The determination of gradients inside a petroleum reservoir is a huge problem for modelling species repartition in order to improve production and future sequestration in empty reservoirs. The shaping of these gradients is mainly determinate by gravitational segregation and thermodiffusion (Soret effect). In spite of a lot of advances, there are few available data on mass transport properties, i.e. diffusion and thermodiffusion coefficients, especially in reservoir conditions; which means the necessity to develop an experimental approach for providing reference data on these transport coefficients as a function of pressure. When a fluid is out of equilibrium by the presence of a gradient, its fluctuations become long-ranged and their intensity diverges for large spatial scales. Another interesting aspect of non-equilibrium fluctuations is their dynamics, which can lead to grab interesting information about fluid properties, especially at small wave vectors. While small fluctuations exhibit diffusive time constants, larger ones can be dominated by the effect of gravity and confinement or viscous forces as shown in Fig.1.

Figure 1: Scheme of the evolution of time constants as a function of wave vectors with typical pictures of fluctuations for the different regimes.

By means of a shadowgraph optical technique and a dynamic differential image analysis, we investigated the dynamics of concentration non-equilibrium fluctuations of two binary mixtures representatives of petroleum fluids in order to evaluate the impact of pressure on mass transport coefficients in various thermodynamic states. Within experimental uncertainties, we observed a linear decrease of mass transport coefficients with the pressure, and found new correlations with the dynamic viscosity. We also evidenced, that the confinement has a dramatic effect on time constants at very small wave vectors (Fig.1). Thus, we proposed a new equation for time constants, integrating this finite size effect in order to get more accuracy on transport coefficients for large solutal Rayleigh numbers. Finally, in the limit of very small solutal Rayleigh numbers, i.e. when viscous forces prevail over confinement effect, the analysis of time constants allow to determine the kinematic viscosity also.