Organic Deposition From Reservoir Fluids: A Thermodynamic Predictive Technique
- Seido Kawanaka (U. of Illinois) | S.J. Park (U. of Illinois) | G.A. Mansoori (U. of Illinois)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1991
- Document Type
- Journal Paper
- 185 - 192
- 1991. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 4.2 Pipelines, Flowlines and Risers, 5.2 Reservoir Fluid Dynamics, 5.2.2 Fluid Modeling, Equations of State, 4.1.2 Separation and Treating, 4.6 Natural Gas, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.3.4 Scale, 1.8 Formation Damage, 4.3.3 Aspaltenes
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Summary. A molecular model is developed to predict onset and amount of organic deposition from reservoir fluids caused by variations in temperature and pressure and introduction of miscible solvents. The model is used successfully to predict the phase behavior and deposition regions of asphaltene in CO2/oil mixtures.
The compounds that constitute complex petroleum crudes, coal liquids, and similar substances are mutually soluble as long as a certain ratio of each kind of molecule (or particle) is maintained in the mixture. Variations in the mixture's temperature, pressure, or composition (such as addition of a miscible solvent) alter this ratio. Then the heavy and/or polar molecules may separate from the mixture either in the form of another liquid phase or as a solid precipitate. Hydrogen bonding and the sulfur (and/or nitrogen) containing segments of the separated molecules may start to aggregate (or polymerize) and to produce the irreversible asphaltene deposits that are insoluble in solvents. Development of predictive techniques of organic deposition to describe the behavior of large organic molecules in hydrocarbon mixtures calls for fundamental detailed analyses of such systems. Major questions of interest in the oil industry are when and how much organics will flocculate out under certain conditions. Because petroleum crude generally consists of a mixture of aromatic and other hydrocarbons (resin, wax, and asphaltenes), each of the constituents of this system can be considered as a continuous or discrete mixture interacting with the other constituents as pseudopure components. The theory of continuous mixtures, the statistical mechanical theory of monomer/polymer solutions, the concept of Hildebrand's solubility parameter, and the concept of pseudoizations are used here to analyze and predict the onset and amount of organic precipitation in petroleum crudes. Because heavy organic particles in petroleum crudes have a wide range of size, or molecular weight, distribution, one may consider each crude family as a heterogeneous (polydisperse) polymer. Then, to predict the behavior of such compounds, one can assume that the properties of their fractions depend on their molecular weights. Mansoori and Jiang initially proposed this treatment of heavy organics in petroleum fluids. In their proposed formulation, the Scott and Magat theory of polymer mixtures, which is the statistical thermodynamic model of the mixture of solvents and heterogeneous (polydisperse) polymers, was used. In this paper, the proposed model of Mansoori and Jiang is applied to predict asphaltene deposition from petroleum fluids. Similar calculations can be performed for deposition prediction of other organic macromolecules.
The petroleum industry defines the asphaltene content of a crude as the normal-pentane-insoluble and benzene-soluble fraction of the crude. The exact chemical structure of asphaltenes is not known. On heating, they are not melted but decompose, forming carbon and volatile products above 30 to 400C. They react with sulfuric acid to form sulfonic acids, as might be expected on the basis of the polyaromatic structure of these compounds. The color of dissolved asphaltene in benzene is deep red at low concentrations. At around 3 ppm asphaltene concentration in benzene, the solution is distinctly yellow.
While solutions to the problems associated with the deposition of nonasphaltic organic compounds from petroleum fluids are mostly understood, the asphaltene deposition problem remains a mystery. The devastating effect of asphaltene deposition in the economy of petroleum processing and oil recovery techniques is well recognized. Asphaltene deposition during oil production and processing is a very serious problem in many areas throughout the world. The presence of asphaltene in petroleum crudes causes a number of severe technological problems. One such problem is the untimely precipitation of asphaltene in the petroleum reservoir; in the wells, tubings, and pipelines; and in the refinery components. Currently, mechanical and chemical cleaning methods are being improvised to remove asphaltene deposits and to maintain production, transportation, and processing of petroleum. According to Long, asphaltenes are highly polydisperse and contain a broad distribution of polar groups in their structure. The average molecular weight of asphaltenes present in petroleum crudes is generally very high. Published molecular-weight data for petroleum asphaltenes range from about 500 to 500,000. The wide range of asphaltene size distribution suggests that asphaltenes are partly dissolved in oil and partly in colloidal state. The colloidal asphaltenes are believed to be dispersed and stabilized primarily by resin molecules present in oil that are adsorbed on asphaltene surface. The degree of dispersion of asphaltenes in petroleum oils depends on the chemical composition of the petroleum. In heavy and highly aromatic crude oils, the asphaltenes are well dispersed, but in the presence of an excess of petroleum ether and similar paraffinic hydrocarbons, they are coagulated and then precipitate. In developing a comprehensive model of asphaltene deposition, we have considered a number of theoretical approaches, including colloidal solution theories, polydisperse polymer solution theories, continuous thermodynamics, and fractal aggregation theories. The theoretical technique proposed here is part of our continuing effort to develop a comprehensive model of asphaltene deposition.
Background of Proposed Model
The statistical mechanical theory of mixtures of high-molecular-weight polymer solutions was originally introduced by Meyer, who used hypothetical lattice cells, one of which may be filled with a segment of either a polymer or a solvent molecule, and discussed the theory qualitatively. later, Flory and Huggins independently developed thermodynamic models of the lattice theory for homogeneous polymer solutions-i.e., the solution containing uniform polymer molecules in a solvent in which the partial molar entropies of mixing are obtained by use of the lattice theory. Furthermore, Flory applied his lattice theory to homogeneous chain-polymer solutions and used the van Laar's rule for calculation of the heat of mixing. Then, by combining the entropy and heat of mixing, he derived the expression of the partial molar free energy for the homogeneous polymer solutions. Later, Scott and Magat proposed a statistical mechanical method to derive expressions for partial molar free energies of heterogeneous polymer solutions.
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