Carbon dioxide miscible gas injection optimization utilizing nitrogen/methane/ethane enrichment

Carbon dioxide miscible injection is one of the non-thermal effective enhanced oil recovery (EOR) methods. In the Middle East, particularly Abu Dhabi, pilot testing is already implemented in the Rumaitha Field for CO ₂ miscible injection. However, CO₂ is expensive to capture/handle, has high compressibility, corrosive, and can cause asphaltene deposition problems. Nitrogen injection is an attractive alternative because nitrogen can be obtained in large quantities, and it is inert. This study investigates means of: improving CO₂ miscible injection by N₂/CH₄/C₂ H₆ enrichment, improving N₂ miscible injection by CH₄/C₂H₆ enrichment, and looking for an ultimate injection gas combination which has minimum MMP value and gives a maximum oil recovery. This can be performed by incrementing one (primary) gas into the other (secondary) gas stream. Minimum miscibility pressure (MMP) and oil recovery factor for nitrogen, CH₄ and C₂H ₆ content addition to CO₂ injection gas were studied using target oils from Abu Dhabi carbonate reservoirs.;The first contact miscibility (FCM) and the minimum miscibility pressure (MMP) (multiple contact miscibility - MCM) of injected gases/oil mixture under reservoir temperature have been determined using Cell to Cell Simulation method and Semi-analytical (key tie lines) method in WinProp module of the Computer Modeling Group (CMG) simulator. In addition, the corresponding oil recovery factors for various injection gas scenarios were estimated by 1-D compositional simulation of slim tube using Builder and GEM module of CMG Simulator. The simulation runs were conducted for different concentration percentages (0-100%) of N₂/CH₄/C₂H₆/HC in injected gas (CO₂/N₂).;Few reservoirs from South East Abu Dhabi were also screened for MMP correlations of pure nitrogen and it was found that some reservoirs possess potential for additional reserves from high-pressure miscible nitrogen injection. Miscibility is predicted to occur with the MCM vaporization mechanism between N₂ /CO₂ injected gas and the targeted reservoir oils (Oil 1 and Oil 2) at MMP pressures higher than 3375 psi. Minimum value of MMP was observed to be 2300 psi in case of injection of ethane (C₂H ₆) with oil 1. Second best MMP values obtained for oil 1 were recorded for CO₂ injection.;A threshold injection gas concentration was observed after that the MMP almost became constant value. Enrichment of N₂ to CO₂ miscible flooding, CH₄ and C₂H₆ diminished the oil recovery for oil 1. However, enrichment of CO ₂ miscible flooding with N₂ showed a constant oil recovery factor for 0-70% N₂ for oil 2. Setting the grid blocks formulations to fully implicit resulted in more realistic oil recovery estimation as compared to explicit grid blocks formulation. The results of this study may help in developing an optimized injection strategy for the candidate reservoirs.