Formation Element Logging Gamma Spectra Numerical Simulation

In the research of geological exploration,particularly in the logging industry,due to high cost and low accuracy of the corresponding physical experiments.With the development of the computer,the Monte Carlo method has become an important research tool in nuclear well logging and it plays a role that can not be ignored in the previous theoretical work.In this paper,the function of the gamma ray contribution to the detector in formation element logging is established by the two-group diffusion theory.Through the corresponding parameter analysis,the results show that the factors affecting the gamma distribution of formation element logging are mainly determined by the length of neutrons in the stratum and the attenuation coefficient of gamma rays in the stratum.The MCNP5 and Geant4 MC software are widely used in elemental logging,and a single-element PGNAA model for rock-forming elements Al,Ca,Fe,H,O,K,Mg,Na,and Si is established.Evaluated two software neutron activation simulation accuracy,the results show that the simulation results of MCNP5 software are closer to those of the IAEA cross-section library;Comparing the MCNP5 and Geant4 simulating the fast neutron excitation gamma energy spectrum,there is a big difference between MCNP5 and Geant4 in the fast neutron excitation gamma ray simulation of rock-forming elements.It provides an important reference for the future research of elemental logging simulation.In order to study the influence of porosity on the γ distribution in formation element logging,the formation element logging simulation model with D-T neutron source was established according to the “Guidelines for Practical Geological Analysis Standard Materials” and the parameters of standard limestone model wells.The MCNP5 program was used to simulate the coupled transport process of neutrons andγ-rays in the formation,and the gamma fluence distributions with different porosities and different source distances were obtained.The results show that the correlation between the fluence rate and porosity of γ-rays is close to the near-source distance related,but at far distance,the γ-ray fluence rate is less related to porosity.Based onthe sensitivity of the aperture and the setting of the corresponding shield,we sets the parameters to study the difference in the position of source.By comparing the performance of different detectors and combining logging conditions and requirements in formation element logging,we suggest that LaBr3 detectors are the most suitable for formation element logging.Then use the MCNP numerical simulation method to simulate the actual logging conditions and evaluate the elemental detection limits.