造岩元素中子γ测井仪器结构优化研究

Optimal design of neutron gamma logging tool for rock-forming elements

为了获得造岩元素中子γ测井仪器较好的计数和能谱,有效地减少干扰信号,需要对该仪器的参数进行优化研究。通过建立造岩元素中子γ测井仪器蒙特卡罗模型,对具有不同外壳材料、硼套结构长度、屏蔽体结构及源距的晶体探测器进行模拟,系统地研究造岩元素中子γ测井仪器外壳材料、硼套结构长度、屏蔽体结构及源距变化和仪器的响应关系。最后分析得出优化后的造岩元素中子γ测井仪器结构参数:外壳材料采用TC11钛钢;硼套长度取为29 cm嵌于TC11钛钢材料仪器外壳上;屏蔽体材料采用钨镍铁,直径取为11.8 cm;源距定为37.6 cm最优。并且通过仪器性能模拟测试确定出优化后的造岩元素中子γ测井仪器的探测深度为38.9 cm,纵向分辨率为40 cm,可在7~40 cm的井眼尺寸下获得较准确的元素干重值。

[Background]The 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.[Purpose]This study aims to obtain better counts and energy spectrum of the rock-forming element neutron gamma logging tool and effectively reduce the interference signal.[Methods]By 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.[Results]The 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.[Conclusions]Through 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.