RIASSUNTO
ABSTRACT
A measurement while drilling tool is described which is designed to provide neutron porosity and density logs of excellent quality. This recently developed nuclear logging tool for Logging While Drilling (LWD) is called the Compensated Density Neutron (CDN* ) tool since it combines two important nuclear measurements in one drill collar. The density measurement is borehole compensated and employs two gain stabilized photomultiplier /crystal scintillator detectors. The use of this type of detector in conjunction with low density ""windows"" in the drill collar enables spectroscopy methods to be employed. The photoelectric absorption factor of the formation is also measured for use in lithology identification. The borehole compensated, predominately epithermal neutron measurement provides a high quality porosity log that is free of the many interferences that degrade a capture gamma ray measurement (e. g. , natural gamma ray background, tool and formation activation, formation and mud salinity). An extensive set of laboratory measurements and mathematical modeling results have been used to quantify environmental effects on both the porosity and density measurements. Emphasis has been placed on the accurate calibration of both these measurements to ensure high quality field data. Comparisons between standard wireline logs and LWD logs are shown. In addition to being fully enclosed within the drill collar body, both the neutron and density sources are retrievable by conventional wireline fishing techniques, greatly minimizing the probability of having to abandon the sources downhole in the event that the drillstring cannot be fished.