Three Dimensional Compositional Variation in Gas Condensate Reservoirs

Abstract

Gas condensate reservoirs are a major player in the oil and gas industry. Proper understanding of reservoir fluid composition and their spatial distribution helps define our resource base and forecast field production profile. In a gas accumulation with substantial vertical span, a composition gradient is expected, where heavier molecules gravitate towards the bottom end and lighter components' concentration increases towards the top of the column. However, in the literature some investigators have reported both condensate content and heavy hydrocarbon components decrease with increasing depth and temperature. Their observations appear to be contradicting the expected gravity driven compositional gradient as per thermodynamic equilibrium. In this paper, we have done an integrated areal and vertical composition trend analysis of some gas condensate reservoirs across a big area i.e. a three-dimensional trend analysis. Field wide areal and vertical reservoir anhydrite content has been mapped. The areal composition gradient that defies expected gravity driven composition gradient, while honoring vertical thermodynamic equilibrium has been reconciled. Post depositional fluid/rock thermochemical reaction and low areal diffusivity has resulted in this areal variation. In-situ H2S generation has been enhanced where reservoir deepens and temperature increases, thus overcoming the activation energy hump. H2S concentration increase has happened at the expense of the hydrocarbon concentration as per the thermochemical reaction stoichiometry. It was found that the vertical compositional gradient is consistent with thermodynamic equilibrium calculations. Some observed trend anomalies have been explained using rock composition and geological information. Thus, a three dimensional compositional gradient was deciphered for reservoirs with such behavior. The aim of this study has been to provide a better quality forecast for reserves, rate, recovery and composition mix for gas condensate fields.