Improving Recovery Mechanism Through Multi-Well Water and Gas Injection in Underground River Reservoirs

Underground river reservoirs are dominated by large-scale elongated caves and are typical fractured–vuggy carbonate reservoirs. This paper established physical models of underground river reservoirs with different filling modes. We first conducted bottom water flooding experiments and then studied multi-well, alternating water flooding and gas injection. The remaining oil distribution patterns and key factors under different filling modes and well locations were studied to clarify the recovery-improvement mechanisms of multi-well water and gas injection. The results show that the remaining oil after bottom water flooding can be categorized into the following five types: “insufficient well control remaining oil”, “attic remaining oil”, “bypass remaining oil”, “residual oil in filling medium”, and “shielded oil in filling medium”. Early water injection effectively recovers “insufficient well control remaining oil”, “bypass remaining oil”, and “residual oil in filling medium”. Gas injection targets included “attic remaining oil”. Late water injection can further improve recovery. When the cave is partially filled, there exists a large amount of “shielded oil in filling medium” that is difficult to recover, reducing recovery by 27% compared to unfilled cases. This study clarified the remaining oil distribution laws and water–gas flooding mechanisms for underground river reservoirs, providing guidance for efficient development.