**4.4.2.5 Influence of molar concentration on diffusion coefficient**

According to literature review, there are two different opinions about the problem whether component concentration has an influence on diffusion coefficient or not at present. Some scholars think that there is an influence of component concentration on diffusion coefficient while others think that there is no influence. Taking the component of injected gas diffusing into liquid phase at 60Ԩ as an example, the relationship of content and diffusion coefficient is shown in fig7, 8 and 9. These figures show that diffusion coefficient of gas changes with the concentration variation of gas diffusing in the liquid phase. Compared with the initial values, the molar concentration changing level of N2,CH4,CO2 are 12.86%,34.087% and 67.262% respectively and the changing level of the diffusion coefficient of the three gases is 0.211%,1.88% and 0.934% respectively at the end of tests. The data above show that the rate of change of concentration differs from that of diffusion coefficient in different systems. N2 has the smallest rate of change while the rate of change of CH4 diffusion coefficient is the largest. Theoretically, the component concentration does have a certain impact on diffusion coefficient. But in engineering application, the impact on the diffusion coefficient can be ignored due to the small rate of change (<2%) under this experimental condition.

The gas injection is applied widely not only in oil-field, but also in condensate gas-field. Hence, further researches need to be done to make sure whether the diffusion phenomena of gas-gas and gas-volatile oil agree with the research result in this paper. The porous media has impact on the phase state of oil and gas, the diffusion in porous media should be the

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Fig. 9. Relationship of CO2 mole fraction in liquid phase and its diffusion coefficient in CO2-

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Fig. 7. Relationship of N2 mole fraction in liquid phase and its diffusion coefficient in N2-oil diffusion experiment

Fig. 8. Relationship of CH4 mole fraction in liquid phase and its diffusion coefficient in CH4 oil diffusion experiment

Fig. 9. Relationship of CO2 mole fraction in liquid phase and its diffusion coefficient in CO2 oil diffusion experiment
