Accurate water saturation calculation is one of the requirements for estimating oil in place. Typically this is performed using resistivity logs and the Archie equation. In this heterogeneous Middle East carbonate field, the resistivity technique is subject to large uncertainties related to m and n values and their unpredictable variability. We propose deriving water saturation directly from NMR logs. Using an oil based mud, we demonstrate that enough contrast exists in the invaded zone between oil mud filtrate and water to deduce water saturation directly from the NMR signal. Various laboratory experiments were performed to show that no water is displaced in the invaded zone, and to determine the NMR signature at representative conditions: whole mud invasion experiments to determine the mud leak-off rate, NMR T2 distribution at Swi with reservoir brine and oil filtrate, flooding with oil filtrate at a representative flow rate, effect of the oil filtrate surfactants, effect of temperature on the T2 distribution. The set of experiments also illustrates why the standard NMR T2 cut-off approach used in log interpretation is difficult to apply. To test the technique and provide a saturation benchmark, a well was cored with a 100% water free oil based mud. Both resistivity and NMR logs were run. For resistivity log interpretation m and n were measured at reservoir temperature and overburden pressure using resistivity measurements on as received plugs from the rig and the FRIM method on soft cleaned, preserved plugs.
The NMR and resistivity logs of the pilot well were interpreted with in-house software for water saturation and results compared to Dean Stark saturation from the oil base core plugs. Water saturation determined from NMR and resistivity are in agreement with cores in some reservoir layers and disagree in others. The possible reasons for this are explored. We conclude NMR logs have the potential to provide accurate Swi information.