可控中子源密度测井仪的密度响应特性与算法研究

在可控中子源密度测井中，脉冲中子源的中子产额存在波动，采用探测器绝对计数的密度算法受地层环境、测量条件的影响较大，因此会导致密度算法的精度较差。为提高密度测量的稳定性和精确性，本文针对可控中子源一体化测井仪的研发需要，建立了适合所研发仪器的密度算法：首先，结合蒙特卡罗数值模拟软件MCNP对新型一体化测井仪及地层进行了建模；然后，利用所建模型，模拟了在不同岩性、孔隙度条件下可控中子源发射的快中子与地层物质的相互作用过程，并通过记录γ探测器的近、远非弹性散射γ计数和俘获γ计数，中子探测器的近热中子及近超热中子计数信息，获得了一体化测井仪探测器计数与地层密度之间的响应关系，并对影响密度算法的主要因素进行了分析；最后，在密度响应特性分析的基础上，提出了使用近超热中子与近热中子计数比、近远非弹γ计数比来提高稳定性和精确性的可控中子源密度测井新算法。采用新密度算法对所建模型进行计算，结果表明，砂岩、灰岩、白云岩3种岩性下计算的密度与真密度非常接近，其相对误差小于6%。与哈里伯顿和斯伦贝谢算法的计算结果相比，本文方法显示了更好的效果：公式参数少、没有探测器的绝对计数、精确度高。

Density Response Characteristics and Algorithm of Density Logging Instrument Using Controllable Neutron Source

In density logging with the controllable neutron source, the number of neutrons emitted by the pulsed neutron source fluctuates from pulse to pulse, which leads to the poor accuracy of the density measurement results under the impact of different formation and detecting conditions. In order to improve the stability and accuracy of the density measurement, a density algorithm suitable for the developed integrated logging instrument with the controllable neutron source was established in this study. Firstly, the modeling of the new integrated logging instrument and the formation was carried out with the Monte-Carlo numerical simulation software MCNP. Then, the interaction process between fast neutrons emitted from the D-T neutron source and formation materials under different lithology and porosity conditions was simulated using the built model. By recording the near and far inelastic scattering γ-ray counts and capture γ-ray counts, near thermal neutrons and near epithermal neutrons counts from neutron detectors, the response relationships between the detector count of the integrated logging instrument and formation density were obtained, and the main factors which affect the density algorithm were also analyzed. Finally, based on the analysis of the density response characteristics, a new algorithm for controllable neutron source density logging which uses the ratio of near thermal neutron counts and near epithermal neutron counts and the ratio of near and far inelastic γ-ray counts was proposed to improve stability and accuracy. The calculation results show that the calculated density of sandstone, limestone and dolomite is very close to the true density, and the relative error is less than 6%. Compared with the calculation results of Halliburton and Schlumberger algorithms, the method in this paper shows better results of less formula parameters, no detector absolute counting, and high accuracy.