asphaltene Distribution Function

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