Asphaltene Precipitation: Phase Behaviour Modelling and Compositional Simulation
Authors: Nghiem Long X. , Kohse Bruce F. , Ali S.M. Farouq , Doan Quang 
Published: December 2007

Abstract
This paper presents results of phase behaviour calculations and compositional simulation of asphaltene precipitation in reservoirs. For phase behaviour calculations, the precipitated asphaltene is represented by a pure solid while the oil and gas phases are modelled with an Equation of State (EOS). Compositional simulation of the dynamics of asphaltene precipitation in porous media includes the flow of suspended solid in the oil phase, deposition of solid through adsorption and entrapment, and plugging. Calculations of asphaltene precipitation for a North Sea oil with hydrocarbon gas, for a Canadian crude with CO2, and for a heavy oil with propane are described. The results are in agreement with laboratory experiments and field observations.

Introduction
Asphaltene precipitation from reservoir fluids during oil production is a serious problem because it can result in plugging of the formation, wellbore and production facilities. Asphaltene precipitation can occur during primary depletion of highly undersaturated reservoirs or during hydrocarbon gas or CO2 injection for improved oil recovery (IOR).
The injection of hydrocarbon gases or CO2 for IOR promotes asphaltene precipitation. Numerous field reports and laboratory studies on this aspect have been published1–8. Precipitation can occur anywhere in the reservoir, although it manifests itself frequently at the production wellbore at solvent breakthrough.
Asphaltene precipitation may also occur during solvent injection into heavy oil reservoirs9. Butler and Mokrys10,11 proposed an in situ solvent extraction process for heavy oils and tar sands called VAPEX. This process uses two horizontal wells (one injector and one producer). The injection of solvent (e.g. propane) creates a solvent chamber where oil is mobilized and drained toward the producer. In addition to the mobilization process, the solvent may also induce asphaltene precipitation, which provides an in situ upgrading of the oil.
The Asphaltene Precipitation Envelope (APE) bounds the region where precipitation occurs12,13. In Refs. 12 and 13, the APE's are referred to as Asphaltene Deposition Envelopes (ADE). In this paper, the term "precipitation" refers to the formation of the asphaltene precipitate as a result of thermodynamic equilibrium, and "deposition" refers to the settling of the precipitated asphaltene onto the rock surface in a porous medium. The onset conditions correspond to points on the APE. Within the APE, the amount of precipitated asphaltene increases as pressure decreases from the upper onset pressure to the saturation pressure of the oil. The precipitation reaches a maximum value at the saturation pressure, and decreases as pressure decreases below the saturation pressure.
Inside the reservoir, after precipitation has occurred, the asphaltene precipitate can remain in suspension and flow within the oil phase, or can deposit onto the rock surface. The main deposition mechanisms are adsorption and mechanical entrapment. The deposited asphaltene may cause plugging of the formation and alteration of rock wettability (from water-wet to oil-wet).
Many thermodynamic models that describe the phase behaviour of asphaltene precipitation have been reported in the literature. These include the use of a liquid solubility model2, a thermodynamic colloidal model14, a thermodynamic micellization model15, a colloidal activity coefficient model16, a variation of a model for wax17,18, or a pure solid model19–21. Nghiem et al.20,21 also describe the incorporation of the pure solid model into an EOS compositional simulator.
This paper focusses on the phase behaviour modelling and compositional simulation of asphaltene precipitation in gas injection process for IOR using a solid model for the asphaltene precipitate. Calculations are performed for three typical IOR process: a North Sea oil with hydrocarbon gas injection, the Weyburn oil with CO2 injection, and the Lindbergh heavy oil with propane injection. These represent examples of the three main solvent IOR processes where asphaltene precipitation normally occurs.