A slow release CO₂ for enhanced oil recovery in carbonate reservoirs

CO₂ is used to swell the oil and increase its mobility by reducing the viscosity and reducing the IFT between the rock and oil. The problem of CO₂ during the EOR process is the gravity override and the displacement efficiency. Foamed CO₂ was introduced to overcome the displacement efficiency problem. The foam can be generated using different types of surfactants. The surfactants will not from a stable foam at HPHT conditions and the reservoir water salinity will impact the foam quality in addition to the surfactant adsorption to the carbonate rocks will reduce the surfactant concentration and will affect the generated foam. In this study we will investigate the use of slow release CO₂ concept. The CO₂ can be slowly released in the carbonate reservoir by injecting low reaction rate fluids (in this case citric acid) that will react with the rock after placement and this will generate CO₂. Coreflooding test will be performed using carbonate core at 100°C saturated with crude oil. CMG will be used to simulate the process of CO₂ generation by injecting the CO₂ in an aquifer underlying the carbonate formation and after that allow the CO₂ to release to the oil zone. The slow released CO₂ from the aquifer will swell the oil and then the oil can be displaced by sea water. Coreflooding results showed that the slow release CO₂ technique gave up to 42 % extra oil recovery from the oil in place after sea water flooding into the carbonate core at 100°C and 1100 psi back pressure. The extra oil recovery was due to the slow resealed CO₂ and oil swelling and also due to the rock dissolution and IFT reduction. The simulation results by CMG showed that the slow release CO₂ technique gave 70% oil recovery from the residual oil more than the sea water injection which confirmed the efficiency of the slow release technique in large scale.