D-T脉冲中子发生器随钻中子孔隙度测井的蒙特卡罗模拟

Monte Carlo Simulation on Compensated Neutron Porosity Logging in LWD With D-T Pulsed Neutron Generator

利用蒙特卡罗方法模拟研究了D-T脉冲中子发生器和^241Am-Be中子源产生的中子与地层的作用过程，以探讨D-T脉冲中子发生器在随钻中子孔隙度测井中的应用价值。模拟结果显示，使用这两种中子源，热中子计数均随源距增加而呈指数下降；孔隙度较小时，两者的计数差异较小，当地层孔隙度达到40％时，D-T脉冲中子发生器产生的热中子和超热中子计数均比。^241Am-Be中子源高很多，其分布范围也更宽，近探测器的源距选择20～30cm，远探测器的源距选择约60～70cm；C-T脉冲中子发生器用于中子孔隙度测井时对地层孔隙度的灵敏度降低，而相同源距条件下探测深度几乎不变。以上结果提示，利用D-T脉冲中子发生器可以进行补偿中子孔隙度测井，在增加源距的同时既可以保证计数统计性，又可以提高灵敏度和探测深度，在随钻测井仪器设计中可以取代^241Am—Be中子源。

The process of neutron interaction induced by D-T pulsed neutron generator and 241 Am-Be source was simulated by using Monte Carlo method. It is concluded that the thermal neutron count descend exponentially as the spacing increasing. The smaller porosity was, the smaller the differences between the two sources were. When the porosity reached 40 %, the ratio of thermal neutron count generated by D-T pulsed neutron source was much larger than that generated by 241 Am-Be neutron source, and its distribution range was wider. The near spacing selected was 20-30 cm, and that of far spacing was about 60-70 cm.The detection depth by using D-T pulsed neutron source was almost unchanged under condition of the same sapcing, and the sensitivity of measurement to the formation porosity decreases. The results showed that it can not only guarantee the statistic of count, but also improve detection sensitivity and depth at the same time of increasing spacing. Therefore, 241Am-Be neutron source can be replaced by D-T neutron tube in LWD tool.