| Summary: | BackgroundThe rock-forming element neutron gamma logging tool is mainly composed of a deuterium-tritium neutron source, a shield, and a lanthanum bromide (LaBr3) scintillator detector. Among them, the instrument shell material, boron sleeve structure length, shield structure, and the distance (source distance) from the bromide crystal detector to the neutron source are main factors that affect the count and energy spectrum of the gamma detector. These factors have a crucial impact on the identification and determination of lithology, and it is necessary to optimize the parameters of the tool.PurposeThis study aims to obtain better counts and energy spectrum of the rock-forming element neutron gamma logging tool and effectively reduce the interference signal.MethodsBy establishing a Monte Carlo model of rock-forming element neutron gamma logging tool, the performances with different shell materials, boron sleeve structure lengths, shielding body structures, and source distances were simulated. And the relationships between the changes of the shell material, boron sheath structure length, shield structure, source distance and the response of the tool were systematically studied. Finally, the optimized structure parameters of the logging tool were obtained.ResultsThe results show that the optimum shell material is TC11 titanium steel, the optimum length of the boron sleeve is set to be 29 cm and the boron sleeve is embedded on the TC11 titanium steel instrument shell, the shielding body is made of tungsten nickel iron with a diameter of 11.8 cm, and the optimum source distance is 37.6 cm.ConclusionsThrough the simulation test of the instrument performance, it is determined that the optimized neutron gamma logging tool of rock-forming elements has a detection depth of 38.9 cm and a vertical resolution of 40 cm, and can obtain more accurate dry weight of elements in the borehole size of 7~40 cm. This study provides a theoretical reference for the design of rock-forming element neutron gamma tool and the logging interpretation.
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