Study Of Gamma Spectra Logging System With Dual Detector And Analysis Method Based On MCNP

Study of Gamma Spectra Logging System with Dual Detector and Analysis Method Based on MCNPGamma ray spectrum logging is mainly used for natural gamma ray,scattered gamma and captured gamma rays in pulsed neutron logging to determine the types and contents of formation elements.It is a way of studying the properties of well sections and formations to solve related problems such as geoscience and environmental sciences.Traditional downhole gamma spectroscopy measurement system normally consists of a single detector with a narrow energy response area.The wide-energy neutron gamma spectroscopy logging usually comprises a multi-probe structure to measure the gamma spectroscopy in the logging,which can record the neutron-captured gamma rays produced by interactions of neutrons and formations and natural gamma-radioactive.Information of formation element types and contents can be obtained by its spectrum analysis.In this study,a wide energy measurement range of the gamma spectrum was achieved by adopting a dual detector system combination of LaBr₃:Ce³⁺and BGO detectors through theoretical analysis and simulation.The wide energy gamma logging system can measure gamma rays of 0.03MeV-3MeV by high energy resolution of LaBr₃:Ce³⁺detector and captures gamma rays of 3MeV-10MeV by high detection efficiency of BGO detector.The model of the logging system was established by the Monte Carlo program MCNP.The structure and material optimization selection principles of the wide energy gamma logging system were studied to determine effects of the shield material,thickness,detector,and neutron source position on the measured spectrum in order to obtain a more ideal gamma spectrum.In the study of natural gamma spectrum analysis,a formula for spectrum analysis calculation by spectrum analysis method was derived.The calibration of the natural gamma ray instrument calibration was completed by establishing a standard calibration well.And the response coefficients of the three characteristic peak energy windows of ⁴⁰K,uranium series and actinide series were obtained and verified by simulation.In the study of the capture gamma spectrum analysis,the linearization and normalization of the captured gamma spectra acquired by the LaBr₃:Ce³⁺detector and the BGO detector were completed.Based on the theory of well logging,the calculation formulas of formation element mass ratio,least square method to obtain the yield and the oxide closure model to obtain the formation factor are derived.By establishing a mixed stratigraphic model,a mixed capture gamma spectrum is obtained by simulation.Element yield,formation factor,and relative sensitivity factor are calculated through a derived formula.The mass ratio of the elements in the formation is finally determined.After the above research work:(1)The geometric structure of the wide energy gamma logging system was determined.The shield is a 14cm tungsten alloy block.The detector is located 35cm above the shell interface with a 2mm boron sleeve.(2)The feasibility of the natural gamma spectrum analysis method of the wide energy gamma logging system was verified.And the results show that the LaBr₃:Ce³⁺detector is a good at collecting natural gamma rays.The average relative error of the mass ratios of uranium,thorium,and potassium obtained from the spectrum analysis was less than 5%.(3)The standard spectrum of elements in the formation is obtained by simulation and linearized,normalized and smoothed.(4)The normalized wide range captured gamma spectrum was verified by spectrum analysis.Through the calculation,it was found that the relative error of the element mass ratio obtained by spectrum analysis of H was up to 7%due to its small relative sensitivity factor.The relative errors of other elements were within 5%.It is basically consistent with the real element mass ratio in the model.The results verify the effectiveness of linearization and normalization of the captured gamma energy spectra collected by the two detectors,and prove the feasibility of the dual-probe wide range gamma spectrum logging system for the calculation of natural and captured gamma spectrum analysis.