Experimental Study for Determining the Diffusion Coefficient of Nitrogen and Methane Gases in Crude Oil

Ghorbani, Mohammad | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53979 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Ghazanfari, Mohammad Hossein
  7. Abstract:
  8. Gas injection is one of the common methods for enhance oil recovery from oil reservoirs. At miscible injection condition, the efficiency of the process is majorly affected by the molecular diffusion coefficient of the gas in the oil. Therefore, the accurate value of this coefficient is an essential for applying in reservoir simulator. However, determination of this coefficient only is possible through experimental methods. In this study, pressure decay method was applied to determine the diffusion coefficient of nitrogen, methane and carbon dioxide gases in crude oil. The reasons for choosing this method are : accuracy in designing the experiment, covering a wide range of pressure values and also determining parameters such as mass transfer coefficient, Henry constant and solubility. For this purpose, an experimental set-up for determining the diffusion coefficient at high temperature and pressure has been designed and built. After preparation and calibration of the set-up, pressure decay tests were performed for nitrogen-crude oil and methane gas-crude oil systems at different values of initial pressure and temperature. The process of gas diffusion in crude oil is modeled and by using the laboratory results, the values of diffusion coefficient and other parameters related to mass transfer are determined. In modeling, the effect of factors such as different boundary conditions, liquid phase swelling, liquid phase advection, and time dependency on the values of the diffusion coefficient and other relevant parameters have been investigated. The results show that time-dependent diffusion coefficient should be used to model the diffusion process of nitrogen gas in crude oil due to the convective mass transfer. Pressure decay behavior in terms of time for the above system can be divided into three zones; named as first, transient and second. The transition times between zones at different values of temperature and initial gas pressure is determined. A mathematical expression for time dependent diffusion coefficient is determined. Examination of diffusion of methane gas in crude oil showed that the boundary conditions did not take into account the surface resistance to mass transfer provide more accurate results for modeling of this process. The values of diffusion coefficient of nitrogen gas in crude oil, in the range of 9.21×〖10〗^(-9) m^2⁄s to 1.97×〖10〗^(-8) m^2⁄s ,and for methane gas in crude oil, in the range of 1.65×〖10〗^(-9) m^2⁄s to 6.03×〖10〗^(-8) m^2⁄s , were determined in the temperature and pressure range used in the experiments. The diffusion of carbon dioxide in light and heavy crude oil is modeled based on the available laboratory results at different boundary conditions, and in each case the diffusion coefficient and mass transfer parameters are determined. In this case, modeling of the diffusion process with boundary conditions that consider mass transfer resistance for the two-phase contact surface were provided a more accurate results. In the relevant modeling, the effect of oil phase swelling and also the effect of oil phase advection is considered. The achievement of this research is in the modeling section that considered the effect of liquid phase advection during pressure decay method in the form of a moving boundary through diffusion process . Also, for the cases that the diffusion coefficient is not fixed, its dependency with time is determined, which is rarely attended in the previous researches. Moreover, By applying the numerical solution of the equations, the simplistic assumptions made in analytical solutions has been avoided, which made the results of this research more accurate
  9. Keywords:
  10. Moving Boundary ; Modeling ; Experimental Model ; Crude Oil ; Pressure Decay Method ; Mass Transfer Coefficient ; Molecular Diffusion ; Effective Molecular Diffusion Coefficient

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