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With the advent of more efficient application of radioactivity to industrial uses, radioactivity logs have become a practical indicator of the presence of hydrocarbons in a formation. Gamma ray and neutron logs have made it possible to log lithology and relative porosity, to locate gas-liquid contacts through casing, and to locate casing collars for subsequent perforating. Modern tools are small enough to run through tubing; they are used to correlate perforation depth with the formation of interest, regardless of the measured hole depth. Neutron devices can be employed to log chlorine intervals outside casing to offer a means of distinguishing hydrocarbon-bearing from nonproducible formations. Significantly better devices, pulsed neutron logs, have been developed for fluid saturation measurements outside cased holes. These logs make it possible to ascertain whether or not reservoirs behind casing are hydrocarbon bearing. By the use of through-casing devices, questionable or unknown accumulations of oil have been discovered.
Acoustic, or sonic, logs, similar to the porosity devices used in open-hole logging, are employed to check cement bonding, isolation or communication of formation fluids, and the level of cement outside the casing. Temperature logs are used in cased holes for locating cement tops. More sophisticated uses include gas-fluid entry surveys. Production logging is the descriptive term for the combination of wireline services used to investigate and evaluate a well’s performance while it is producing.
This training manual provides the student with a broad background in cased-hole logging. Through clearly stated objectives, helpful self-tests, readable language and descriptive illustrations, this book gives a solid overview of logging methods. No attempt is made to go into the more detailed, highly sophisticated cased-hole logging techniques.