Initial state of petroleum reservoirs: A comprehensive approach

Abstract

After their migration into the trap, the fluids are shaped by various forces; among them the gravity has the most striking effect and was widely studied. In many cases there is evidence for the contribution of other forces like thermal gradient and fluxes. The fluxes originate from migration, leakages, natural convection, chemical reactions and biodegradation phenomena. Taking into account all the phenomena in order to establish a consistent picture of fluid distribution in the field is an important challenge for petroleum industry. The aim of this paper is not to explain where these fluxes come from but to model the consequences on the fluid distribution and on the pressure field in the reservoir. Reciprocally the actual fluid distribution and the pressure gradient can be used to assess the connectivity of the different panels and layers. There is already a number of PVT software available for calculating the compositional gradient within a reservoir but none of them take external fluxes into account. With the available software the calculated profile differs from the actual one. The discrepancies are often due to flux of light component at the reservoir boundaries. In this paper, we present the modeling of the stationary state of a reservoir submitted to all possible external forces. The basic equations are developed and discussed. Drawing on a field example, this paper provides a methodology for dealing with complex reservoir fluid systems. The model matches the observed compositional gradient and corresponding PVT properties. It allows reliable connectivity assessment and it gives reliable values of the magnitude of external fluxes or hydrocarbons alteration rates.