Simplified Compositional Formulation for Modified Black-Oil Simulators

Abstract

ABSTRACT This paper gives an evaluation of simplified compositional PVT formulations presently being used in modified black-oil reservoir simulators. In particular we consider a promising four-component general black-oil (GBO) formulation. This formulation has been successfully used in full-field simulations of several North Sea reservoirs containing near-critical fluids with composition that varies areally and with depth. The four "components" used in the GBO formulation are: non-volatile surface oil volatile surface oil surface hydrocarbon gas injection gas (e.g., N2) This formulation also models revaporization in reservoirs undergoing dry gas and/or non-hydrocarbon gas injection. We first review the standard black-oil (BO) model and the modified black-oil model (MBO) based on solution oil-gas ratio (rs). Next, the GBO is introduced for reservoirs where only depletion is considered and therefore the injection gas component is not present. Finally, the four-component GBO formulation is developed for reservoirs undergoing gas injection and where the initial fluid composition may vary throughout the reservoir. The GBO model requires PVT properties of hypothetical "binary" mixtures which are used to obtain actual fluid behavior. In the appendices we give methods for calculating these binary PVT properties based on correlations, material balance equations, and non-linear regression.