Abstract

Study of three phase equilibrium has been seen from many literatures. Most research onmodeling ofasphaltene precipitation has remained in qualitative description stage. Even though quantitative calculation of asphaltene precipitation can be seen from some reports. However, quantitative calculation of asphaltene precipitation with three phase flash algorithm is unstable because gas-liquid-solid material balance equations is highly non-linear equations. In the light of general view that there are not heavy hydrocarbons with high molecular weight including asphalt in gas phase, technique of stable asphaltene precipitation calculation is put forward by the paper. The recommended technique in the paper separates gas-liquid-solid equilibrium into two parts: gas-liquid and liquid-solid equilibrium. Liquid phase is a bridge between gas and solid phase and solid precipitates always from liquid phase. Gas-liquid equilibrium is described through EOS and fugacity of asphaltene determined from gas-liquid equilibrium should be compared and accord with fugacity of pure solid phase of asphaltene. In this way, modeling of three phase equilibrium becomes easier and more stable. The validation of the model with experimental data is described also in the paper. Additionally, the paper puts forward gas-liqiud-asphaltene material balance equations which can be deduced from general three phase material balance equations and are not similar to them in forms.

Introduction

During oil production, Asphaltene precipitation will cause serious problems because it can result in plugging of the formation, wellbore and production facilities. Many literatures1,2,3,4 gave the description of asphaltene problems and remedies throughout the world. Currently, many cleaning methods of wellbores are being improvised to maintain production, but these methods are time-consuming and expensive. Asphaltene precipitation also occurs frequently during enhanced-oil-recovery by gas injection which impedes seriously the oil recovery. A model for predicting asphaltene precipitation is highly desirable because it would allow the design of production plan with which asphaltene precipitation can be minimized.

Some literatures5,6 presented modeling technique of asphaltene precipitation with EOS. The presented technique can give quantitative calculation of asphaltene precipitation with three phase flash algorithm7. The quantitative calculation with three phase algorithm is unstable because of highly non-linear equations of three phase material balance equations.

According to generic opinion that there are not heavy hydrocarbons with high molecular weight including asphalt in gas phase, new technique of asphaltene precipitation calculation is presented in the paper. The suggested technique in the paper separates vapor-liquid-solid equilibrium into two parts -vapor/liquid and liquid/solid equilibrium. Liquid phase is a bridge between gas and solid phase and solid precipitates always from liquid phase. Vapor/liquid equilibrium is described through EOS and fugacity of asphaltene calculated from vapor/liquid equilibrium should be compared and accord with fugacity of pure solid phase of asphaltene. modeling of asphaltene precipitation with the present technique becomes stable. The validation of the model with experimental data is also proved in the paper.

Mechanism of Asphaltene Precipitation

Many literatures8,9,10,11 hold the same view that asphaltenes are heavy hydrocarbons which are in colloidal suspension in the oil, stabilized by resins adsorbed on their surface. Changes in pressure, temperature and composition may cause asphaltene precipitation. The mechanism of asphaltene precipitation is still under investigation. There are two possibilities2:

  1. the asphaltene/resin micelles precipitate essentially unaltered and

  2. there is a dissociation of the asphaltene/resin micelles which cause the asphaltene precipitation.

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