Multiphase flow characteristics and EOR mechanism of immiscible CO2 water-alternating-gas injection after continuous CO2 injection: A micro-scale visual investigation

Water-alternating-gas (WAG) injection has been widely applied in fossil fuel reservoirs to enhance oil recovery (EOR) and CO2 capture and storage (CCS) efficiency. However, no systematic investigation on the multiphase flow characteristics and EOR mechanism of WAG injection after continuous CO2 injection has been conducted at the micro-scale. Therefore, a visual micromodel was developed using CT scanning and microelectronic photolithography. Subsequently, micro-scale visual experiments were conducted to simulate the water injection, CO2 injection and WAG injection processes. Then, the multiphase flow characteristics and residual oil trapping mechanism were observed and analyzed. Finally, the volumetric sweep efficiency (VSE), oil displacement efficiency (ODE), residual oil distribution, and improvement effects of WAG-EOR and WAG-CCS were quantitatively studied. Experimental results revealed the flow characteristics of oil-water, oil-gas, and oil-gas-water were different. Pore throat morphology, wettability, and injection parameters all affects the multiphase flow process. Six types of residual oil were trapped and observed at various locations. The multiphase flow characteristics of jumping displacement, multiple displacement, and compression-release were identified. The VSE of water injection, gas injection, and WAG injection were 68.0%, 53.5%, and 86.0%, respectively. WAG injection significantly reduced cluster-shaped and island-shaped residual oil and oil film/ring. Therefore, the WAG-EOR mechanism is a combination of the macroscopic VSE of water injection and the microscopic ODE of CO2 injection. WAG injection after continuous CO2 injection increased oil recovery by 23.15% and CCS by 0.174 PV. However, the improvement effects of EOR and CCS capacity were dominated by the first and second WAG injection cycles, respectively.