A Novel Method for Enhancing Oil Recovery by Thickened Supercritical CO2 Flooding in High-Water-Cut Mature Reservoirs

High-water-cut mature reservoirs typically serve as the “ballast” for ensuring China’s annual crude oil production of 200 million tons. Despite the use of water flooding and chemical methods, over 40% of crude oil remains unexploited. It is critical to develop efficient revolutionary technologies to further enhance oil recovery (EOR) by a large percentage in high-water-cut mature reservoirs. To address this issue, the potential of vertical remaining oil in Daqing Oilfield is first analyzed from massive monitoring data. Using molecular dynamics simulation to design optimal synthetic routine, a copolymer without fluorine or silicon is synthesized by modifying vinyl acetate (VAc) with maleic anhydride (MA) and styrene (St), and treated as a supercritical CO₂ (scCO₂) thickener. The underlying EOR mechanism of the scCO₂ thickener is thereafter clarified by high-temperature, high-pressure oil displacement experiments. The EOR effect by thickened scCO₂ flooding in a typical high-water-cut mature reservoir is predicted, and future technological advancements of the technique are ultimately discussed. Results show that the vertical remaining oil enriched in weakly swept zones is a primary target for further EOR in high-water-cut mature reservoirs. The copolymer typically exhibits good solubility, strong dispersion stability, and high thickening effect in scCO₂. Under an ambient pressure of 10 MPa and a temperature of 50 °C, the dissolution of copolymer at a mass concentration of 0.2% can effectively increase the viscosity of scCO₂ by 39.4 times. Due to the synergistic effect between expanding vertical swept volume and inhibiting gas channeling, crude oil recovery can be further enhanced by 23.1% for a typical high-water-cut mature reservoir when the scCO₂ viscosity is increased by 50 times. Our understandings demonstrate that the thickened scCO₂ flooding technology has significant technical advantages in high-water-cut mature reservoirs, with challenges and future development directions in field-scale applications also highlighted.