Theory, Response, And Calibration Of An MWD Neutron Porosity Sensor Employing Sidewall-Mounted Li6 Glass Scintillation Neutron Detectors And Spectral Processing

The Modular Neutron Porosity (MNP) sensor is a newly-developed neutron porosity tool for Measurement While Drilling (MWD) applications. The MNP sensor employs new detector technology in which sidewall-mounted Li6 glass scintillators coupled with a 256-channel spectrum analyzer are used to detect neutrons. The small size of these detectors, the sidewall mounting scheme, and the source-to-detector spacings combine to produce very good porosity sensitivity at relatively high porosities, improving on what is commonly considered to be a limitation of neutron porosity logging. The tool also incorporates innovative gain stabilization and signal processing techniques. To characterize the response of the tool, laboratory formations of varying lithologies, porosities, and hole sizes have been constructed. Extensive computer modeling supplements the laboratory measurements, and provides information not readily available experimentally. With the analytic model and the laboratory formation data, a set of environmental correction charts has been constructed. The MNP sensor has been extensively field tested, with comparisons made between MNP logs and wireline neutron porosity logs.