Oil Composition Variation in a Large Deep Water Field

Abstract

Abstract A new computational model for the non-isothermal gravitational compositional equilibrium is developed and presented. The mathematical formulation is based on the works of Bedrikovetsky (gravity and temperature using irreversible thermodynamics) and Whitson (algorithm). The computational model is validated on published data and previous simplified models. An application case is presented for a reservoir in a large deep water field in Brazil. The magnitude of the calculated oil composition variations is sufficient to explain most observed data. The results suggest that the reservoir is partially connected and that the temperature effect can be as important as the gravity effect on the oil composition variation. The changes are significant and the methodology applied is an example of the application of thermodynamic data to the evaluation of reservoir connectivity and fluid properties distribution under the conditions approaching those encountered in natural reservoirs. Introduction Compositional variations along the hydrocarbon column are observed in many reservoirs around the world. They may affect reservoir/fluid characteristics considerably, such as viscosity, total hydrocarbon volume in place and the development of miscibility, leading to different field development strategies. These variations are caused by many factors, such as gravity, temperature gradient, rock heterogeneity, and hydrocarbon genesis and accumulation processes. In the cases where thermodynamic associated factors are dominant, the existent gravitational compositional equilibrium (GCE) models, which do not properly account for the temperature gradient effect, allow the explanation of most observed variations. However, it is noted that in some cases, the thermal effect could have the same order of magnitude as the gravity effect. In this paper, a non-isothermal compositional gravitational model is developed and presented. The model is validated and applied to some fields in Brazil, among them a large deep water field.