Abstract
Thermodiffusion (the Soret effect) is important for the study of compositional variation in hydrocarbon reservoirs. The development of research history, theoretical modeling and applications to multicomponent hydrocarbon mixtures is included in this work. The Firoozabadi model appears to be an appropriate model for thermodiffusion estimation for hydrocarbon mixtures, and it is derived based on the equation of entropy generation rate and four postulates in non-equilibrium thermodynamics. Two equations of state, the Peng-Robinson Equation of State (PR-EoS) and the volume translated Peng Robinson Equation of State (vt-PR-EoS), have been used to estimate the thermodynamic properties of mixtures. In this work, different cases are presented: first, a new thermodiffusion cell designed to perform high pressure measurements in a porous medium has been validated at atmospheric pressure. Two systems were investigated, (1) 1,2,3,4-tetrahydronaphtalene (THN) and n-dodecane (nC12), and (2) isobutylbenzene (IBB) and n-dodecane at 50% of mass fraction. Experimental results revealed an excellent agreement with benchmark values and a good agreement with theoretical data. Second, the thermal expansion and concentration expansion coefficients and the viscosity of mixtures are necessary properties for the determination of the thermodiffusion coefficient. The densities of binaries of nC12, IBB and THN for pressures from 0.1 to 20 MPa and a temperature centred on 25⁰ were measured. By an derivative method, the thermal expansion and concentration expansion coefficients were determined. Viscosities were directly measured using a high pressure high temperature viscometer. Finally, the thermosolutal convections of two ternary mixtures, methane (C1), n-butane (nC4) and n-dodecane (nC12) at a pressure of 35.0 MPa and nC12, THN and IBB at atmospheric pressure, in a porous medium, were investigated over a wide range of permeability. The effect of permeability in the homogeneous and heterogeneous porous media on fluid transport was studied with consideration of thermodiffusion and molecular diffusion. In the analysis of the homogeneous porous medium, it was found that, for permeability below 300 mD, the thermodiffusion for both mixtures was dominant; and above this level, buoyancy convection became the dominant mechanism. Also, the viscosity was found to influence the evaluation of the molecular and thermodiffusion coefficients. In the case of heterogeneous porous medium, the impact of permeability ratio on the composition of the mixture components, velocity in the porous medium and on the separation ratio was investigated. It was found that the heterogeneity of porous medium has a significant influence on the composition of the mixture components.
Highlights
In a fluid mixture, diffusion is the general term used to describe the motion of one species with respect to another
Several experimental methods were developed on the ground and in microgravity to measure both molecular diffusion and the Soret coefficients of binary mixtures at atmospheric pressure (Bou-Ali et al, 2003, Wittko and Kohler, 2003, Van Vaerenbergh and Legros, 1998), in order to provide a good understanding of separation processes and molecular interaction
The measured mixture densities have been compared with densities which were calculated using Peng-Robinson Equation of State (PR-EoS) and vt-PR-EoS for pressures varying from 0.1 MPa to 20 MPa
Summary
Diffusion is the general term used to describe the motion of one species with respect to another. Since conditions under which crude oil is found underground imply high pressure (HP) and diffusion in a porous medium, it is considered important to analyze the influence of the pressure and the interaction with a porous medium on the transport properties of liquid mixtures. Due to limited energy resources, the characterization of petroleum reservoirs has gained a lot of interest In this context especially knowledge of transport properties of hydrocarbon mixtures such as linear alkanes and organic ring compounds is very important (Montel, 1998 and Van Vaerenbergh et al, 2005). Studies based on the thermodynamics of irreversible processes have shown that thermodiffusion in liquids, along with the effect of natural convection, can greatly influence the composition distribution in hydrocarbon reservoirs. They extended the dual-porosity model to a dual-porosity/dual-permeability model, where the simulation of fractured reservoirs involves discretization of the solution domain into two continua, one the domain representing the primary matrix, and a secondary domain representing fractured formulations
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