Simulation Study on CO 2 Water-alternating-Gas Enhanced Oil Recovery in a Low-permeability and High-heterogeneity Oil Reservoir

For low-permeability and high-heterogeneity oil reservoirs, the water cut may increase rapidly by water injection due to the water channeling effects. Water-alternating-gas (WAG) injection can reduce the relative permeability of the gas and water phases, reducing the water and gas channeling phenomena compared to single water flooding or gas flooding. Thus, WAG injection has been widely applied in low-permeability and high-heterogeneity oil reservoirs as an enhanced oil recovery (EOR) method. This study presents a simulation study on the performance evaluation of WAG-EOR by CO ₂ injection in a low-permeability and high-heterogeneity oil reservoir, with an oil recovery of 23.6%, an average water cut of 61.3%, and a reduction of reservoir pressure of 53.6% of the initial reservoir pressure at the current stage after water flooding. The 20-year long-time history matching study is conducted based on the field production data and the precise PVT simulation with experimental results. Then a field scale design of the WAG-EOR by CO ₂ injection is presented, and the effect of the operation parameters on the performance of CO ₂ -WAG EOR, including the pressure raising level by water or gas injection in advance, well network design, water/gas alternating slug ratio, and slug size, are investigated during the simulation.The results show that CO ₂ WAG has a maximum oil recovery of 17.9% compared to water flooding in a 30-year prediction period, making it a feasible method for the enhanced oil recovery with CO ₂ capture and storage (CCS) method for the low-permeability and high-heterogeneity oil reservoir.