Place de la thermodiffusion dans l'évaluation de l'état initial des gisements pétroliers

After their migration into the trap, the fluids are shaped by various forces, among them, gravity has the most striking effect and was widely studied. Yet, in the case of diphasic reservoirs, the observed variations of composition are significantly stronger than those calculated with this approach. The contribution of other forces like the thermal gradient is more difficult to assess because of the lack of reliable data on thermodiffusion coefficients. At a first look, the effect of geothermal gradient does not account for the gap between data and gravitational profile. In the case of a complex diphasic fluid column in a reservoir from Guinea Gulf, we concluded to the presence of a dynamic transition zone between the oil and the gas zone due to thermo-convection in the gas phase. As a consequence, an increase of the compositionnal gradients in the oil zone close to the interface is likely. The on-going studies on thermodiffusion phenomena should soon provide a reliable modelling of such phenomena.