Investigating the Impact of Molecular Diffusion in Natural Gas Storage Process in an Iranian Depleted Fractured Oil Reservoir

Iranian oil reservoirs are mainly fractured. Performance of such reservoirs could be significantly different form the conventional reservoirs. That is mainly due to some additional flow displacement mechanisms, which take place in fractured reservoir. One of these mechanisms is molecular diffusion, which could play a key role on oil production. The impact of molecular diffusion on flow displacement during gas injection for either displacement or storage purposes would be more pronounced in a fractured reservoir compared to that in a conventional reservoir. It is mainly because of different oil trapping types and huge difference between contact area of the in situ oil and the injected gas phase in these two kinds of reservoir. In this study, the process of natural gas storage in an Iranian heavy oil fractured reservoir isinvestigated. In this paper, specifically the impact of molecular diffusion on the primary oilproduction and different stages of gas storage process are studied using a commercial simulator. The results of this study show that molecular diffusion could play an important role on injection/ production rates as well as the average reservoir pressure during injection process. This impact would be more pronounced when a gas with different composition compared to that in the reservoir is used for storage purposes.