A NEW APPROACH OF COMPOSITIONAL SIMULATION FOR A VOLATILE OIL RESERVOIR MODELING

Abstract

This paper proposes a new compositional simulation approach for a volatile oil reservoir
modelling. The proposed formulation has an implicit equation for the oil-phase pressure
and water saturation, an explicit equation for the hydrocarbon saturation, and explicit
equation for the overall composition of each hydrocarbon component that satisfies
thermodynamic equilibrium. An Equation of State for phase equilibrium and property calculations
is used in this new formulation. Interfacial tension effects are included in this
approach to characterise the thermodynamically dynamic nature of the relative permeability.
A two-dimensional relative permeability algorithm is included which handles lumped
hydrocarbon phase hydrocarbon phase as well as individual phase flows. For each grid
block two equations are required, namely total hydrocarbon and water-phase flow equations.
These equations are highly non-linear and they are linearised by using Newton-
Raphson method. The resulting equations are solved by an efficient Conjugate Gradient
based iterative technique to obtain pressures and saturations simultaneously, and hydrocarbon-
phase saturations are deduced from their respective equations.
The new compositional simulation approach is validated through analytical and numerical
methods. It is demonstrated in this present paper that the results are compared
favourably with analytical techniques and published numerical results. They also confirm
that the proposed codified formulation is unconditionally stable and it is as stable as fully
compositional model yet the computational cost reduction was substantial.