Compositional Simulation of the VAPEX Process
Authors: Nghiem L.X. , Kohse B.F. , Sammon P.H. 
Published: March 2010

Abstract
This paper describes important features of an equation of state compositional simulator that are required for the simulation of the VAPEX process: (1) asphaltenes precipitation, and (2) fluid mixing by molecular diffusion and convective dispersion. The asphaltenes precipitate is modelled as a pure solid that may flow as a suspension in the oil phase, or deposit onto the rock surface. It is shown that this model is capable of modelling the important mechanisms of asphaltenes precipitation in the VAPEX process. The growth of the solvent chamber is governed by molecular diffusion and convective dispersion, with the latter being the dominant mechanism. Sensitivity runs are performed with different dispersivity coefficients, showing their effect on fluid mixing and chamber growth.

Introduction
The VAPEX process, introduced by Butler and Mokrys(1, 2) for the recovery of heavy oils and bitumens, has received a lot of interest in the past few years. This process is attractive in thin reservoirs, where the application of steam injection is impractical because of potential excessive heat loss to the under- and overburden. The VAPEX process utilises two horizontal wells, as in the Steam Assisted Gravity Drainage (SAGD) process, but with the steam chamber replaced by a hydrocarbon vapour chamber. The desirable solvent for the VAPEX process is propane, although butane and mixtures of propane and butane have also been considered(3). The important physics in the VAPEX process include the mixing of solvent and oil at the vapour chamber boundary, and possible precipitation of asphaltenes. The mixing of solvent and oil is through molecular diffusion and convective dispersion. Asphaltenes precipitation occurs particularly with propane-rich solvents, and provides an in situ upgrading of heavy oil and bitumen.
An equation of state (EOS) compositional simulator has been enhanced to include the modelling of asphaltenes precipitation, molecular diffusion, and convective dispersion. Simulation of a typical VAPEX process using Lindbergh oil is performed to study asphaltenes precipitation, and mixing through molecular diffusion and convective dispersion. This paper presents the equations used in the modelling, and shows that the model can reproduce the important phenomena associated with asphaltenes precipitation and fluid mixing observed in laboratories.

Modelling Methods
Phase Equilibrium
The modelling of the phase behaviour of asphaltenes with a pure solid model has been described in previous publications(4, 5), where the effects of pressure and composition on asphaltenes precipitation were examined. The key step in the modelling of asphaltenes is the split of the heaviest component in the oil (e.g., C31+) into a non-precipitating component (C31A+) and a precipitating component (C31B+).These two components have identical critical properties and acentric factors, but different interaction coefficients with the light components. The precipitating component has larger interaction coefficients with the light components. With larger interaction coefficients, the precipitating component is more "incompatible" with the light components, and tends to precipitate as the amount of light component in solution increases. The oil/gas/solid phase equilibrium equations are:
Equation (1a) Available In Full Paper
Equation (1b)Available In Full Paper