A Comparative Study of Dual-Mode High-Resolution NMR Logging to Enhance Residual Oil Detection Below Free Water Level in Heterogeneous Carbonate Reservoirs: Study Case of Giant Offshore Oil Field, Abu Dhabi - UAE

Abstract Quantifying residual oil saturation (ROS) below the free water level (FWL) remains challenging in mature carbonate reservoirs, where conventional logging is limited in separating movable and immovable oil. This study compares two advanced high-resolution nuclear magnetic resonance (NMR) logging approaches to enhance ROS evaluation without relying solely on core data. Two complementary NMR techniques were deployed in separate wells within the same reservoir. The first well used simultaneous T₁–T₂ mapping, capturing fluid clusters with improved vertical resolution. The second well applied T₂ relaxation with diffusion editing, enabling multi-shell radial profiling to distinguish oil, water, and filtrate. Rigorous data processing included cluster analysis and cross-plotting to quantify movable and immovable oil fractions under sub-FWL conditions. To validate consistency, results were integrated with lithology scanning and petrophysical synthesis. In Well-1, T₁–T₂ mapping clearly identified two oil-dominated clusters below the FWL, with intensity decreasing in deeper intervals, consistent with expected residual oil behavior. Despite challenges from oil-based mud, fluid separation remained reliable, confirming the method's robustness. In Well-2, diffusion-edited T₂ profiling successfully captured radial variations, showing a distinct light oil zone above and more filtrate-invaded rock deeper. This technique confirmed invasion zones that might distort conventional saturation logs. Combining both methods reduced uncertainty in ROS estimates and clarified the distribution of movable versus immovable oil, enhancing confidence in the saturation model below the FWL. This field comparison demonstrates the clear advantage of integrating vertical (T₁–T₂) and radial (diffusion-edited T₂) NMR dimensions for more accurate residual oil mapping in complex carbonates. The findings offer practical guidelines on choosing the right NMR strategy to improve reservoir understanding, recovery forecasting, and late-life production decisions—while minimizing dependence on core data. This integrated approach provides a cost-effective solution to tackle ROS uncertainty and optimize reservoir management under challenging conditions.