350 kV高压X射线密度测井探测深度特性及影响

Depth-of-investigation characteristics and influence of 350-kV high-voltage X-ray density logging

密度测井中使用可控X射线源替代137Cs放射源已经成为新的趋势。X射线源强度受靶基上高压影响较大,当高压为350 kV时密度测量不确定度可以保持在0.01 g·cm-3。为探究350 kV高压X射线密度测井仪器的探测深度特性及影响,利用蒙特卡罗方法研究了不同源距探测器在20%含水石灰岩地层中的探测深度;并且通过与137Cs源密度测井仪探测特性的对比,分析了二者探测深度差异的原因。另外通过模拟分析了泥饼和地层对探测器的贡献度以及不同探测器的密度偏差,研究了井壁泥饼对密度测井响应的影响。研究结果表明:X射线密度测井仪的探测深度随着源距的增加而变大;与137Cs源密度测井仪相比,X射线密度测井仪散射粒子主要集中于井壁1~3 cm处,从而造成二者探测深度差异;此外,泥饼和地层对不同源距探测器的贡献度不同,探测器密度偏差随着源距增加而减小。

[Background]The use of controlled X-ray sources instead of 137Cs radioactive sources in density logging has become a new trend.The high voltage on the target substrate significantly affects the intensity of the X-ray source,and the density measurement uncertainty can be maintained at 0.01 g∙cm−3 when the high voltage is 350 kV.[Purpose]This study aims to examine the depth-of-investigation characteristics and influence of a 350-kV highvoltage X-ray density logging instrument.[Methods]The depth of investigation of various source distance detectors in 20%water-bearing limestone formation was studied using the Monte Carlo method.By comparing the investigation characteristics of 350-kV high-voltage X-ray source and 137Cs source density logging,the reasons for the differences in the depth of investigation among them were analyzed.Moreover,the contribution of mudcake and formation to the detector and the density deviations of various detectors were analyzed via simulation.Finally,the influence of mudcake on the density logging response of the well wall was explored.[Results]The results indicate that the depth of investigation of X-ray density logging instrument increases with the augment of source distance.Compared to the 137Cs source density logging,the scattered particles of the X-ray density logging are mainly concentrated at 1~3 cm from the bore wall,resulting in the depth-of-investigation differences between the two techniques.Furthermore,the contributions of mudcake and formation to different source distance detectors are different,and the detector density deviation decreases with the increase in source distance.[Conclusions]This study affords a theoretical basis for the depth-of-investigation characteristics and influence of 350-kV high-voltage X-ray density logging.