Evaluation of CO2-EOR and Sequestration in Alaska West Sak Reservoir Using Four-Phase Simulation Model

Injection of CO2 into West Sak reservoir of Alaska was studied. The study was conducted with the objective to store CO2 and enhance the oil recovery. Due to low reservoir temperature, mixing of possible injected mixtures, including pure CO2 and HC-enriched CO2, and the in-place hydrocarbon will result in development of three hydrocarbon phases, gas, oil-rich and CO2-rich phases, at certain pressures encountered in reservoir. Commercially available simulators are incapable of appropriate identification of this new phase and using them will result in unrealistic results and wrong predictions. A compositional EOS simulator, capable of handling up to four phases, including two HC liquid phases, was used. A three dimensional simplified model of the reservoir was built, based on reservoir core data, to study the oil displacement by various injected fluids including pure CO2. Sensitivity of the results to the relative permeability parameters of the second liquid phase was studied. Effect of using WAG with different water to gas ratios on the oil recovery and CO2 sequestration was also investigated. Various simulations runs resulting from combining different water-CO2 injection plans, well completion schemes, well spacing and production constraint were conducted to propose an injection-production strategy that can optimize the oil recovery and CO2 storage. Results show that the second liquid phase significantly affects both breakthrough time and total oil recovery. The sensitivity analysis of relative permeability parameters shows its profound effect on the results, emphasizes the need to use accurately measured relative permeability data to produce accurate results.WAG scheme improves the sweep efficiency but decreases the amount of stored CO2.Within the range investigated, 880 -1240 ft, we found higher recovery rate for smaller well spacing but negligible change in ultimate recovery. West Sak contains 7 to 9 billion barrels of original oil in place. Its high viscosity makes the primary recovery scheme inefficient. Although waterflooding was successful to a certain degree but CO2 injection can be a successful option to increase recovery and store considerable mass of CO2 that might be produced from coal-bed gasification in Alaska. Although CO2 cannot develop full miscibility with West Sak oil; however, it causes the oil to swell, oil viscosity to decrease, and recovery to increase.