Wang Hu, Wu Wensheng, Wang Ruigang, Ge Yunlong
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China.
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China.
Appl Radiat Isot. 2019 Aug;150:110-119. doi: 10.1016/j.apradiso.2019.05.023. Epub 2019 May 17.
Formation density is one of the most important parameters in formation evaluation. The radioisotope chemical sources are widely used in conventional nuclear logging tools. Considering security and environmental risks, the pulsed neutron generators have successfully replaced the Am-Be source in neutron porosity logging tools. However, the Cs-137 is still mainly used for bulk density measurement. Recently, there is a growing interest in measuring bulk density with the pulsed neutron generators. Although the neutron-gamma density (NGD) is also calculated by gamma-ray attenuation, the neutron transport correction is needed for neutron-induced inelastic gamma rays. A new method of NGD calculation is developed based on the coupled-field theory. This new method does not require additional neutron transport correction. Additionally, a structural model of NGD logging tool is designed by Monte Carlo method, which includes two gamma-ray detectors and two epithermal neutron detectors. According to the proposed method, the corrected inelastic gamma-ray count rate response can eliminate the effect of neutron transport and meet the law of gamma-ray attenuation. The Monte Carlo simulation results indicate that the accuracy of NGD measurement is 0.02 g/cm, which is close to the conventional gamma-gamma density logging (0.015 g/cm). The NGD measurement is insensitive to porosity and formation fluids in the pore space.
地层密度是地层评价中最重要的参数之一。放射性同位素化学源广泛应用于传统核测井仪器中。考虑到安全和环境风险，脉冲中子发生器已成功取代中子孔隙度测井仪器中的镅铍源。然而，铯-137仍主要用于测量体积密度。最近，使用脉冲中子发生器测量体积密度的兴趣日益浓厚。虽然中子伽马密度（NGD）也是通过伽马射线衰减计算得出的，但对于中子诱发的非弹性伽马射线需要进行中子输运校正。基于耦合场理论开发了一种新的NGD计算方法。这种新方法不需要额外的中子输运校正。此外，采用蒙特卡罗方法设计了一种NGD测井仪器的结构模型，该模型包括两个伽马射线探测器和两个超热中子探测器。根据所提出的方法，校正后的非弹性伽马射线计数率响应可以消除中子输运的影响，并符合伽马射线衰减规律。蒙特卡罗模拟结果表明，NGD测量的精度为0.02 g/cm，接近传统伽马-伽马密度测井（0.015 g/cm）。NGD测量对孔隙度和孔隙空间中的地层流体不敏感。
