Abstract Molecular diffusion of gases in heavy oils is one of the most important physical parameter governing cold production processes such as solution gas drive or Vapex. Indeed, the way bubbles are able to grow by diffusion will have a direct impact on gas mobilization and consequently on oil recovery. Furthermore, the importance of this parameter is emphasized by the very low gas availability characterizing extra heavy oils.
Surprisingly, publications of experimental data concerning gas diffusivity in heavy oils are relatively rare. Furthermore, published values can vary by different orders of magnitude. This is probably due to the fact that this kind of measurement is quite tricky. Indeed gas leakages in high pressure cells could be in the same order of magnitude than gas diffusion in oil. To study very viscous oil (extra heavy oils), corresponding to very low diffusivity, it is thus necessary to carefully analyze the obtained results.
The aim of this work is to characterize experimental fickian diffusivity of methane in heavy oils using two different concepts in order to be able to validate the experimental results. In this paper we present the two different methods (thermophysics and analytical) we have settled. Experimental results clearly indicate that diffusivity in extra heavy oils is lower than usually considered for ‘conventional’ oil.
From various experimental results we also propose a first mathematical model to simulate the influence of temperature on gas diffusivity.
Introduction The molecular diffusion of gases in heavy oils is one of the most important physical parameter governing cold production processes such as solution gas drive or Vapex.–The solution gas drive process is a pressure driven mechanism. The dissolved gas nucleates in bubbles when the pressure is reduced below the bubble point pressure as depicted by various works1,2. The bubble growth mechanism3 is mainly due to mass diffusion but is also controlled by IFT, oil viscosity, solubility parameters...During post nucleation and bubble growth phase, it is essential to have a good knowledge of diffusivity in order to correctly model the evolution of the gas phase occurring in the depressurization process. Indeed, the way bubbles are able to grow by diffusion will have a direct impact on gas mobilization and consequently on oil recovery. Furthermore, the importance of this parameter is emphasized by the very low gas availability characterizing extra heavy oils.
–For the Vapex process (injection of solvent in heavy oil), the molecular diffusion of the solvent is the rate controlling mechanism.
Oddly enough, publications of experimental data concerning gas diffusivity in heavy oils are relatively rare. Furthermore, published values can vary by different orders of magnitude. This is probably due to the fact that this kind of measurement is quite tricky (difficult and time consuming). Indeed gas leakages in high pressure cells could be in the same order of magnitude than gas diffusion in oil.
Many experimental methods have been developed to measure the diffusivity of gases in liquids but none of these methods seems to be considered as referential. Diffusion of gases in thermodynamically stable state can be estimated from NMR measurements and more precisely T2 relaxation4.
Nowadays the most used method has been proposed by Riazi5 using a PVT cell. Later on Zhang and his co-workers2 have adapted and simplified the experimental technique developed by Riazi for measurements in heavy oil systems. The concept takes birth in a mathematical model developed by Crank6 from the gas recombination kinetics data. This method is correct for liquids with low Gas Oil Ratio like heavy oils as diffusion length must be neglected. Zhang2 preferred to follow a method by monitoring the pressure change in the PVT cell.