Study on Distribution of Remaining Oil in Fractured-Vuggy Reservoirs Considering Influence of Connected Structure Patterns

The karst fractured-vuggy reservoirs in the Tarim Basin of China have abundant reserves, and show enormous development potential. This type of reservoirs is highly heterogeneous, with multiple storage types such as matrix, fractures, and karst caves coexisting, and the fractured-vuggy combination patterns and the oil-water relationships are very complex. The unclear understanding of the distribution of remaining oil leads to large differences in development effects. How to achieve efficient development of karst fractured-vuggy reservoirs is a serious technical challenge. In this article, based on the typical connected structure patterns of fractured-vuggy units, and by using 3D printing technology, 2D and 3D simulation physical models of fractured-vuggy combinations that meet geometric similarity, motion similarity, dynamic similarity, and characteristic parameter similarity were designed. Indoor water and gas flooding displacement experiments were conducted to study the fluid flow mechanism and remaining oil distribution in the fractured-vuggy combinations, and to reveal the control mechanism of the fractured-vuggy connected structure. The study shows that under fracture injection and cave production mode, the oil-water displacement is more uniform, the remaining oil is retained less, and the water injection effect is better. Due to the high flow rate of gas in the fractured-vuggy medium and the easy occurrence of gas channeling, the gas injection effect under the cave injection and fracture production mode is significantly better than that of the fracture injection and cave production mode. For situations with multiple fractures and caves, during water injection and gas injection development, the efficiency of producing nearby caves is higher, and the remaining oil in distant caves and the top of fractures is distributed in a continuous pattern. The connection relationship between fractures and caves has a significant influence on the distribution of remaining oil, but the injection rate has little effect on the distribution of remaining oil. The water-flooded remaining oil in the 3D fractured-vuggy physical model is mainly distributed in three patterns: attic pattern, low-amplitude residual hill pattern, and blind end pattern. This article provides a method reference for studying the distribution pattern of remaining oil and the optimization design of development plans for similar types of reservoirs.