Summary.

A molecular model is developed to predict onset and amount of organic deposition from reservoir fluids caused by variations intemperature and pressure and introduction of miscible solvents. The modelis used successfully to predict the phase behavior and deposition regions of asphaltene in CO2/oil mixtures.

Introduction

The compounds that constitute complex petroleum crudes, coalliquids, and similar substances are mutually soluble as long as acertain ratio of each kind of molecule (or particle) is maintained inthe mixture. Variations in the mixture's temperature, pressure, orcomposition (such as addition of a miscible solvent) alter thisratio. Then the heavy and/or polar molecules may separate from themixture either in the form of another liquid phase or as a solidprecipitate. Hydrogen bonding and the sulfur (and/or nitrogen)containing segments of the separated molecules may start toaggregate (or polymerize) and to produce the irreversible asphaltenedeposits that are insoluble in solvents. Development of predictivetechniques of organic deposition to describe the behavior of largeorganic molecules in hydrocarbon mixtures calls for fundamentaldetailed analyses of such systems. Major questions of interest in the oil industry are when and howmuch organics will flocculate out under certain conditions. Becausepetroleum crude generally consists of a mixture of aromatic andother hydrocarbons (resin, wax, and asphaltenes), each of the constituents of this system can be considered as a continuous ordiscrete mixture interacting with the other constituents as pseudopurecomponents. The theory of continuous mixtures, the statistical mechanicaltheory of monomer/polymer solutions, the concept of Hildebrand's solubilityparameter, and the concept of pseudoizations are used here to analyze andpredict the onset and amount of organic precipitation in petroleum crudes. Because heavy organic particles in petroleum crudes have a widerange of size, or molecular weight, distribution, one may considereach crude family as a heterogeneous (polydisperse) polymer. Then, to predict the behavior of such compounds, one can assume thatthe properties of their fractions depend on their molecular weights. Mansoori and Jiang initially proposed this treatment of heavyorganics in petroleum fluids. In their proposed formulation, the Scottand Magat theory of polymer mixtures, which is the statisticalthermodynamic model of the mixture of solvents and heterogeneous(polydisperse) polymers, was used. In this paper, the proposedmodel of Mansoori and Jiang is applied to predict asphaltenedeposition from petroleum fluids. Similar calculations can be performedfor deposition prediction of other organic macromolecules.

Asphaltene Deposition

The petroleum industry defines the asphaltene content of a crudeas the normal-pentane-insoluble and benzene-soluble fraction of thecrude. The exact chemical structure of asphaltenes is notknown. On heating, they are not melted but decompose, formingcarbon and volatile products above 30 to 400C. They react with sulfuricacid to form sulfonic acids, as might be expected on thebasis of the polyaromatic structure of these compounds. The color of dissolved asphaltene in benzene is deep red at lowconcentrations. At around 3 ppm asphaltene concentration in benzene, thesolution is distinctly yellow.

While solutions to the problems associated with the deposition of nonasphaltic organic compounds from petroleum fluids are mostlyunderstood, the asphaltene deposition problem remains a mystery. The devastating effect of asphaltene deposition in the economy of petroleumprocessing and oil recovery techniques is well recognized. Asphaltenedeposition during oil production and processing is a very serious problemin many areas throughout the world. The presence of asphaltene inpetroleum crudes causes a number of severe technological problems. One suchproblem is the untimely precipitation of asphaltene in the petroleumreservoir; in the wells, tubings, and pipelines; and in the refinerycomponents. Currently, mechanical and chemical cleaning methods are beingimprovised to remove asphaltene deposits and to maintain production, transportation, and processing of petroleum. According to Long, asphaltenes are highly polydisperse andcontain a broad distribution of polar groups in their structure. Theaverage molecular weight of asphaltenes present in petroleum crudesis generally very high. Published molecular-weight data forpetroleum asphaltenes range from about 500 to 500,000. The wide range of asphaltene size distribution suggests that asphaltenes are partlydissolved in oil and partly in colloidal state. The colloidal asphaltenesare believed to be dispersed and stabilized primarily by resinmolecules present in oil that are adsorbed on asphaltene surface. The degree of dispersion of asphaltenes in petroleum oils dependson the chemical composition of the petroleum. In heavy andhighly aromatic crude oils, the asphaltenes are well dispersed, but inthe presence of an excess of petroleum ether and similar paraffinichydrocarbons, they are coagulated and then precipitate. In developing a comprehensive model of asphaltene deposition, we have considered a number of theoretical approaches, includingcolloidal solution theories, polydisperse polymer solutiontheories, continuous thermodynamics, and fractal aggregationtheories. The theoretical technique proposed here is part of ourcontinuing effort to develop a comprehensive model of asphaltenedeposition.

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 witha segment of either a polymer or a solvent molecule, and discussedthe theory qualitatively. later, Flory and Hugginsindependently developed thermodynamic models of the lattice theory forhomogeneous polymer solutions-i.e., the solution containinguniform polymer molecules in a solvent in which the partial molarentropies of mixing are obtained by use of the lattice theory. Furthermore, Flory applied his lattice theory to homogeneouschain-polymer solutions and used the van Laar's rule for calculation of theheat of mixing. Then, by combining the entropy and heat of mixing, he derived the expression of the partial molar free energyfor the homogeneous polymer solutions. Later, Scott and Magatproposed a statistical mechanical method to derive expressions forpartial molar free energies of heterogeneous polymer solutions.

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