Abstract

Asphaltene deposit formation is a severe problem frequently faced during crude oil production that affects the reservoir, production tubing and facilities, depending on the stage where this critical point is reached. This point corresponds to the pressure at which asphaltene particles start to form and directly related to the oil chemical physical stability.

Modeling of asphaltene precipitation in the reservoir porous media and in the oil well tubing of an Iranian south oilfield is the purpose of this study. For the reservoir, the modeling technique based on the representation of the precipitated asphaltene as a pure dense phase and division of the heaviest component in the oil into non-precipitating and precipitating components. A set of dummy experimental data was used to tune an equation of state based model that represents the asphaltene as pure component solid. Results of the work show that the chosen model could accurately represent the behavior of the fluids of the reservoir.

For the oil well tubing, a method based on the change of well flow conditions has been used to predict the onset of asphaltene precipitation. The flowing wellhead pressure (FWHP) data from the case study well gathered during the production history of the well. These data generated useful information concerning the well flow conditions that could influence the onset of asphaltene precipitation.

Introduction

Asphaltene deposit formation is a severe problem frequently faced during crude oil production that affects thereservoir, production tubing and facilities, depending on the stage where this critical point is reached. This point corresponds to the pressure at which asphaltene particles start to form and directly related to the oil chemical physical stability.

The exact chemical structure of asphaltenes is not known. On heating, they are not melted but decompose, forming carbon and volatile products above 300 to 400 C. 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(1).

Asphaltenes are highly polydisperse and contain abroad 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 range from about 500 to 500000.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 paraftinic hydrocarbons, they are coagulated and then precipitate(1).

Reservoir Description

The reservoir of this field started producing in 1989 under primary depletion.

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