Numerical Simulation Of Pulse Neutron Density Logging Method And Instrument Design

In order to remove the radioactive chemical source in density logging,improve the measurement accuracy of pulsed neutron source density logging,and promote the development of pulsed neutron source density logging,A new method of pulsed neutron density logging which can eliminate the influence of hydrogen index?HI?and inelastic scattering cross section(?_in)is established in this paper.Firstly,the temporal and spatial characteristics of neutron and gamma fields in the process of pulsed neutron source density measurement are simulated by MCNP5,which provides a basis for the subsequent design of pulse sequence and source spacing.Secondly,based on neutron coupled field theory,a new pulsed neutron density log which can eliminate the influence of HI and?_inn is deduced theoretically.The structure of logging tool is optimized for the new method of pulsed neutron density.Finally,the application effect of the new method of pulsed neutron density logging in MWD environment is studied by establishing various formations.The results show that pulsed neutron source density logging also has high measurement accuracy.The density measurement by non-elastic gamma ray is influenced by the spatial distribution and lithology of non-elastic gamma source,which is mainly influenced by HI and?_in.The temporal and spatial distribution of the particles show that the fast neutron and inelastic gamma need to be measured during the pulse period,and both decay quickly with the source distance,so the source distance can't be too large.Based on the theory of fast neutron coupling field,a new method of pulse neutron density logging is deduced to correct the influence of?_inn and HI by non-elastic gamma counting ratio and fast neutron counting ratio.A detection system of pulsed neutron source,two fast neutrons and two gamma detectors was designed.The helium-4?He-4?fast neutron detector and LaBr₃ Scintillation gamma detector are optimized.The fast neutron shielding combination of tungsten and nickel and the gamma-ray shielding combination of tungsten and lead with the best shielding effect are given.The simulation results under different HI,lithology and pore fluid types show that the new pulse neutron density logging method corrects the spatial distribution of gamma source and the influence of lithology by non-elastic gamma counting ratio and fast neutron counting ratio,and the application effect is very good in the drilling environment.For gas-bearing formation,there is obvious error when the formation density is less than 1.9g/cm3,but the formation density is generally higher than 2.0g/cm³.The change of borehole diameter will affect the measurement results.The measurement density decreases with the increase of borehole diameter.The relationship between measurement error and borehole clearance is quadratic function.This study enriches the theoretical methods of pulsed neutron density logging,provides a basis for the development of pulsed neutron density logging tools,and promotes the development of pulsed neutron density logging in China to a certain extent.