Tool design, physics and interpretation of neutron-gamma density
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Chemical radioactive sources pose health, safety, and environmental risks. Pulsed neutron generators have replaced Americium/Beryllium sources for the measurement of neutron porosity. However, Cesium 137 (Cs-137) is still mainly used to measure bulk density. Neutron-Gamma density is a new radioisotope-free measurement of density based on neutron-induced inelastic gamma rays. The first part of this report reviews relevant literature to the Neutron-Gamma density measurement and to the modeling of nuclear logging tools. The second part of this report investigates the nuclear physics behind Neutron-Gamma density and presents the development of a tool design optimized for the measurement. The third part of this report regards the development of a real-time interpretation algorithm. The objective of the algorithm is to correct for the changes in spatial distribution and source strength of the neutron-induced gamma ray source. These source variations are caused by fast neutron transport. Therefore, the interpretation algorithm has inputs of fast neutron and gamma ray counts. We achieve an accuracy of 0.019 g/cm3 in clean formation and 0.034 g/cm3 in shale and shaly formations. In the last part of this report, we study some of the measurement limitations regarding the density range and the influence of standoff. The algorithm does not accurately estimate higher densities (densities greater than 2.89 g/cm3) and standoff should be kept to a maximum of 0.25 inch for light mud. Finally, the depth of investigation of Neutron-Gamma Density is twice the depth of investigation of Gamma-Gamma Density. This work is presented as part of the PhD fast track option and will be extended to a PhD dissertation in the future.