Research on instrument structure scheme for improving the sensitivity of wireline D-T source porosity logging

Abstract

Utilizing a D-T source to replace the chemical source is the development trend of nuclear logging technology. However, the wireline porosity logging with a D-T source has a lower sensitivity. To solve this problem, the key factors on the sensitivity of neutron porosity logging are analyzed. Combining the numerical simulation method, special structures such as neutron-source deceleration block and detector shielding sleeve are designed to improve the sensitivity of D-T source wireline porosity logging tool, and the detector-spacing selection of the improved instrument was discussed. Study indicates that the sensitivity of neutron porosity logging mainly depends on the fast-neutron deceleration length, the proportion of formation information in the detector count and the detector-spacing. In the instrument design, the tungsten neutron-source deceleration block is adopted to reduce the energy of neutrons emitted by source, thereby shortening the fast-neutron deceleration length and improving the logging sensitivity. Additionally, the detector shielding sleeve made of boron carbide can absorb thermal neutrons from the borehole to increase the proportion of formation information in the detector count, which can also effectively improve the sensitivity. Based on the above structure design, it was found that when the near detector-spacing was set at 30 cm and the far detector-spacing was set at about 70 cm, the performance of the instrument can reach the best. This study can provide theoretical guidance for the application of D-T source in the wireline porosity logging tool in the future.