Evaluation of CO2-EOR Gas Injection in a Heavy-Oil Naturally Fractured Reservoir

This paper investigates the use of CO₂ as an EOR solvent for a heavy oil and high permeability naturally fractured reservoir complex in Mexico. The complex is under partial pressure maintenance by Nitrogen injection. First geological features and production performance are analyzed to discern peculiar pressure trends caused by natural depletion and N₂ injection in order to establish the nature of prevailing fluid communication and identify a confined site for CO₂ injection testing. An East Block in the North fields due to its unique dynamic faulting characteristics is found nearly compartmentalized to serve as a suitable site for CO₂-EOR injection studies. Second, a finely-gridded dual permeability compositional simulation sector model with local grid refinement and boundary flux scheme is constructed and a calibrated 8-component EOS along with full tensor molecular diffusion is implemented to model CO₂-EOR process mechanisms. CO₂ and N₂ injections into the gas cap at varying rates and huff-n-puff injection in the oil column are simulated. The impact of injection rate is illustrated, where injection of CO₂ at low rates promotes diffusion and is shown to drain more of the matrix oil. The huff-n-puff simulation cases also indicate increased oil recovery and reduced matrix oil saturation by CO₂ injection as compared with N₂ injection due to a combination of oil swelling, reduced oil viscosity and partial miscibility with CO₂. The paper concludes that the efficiency of CO₂ injections is more pronounced at higher reservoir pressures and with no or less volumes of prior injected N₂.