For fault-controlled fractured-vuggy reservoirs, the development characteristics of bottom water flooding and water and gas injection flooding under multi-well conditions must be clarified due to the structural complexity. To address this issue, we designed and manufactured fault-controlled physical experimental models based on the geological model of Tarim Oilfield and conducted flooding experiments. The results demonstrate significant variations in bottom water flooding characteristics due to differences in flow capacity within fractures, cavity area in caves, and filled area in caves under different filling modes. Different bottom water rates exhibit varying abilities to overcome gravity and breakthrough capillary resistance, significantly impacting the bottom water flooding characteristics. During the bottom water flooding period, the positioning of production wells primarily affects the macroscopic sweep range, while the filling modes significantly influence the distribution of remaining oil within individual caves. Throughout the three periods of multi-well water and gas injection, the early water injection stage mainly focuses on mobilizing “insufficiently controlled remaining oil” and some “attic remaining oil,” and the middle gas injection stage primarily targets the attic remaining oil. Finally, the late water injection stage aims to lift the oil–gas–water interface and improve oil displacement efficiency. Furthermore, different water and gas flooding directions affect displacement resistance in each channel and the longitudinal sweep range. This paper identifies the direction for the life cycle production of fault-controlled fractured-vuggy reservoirs and presents a mechanistic explanation.

Attic karst cave is one of the most common fractured-vuggy structure in Tahe oilfield. In this study, a typical attic-shaped cave was extracted from the geological section of Tahe Oilfield. Under the guidance of similarity criteria, physical experiment models of attic karst cave reservoir filled with oil-wet and water-wet filling medium were designed and fabricated using coated resin sand and cemented quartz sand. Water flooding experiments at different injection rates were carried out using the attic karst cave model filled with filling medium of various wettability. The water flooding characteristics and distribution law of residual oil in the different models were determined by analyzing oil displacement performances and the dynamic production curves. Results showed that when the filling medium is water-wet, water injection rate has a trivial impact on the residual oil distribution pattern. While for the oil-wet media filled situations, the lower the injection rate, the lower the swept area. However, at high injection rates, the spread scope is large but the displacement efficiency remains relatively low. In addition, it was noted that the difference in the wettability of the filling medium caused a difference in the location of the residual oil.

Summary: Thermal recovery techniques serve as the primary approach for developing heavy oil due to its high viscosity and poor flowability. In this study, we established a high-temperature and high-pressure 3D physical experimental and numerical model based on the unique reservoir characteristics of the sand-mud interlayer in the Long Lake oil sands of Canada, using similarity criteria. Physical and numerical experiments employing steam-assisted gravity drainage (SAGD) were conducted to investigate the impact of sand-mud interlayer properties on the expansion limit of steam chambers during SAGD development. The results indicate that the expansion mode and limit of the steam chamber play a decisive role in heavy oil mobilization. Notably, heat loss during steam chamber expansion and the flow resistance caused by the interlayer are critical factors influencing the SAGD process. The presence of the interlayer extends the mobilization range in the lower portion of the reservoir, but it also limits the upward expansion of the steam chamber, resulting in a reduced mobilization range above the interlayer. Moreover, the steam chamber above the interlayer exhibits a distinct expansion pattern, featuring concave sides and a convex middle, resembling a “positive triangle.” Furthermore, the properties of the sand-mud interlayer and production parameters significantly affect the expansion limit of the steam chamber. Permeability and position exert a substantial impact on recovery, whereas thickness has a minor influence. Specifically, at an injection rate of 20 mL·min–1, steam quality of approximately 0.7, and a production/injection ratio of approximately 1.0, the steam chamber can successfully penetrate interlayers with a thickness of either 3.5 m and a permeability of 100×10−3 μm2 or 4.5 m and a permeability of 200×10−3 μm2.

Weathered crust karst reservoirs with intricately interconnected fractures and caves are common but challenging enhanced oil recovery (EOR) targets. This paper investigated the remaining oil distribution rules, formation mechanisms, and EOR methods through physical experiments on acrylic models resembling the geological features of weathered crust reservoirs. Acrylic models with precision dimensions and morphologies were fabricated using laser etching technology. By comparing experiments under different cave filling modes and production well locations, it was shown that a higher cave filling extent led to poorer bottom water flooding recovery due to stronger flow resistance but slower rising water cut owing to continued production from the filling medium. Multi-well water and gas injection achieved higher incremental oil recovery by alternating injection–production arrangements to establish new displacement channels and change drive energy. Gas injection recovered more attic remaining oil from upper cave regions, while subsequent water injection helped wash the residual oil in the filling medium. The findings reveal the significant effects of fracture cave morphological configuration and connectivity on remaining oil distribution. This study provides new insights and guidance for EOR design optimization catering to the unique features of weathered crust karst fractured vuggy 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.