地层密度对脉冲中子孔隙度测量的影响及校正方法

Formation density affects and corrections in pulsed neutron porosity logging

在井下脉冲中子孔隙度测量中,岩性或地层密度对含氢指数测量的影响相对于化学源显著增强,地层的非弹性散射影响不可忽略,必须发展相应的校正方法以确保测量结果的准确性。在无限均匀介质中,对中子的减速长度与地层体积密度及含氢指数的关系进行了推导和数值模拟分析,得到经过地层体积密度校正后的计数率比值与地层含氢指数的相关性;继而对近、远探测器的计数率比值进行体积密度校正,提高了对地层孔隙度的灵敏度,显著降低了岩性和泥岩效应的影响。

[Background] The inelastic scattering reaction between neutron and formation element in pulsed neutron porosity logging can not be neglected. Compared to chemical source, the influences of lithology and density on the measurement of hydrogen index increase, so the measured apparent porosity may be affected. [Purpose] This study aims to investigate the formation density influence and corresponding correction method. [Methods] First of all, the relationship of slowing-down length of neutrons in the infinite homogeneous formation between bulk density and hydrogen index was deduced, and numerical simulation was performed. Then the correlation between the quotient of counting rate after correction of formation density and the hydrogen index of the formation was obtained. Finally, the quotients of far detector counting rate and near detector counting rate were used to correct the formation density, and the result of the experiment was analyzed. [Results] The relation between the bulk density corrected count ratio and hydrogen index was obtained, and the sensitiveness of pulsed neutron porosity logging got improved. [Conclusions] Correcting count ratio of near detector and far detector with bulk density by means of numerical simulation can reduce the influence of bulk density, enhance the detection sensitivity of porosity and reduce the influence of lithology and shale effect.