脉冲中子伽马密度测井密度敏感度研究

On Density Sensitivity of Pulsed Neutron-Gamma Density Logging Tools

地层密度敏感度是决定脉冲中子伽马密度测井仪器源距的一个重要参考指标,对源距的优化可以提高该仪器的地层密度分辨率。基于脉冲中子伽马密度测量中的中子伽马场耦合理论讨论仪器伽马响应与地层含氢指数和体积密度的关系,推导地层密度灵敏度与近源距及探测器间距之间的关系,并结合蒙特卡洛模拟计算结果加以验证。理论分析和模拟计算结果表明,当地层密度一定时,近、远非弹性散射伽马射线响应的对数随含氢指数的增加而增加,在相同含氢指数的情况下,近、远非弹性散射伽马射线响应随地层体积密度的增加而增加;探测器间距保持不变时,密度灵敏度随近源距的增加而降低;近源距保持不变时,密度灵敏度随探测器间距的增加而增加;探测器间距对仪器地层密度灵敏度的影响大于短源距。

Density sensitivity is an important indicator which depends on the source-detector distance of a pulsed neutron-gamma density logging tool,so optimizing the source-detector distance can improve the formation density resolution provided by the tool.Based on the neutron-gamma coupled field theory,this paper analyses the relationship between gamma response and hydrogen-bearing indicator and formation density,and derives the relationship between formation density sensitivity and near source-detector distance and detector spacing,and verifies the result with Monte Carlo numerical simulation.Theoretical analysis and simulation results show that,at a constant density,the logarithmic responses of near and far inelastic gamma rays increases with the increasing hydrogen indicator;at a constant hydrogen indicator,the response of near and far inelastic gamma rays increases with increasing formation density;when the detector spacing is fixed,density sensitivity decreases with the increase of the near source-detector distance;and when the near offset is fixed,density sensitivity increases with the increase of the detector spacing.Formation density sensitivity is more strongly affected by the detector spacing than the near source-detector distance.