Miscible WAG-CO2 Light Oil Recovery from Low Temperature and High Pressure Heterogeneous Reservoir

Abstract

Abstract Brazilian pre-salt oils should contain a variable concentration of dissolved CO2, which is produced in the gas phase. A sustainable production of these oils requires a destination for the produced gas. As offshore fields have limitations associated to gas manipulation, storage and exportation, miscible CO2 flooding is an Enhanced Oil Recovery (EOR) method with great potential to be employed in pre-salt reservoirs. In fact, miscible CO2 injection in highly heterogeneous reservoirs should increase the oil production. As CO2 flooding is related to high gas mobility, an alternative EOR method is water-alternating- gas injection (WAG). The WAG with CO2 (WAG-CO2) is advantageous in this scenario due to the sea water available apart from enabling the control of gas mobility. In addition, using CO2 as an injection fluid is a viable solution for the GHG problem. Before implementation of WAG injection in a field, the prediction of the oil production through reservoir simulation is recommended. Simplified Black-Oil modeling is not appropriate to simulate a reservoir with light oil and miscible CO2 injection. The rigorous simulation of WAG injection requires a compositional model. Also, hysteresis of relative permeability should be implemented in the simulation model to consider the cyclic hysteresis of the three-phase relative permeability. The case studied has analogous characteristics of pre-salt reservoirs such as light oil with 8% molar of CO2 and initial reservoir conditions of low temperature and high pressure. The phase behavior of the oil and CO2 mixture is represented by an Equation of State that contemplates the swelling resulting from the CO2 dissolution in the oil. As hysteresis modeling requires some uncertain experimental parameters, a sensitivity analysis is carried out to determine the influence on oil recovery. The importance of using the reservoir simulation as a tool to predict the oil recovery before implementing WAG injection in practice is evidenced. It also demonstrated that the sensitivity analysis of the required uncertain parameters to consider the hysteresis is a viable approach to give the pessimistic and optimistic scenarios of light oil recovery prediction under WAG injection.