Study on Liquid Extraction Patterns in Offshore Strong Bottom-Water Reservoirs during Ultra-High Water Cut Stage under Different Geological Models

A typical reservoir with strong bottom water in Caofeidian Oilfield Group in Bohai Sea, the main oil-bearing horizons are Neogene Minghuazhen Formation and Guantao formation, which are fluvial facies deposits. The reservoir presents high porosity and permeability characteristics, crude oil viscosity is 30-425 mPa·s, and bottom water energy is more than 100 times of water body. Since the development of the oilfield, the natural energy of horizontal wells has been used for exploitation. At present, it has entered the stage of double high development (comprehensive water cut 97.5%, recovery degree 15.6%). Liquid extraction has become a key measure to stabilize production, but due to the influence of reservoir heterogeneity, there is a technical bottleneck that is difficult to determine the liquid extraction range. Four types of bottom water distribution patterns are identified through detailed reservoir description: thick layer strong bottom water, local interlayer thin bottom water, local thin bottom water and remote supply of thin reservoir. Combining with the percolation boundary, the mechanism of water flooding is studied, and it is found that the characteristics of pressure drop funnel of different modes are significantly different: the liquid production capacity of a single well in the thick bottom water area is 2000–3200 m3/d, and that in the thin bottom water area is only 940–1500 m3/d. The decline model based on water driven reserves reveals that the decline rate of thick bottom water is large due to wavelet and high displacement characteristics, while the decline rate of thin bottom water is small due to large wave and high displacement characteristics. The research shows that the bottom water reservoir in the ultra-high water cut stage still has the potential of liquid extraction. Through the model classification control, it is determined that 36 wells can implement the liquid extraction operation, the liquid extraction range of a single well is up to 15 m3/d, and the oil increase is expected to be 30000–50000 t/well. In the thick bottom water area, the sweep efficiency is improved by expanding the pressure drop funnel, while in the thin bottom water area, the stable production cycle is extended by using large waves and characteristics. The research results establish the quantitative relationship between bottom water distribution pattern and development performance, and provide reference for the development of similar offshore oilfields.