A Complex PVT Modeling Study

Abstract

ABSTRACT Oseberg, which is one of the major Norwegian North Sea oil fields, will be produced by injection of associated gas and gas supplied through a pipeline from the Troll field. The two gases have different compositions and neither of them is in thermodynamical equilibrium with the in situ oil. By static multiple contact experiments it has been demonstrated that large component exchanges will occur. Other factors of importance for the reservoir process is a pronounced compositional variation with depth in the oil zone and the existence of a gas cap. A number of relevant measurements and PVT experiments (constant composition expansions and multiple contacts) have been performed in order to obtain data for tuning a qubic Equation of State (EOS) PVT program which is composed of a PVT model and a set of equations. The PVT model consists of a set of well defined pure and pseudo components. Since the PVT program will form the basis for dynamical compositional simulations all the fluids involved must be characterized within a fixed component system. Various approaches have been tried, among them a multivariate lumping scheme. After regression upon 49 experimental data from 4 fluid samples, four PVT models were obtained; two with 6 and two with 22 components. The paper will discuss the choice of experiments and the ideas behind the selection of models, including the choice of pseudo components and regression variables.