ABSTRACT

The NLL has been used extensively for evaluation and recompletion of old wellsbut has found few applications in new wells. Some very interesting examples have been found that demonstrate the value of running the log when a well is first completed or even before casing is set. Base logs run early in the history of a well make it possible to observe progressive changes in fluid saturation under conditions where the log would otherwise have little value. Another interesting application is the comparison of logs in open hole with thoserun at varying intervals subsequent to casing, cementing and production. Since the log is essentially unaffected by casing and cement, it is feasible to identify permeable zones and to measure the progressive changes in saturation whenever the different fluids have differing thermal neutron capture characteristics. Further, by deliberately selecting the drilling fluid filtrate to match the capture cross section of reservoir brine, one can measure residual oil saturation so as to identify immovable hydrocarbon deposits or to establish water flood sweep efficiency. Such saturation knowledge is absolutely essential in planning tertiary recovery projects. It is pointed out that the cost of running Neutron Lifetime Logs in selected new wells could frequently be justified by the consequent improvement in the accuracy and reliability of later cased hole log evaluation. Examples are cited where logs in open holes or in newly completed wells are recommended.

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