Logging devices for determining the points of entry and the dynamic conditions of movement of oil, water and gas provide information whereby efficiency and economy of hydrocarbon exploitation may be optimized. So-called "production logging" instruments are mainly used to make measurements within the borehole. Nuclear logs, especially the Neutron Lifetime Log, can detect fluid levels and sense the direction of fluid flow outside of the casing. The vertical movement of water outside casing can be observed by activation of oxygen as the fluids move past the 14 mev neutron source in the direction of a gamma ray detector. This makes it possible to locate the actual origin of produced water when channeling occurs. A new NLL instrument has been developed for logging through 2-1/2 inch tubing. Production logging techniques using this instrument to supplement other production logs are discussed. The new small diameter NLL, of course, can also be used for conventional fluid definition in the same manner as the 3-5/8 inch tool.

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