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>NMR RADIAL SATURATION PROFILING FOR DELINEATINGOIL-WATER CONTACT IN A HIGH-RESISTIVITY LOW-CONTRASTFORMATION DRILLED WITH OIL-BASED MUD
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NMR RADIAL SATURATION PROFILING FOR DELINEATINGOIL-WATER CONTACT IN A HIGH-RESISTIVITY LOW-CONTRASTFORMATION DRILLED WITH OIL-BASED MUD
One of the most difficult tasks in petrophysics is theproper evaluation of high-resistivity low-contrastreservoirs; a variation of the well-known low-resistivitylow-contrast shaly sand problem. A major challenge inthese reservoirs is the identification of the oil-watercontact, which is usually masked by the lack ofresistivity contrast between the hydrocarbon and waterzones. Changes in formation water salinity, from lowvalues below the contact to higher values in the oilcolumn, often adds another level of complexity to analready challenging problem. The oil-water contactmust be known so that the correct formation watersalinities are used for water saturation calculations.Advanced NMR fluid typing methods can be used tocorrectly identify reservoir fluids. Care must be takenin the interpretation of NMR data, especially in oilbasedmud. Given the intrinsically shallow depth-ofinvestigationof the NMR logging measurements, it isquite possible that invading oil-based mud filtrate getsmisinterpreted as native oil.Radial fluid saturation profiles, obtained from NMRmeasurements made at multiple depths-of-investigationoffer a potential solution to the problem. By analyzingthe fluid saturation trends as a function of radialdistance from the wellbore, one can reliably determinethe fluid contacts. The methodology does not requireinvasion-free measurements: the critical requirement ischanging invasion trends within the radial distancerange covered by the measurements. Robustinterpretation of radial saturation changes demands veryhigh signal-to-noise-ratio NMR data, forcing the use ofstationary measurements.This paper documents the results of a year-long projectthat started with off-the-shelf solutions, and ended upwith highly customized stationary NMR measurementsdesigned for fluid contact delineation in a highresistivitylow-contrast formation. The results verifiedby formation tester fluid samples and production testsprove that NMR is indeed a practical choice for contactdetermination in such a complex reservoir. The paperalso outlines the limitations of the methodology, theknowledge of which is needed for the optimal use of thetechnology.
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