Logging Identification Method for Water‐Flooded Layers in Tight Sandstone Oil Reservoirs: A Case Study of Chang 4 + 5 Oil Reservoir in Western Ordos Basin

ABSTRACT In the middle to late stages of tight sandstone reservoir development, infill adjustment is an important means to maintain stable production and enhance ultimate recovery. Qualitatively characterising and quantitatively determining the water‐flooded layer level are of great significance. Conventional well logging interpretation of water‐flooded layers can only qualitatively distinguish the degree of water flooding based on the response characteristics of logging curves, but cannot quantitatively identify the water‐flooded level. In contrast, the nuclear magnetic resonance T 2 spectrum of water‐flooded oil can reflect changes in rock fluid composition, allowing for the identification of oil saturation under water‐flooded conditions through variations in T 2 spectrum amplitude and position. This study, through scanning electron microscopy, petrophysical characteristics, and nuclear magnetic resonance experiments of water‐flooded oil, analyzes changes in reservoir resistivity and nuclear magnetic resonance T 2 spectra during oil‐displacing‐water and water‐displacing‐oil processes in low‐porosity and ultra‐low‐permeability cores under experimental conditions. By integrating segmented intervals of the T 2 spectrum and studying the relationship between spectrum position and water‐flooded oil saturation, the oil‐bearing factor of water‐flooded layers is calculated. Simultaneously, qualitative identification of water‐flooded layers is achieved using resistivity variation characteristics, combined with methods such as comparison of new and old well curves, resistivity comparison of calcareous interlayers, and resistivity reconstruction. Quantitative interpretation of the water‐flooded level in reservoirs is performed by constructing identification charts based on the relationship between water production rate, water displacement efficiency, and residual oil saturation. By comprehensively evaluating the water‐flooded level of reservoirs using multiple methods, the interpretation accuracy is improved, and the consistency between interpretation results and the production performance of infill wells reaches 92%. This method can be widely applied to the remaining oil potential exploitation in low‐permeability tight sandstone water‐flooding reservoirs.