Advantages of CO2 in enhanced oil recovery for Iranian oil reservoirs offers a unique opportunity to boost incremental oil recovery and reducing emissions of greenhouse gas through geological sequestration. Moreover, because of the dominant role of gravity force in gas injection process in fractured reservoirs, and also relatively low minimum miscibility pressure (MMP) of CO2, therefore, miscible CO2 injection with Gas Oil Gravity Drainage (GOGD) mechanism can be considered as an efficient method in most carbonate reservoirs.

In this paper, different production strategies such as vertical and horizontal immiscible and miscible gas injection were applied to find the optimum EOR method in one of the under-saturated Iranian fractured reservoirs.

To aim this goal, a study with simulation approach was conducted while the effect of gravity drainage on recovery was investigated besides other mechanisms. A complete analysis of phase behavior, PVT properties of reservoir fluid and CO2 mixture was done. These data were applied in designing of the slim tube and full field simulation model. A fully compositional reservoir simulator was employed to understand the reservoir characteristics, also, verifying the suitability of CO2 injection.

Material balance analysis indicated the original oil in place (OOIP) is about 910 million STB with a relatively strong water support. The maximum oil production of 60,000 STB/D was assumed in all simulation cases. Reservoir simulation results revealed that additional oil recovery could be obtained from this fractured reservoir by miscible gas injection, while gravity drainage was the main displacement mechanism. In case of vertical well production, the model predicted an increase in oil recovery about 24 and 28% OOIP for immiscible and miscible CO2 injection respectively. Considering horizontal producers and gravity drainage as the dominant production mechanism, immiscible and miscible gas injection will enhance oil production more than 28 and 36% OOIP respectively.

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