Monte Carlo Simulation Of Formation Element Logging Based On D-T Neutron Source

In recent years,the demand for unconventional oil and gas reservoir exploitation has gradually increased in China,and conventional logging methods are difficult to carry out reservoir evaluation of complex lithology.The application of formation element logging technology in reservoir evaluation of complex lithology is very significant.Compared with foreign countries,the logging of formation elements in China is slightly behind,With the China National Petroleum Corporation logging company and Daqing Oilfield developed the domestic formation element logging tool,China has announced the breaking of the monopoly of logging elements in foreign strata.However,compared with foreign logging instruments,the domestic formation element logging tool has lower precision.Therefore,the optimization of the parameters of the formation element logging tool is urgently needed.The deconvolution method for the capture energy spectrum has not been perfected yet.The focus of this paper is to optimize the parameters of the formation element logging tool and the method of the spectrum of the capture energy spectrum.After reading a large number of related literatures and comparing Geant4 and MCNP software,it was found that the simulated gamma spectroscopy obtained by MCNP software can be well matched with the IAEA database.Then the formation element logging model was constructed using MCNP5 software,introduced neutron source energy spectrum and angular yield data,an anisotropic D-T neutron source model is constructed.Through the set formation element logging model,the effects of the type of scintillation crystal,the size of the scintillation crystal,the source distance of the detector and the material and size of the fast neutron shielding on the energy response of the logging tool were investigated.Then,starting from the interaction between neutron and matter,the relative element yield,relative sensitivity factor and the method of obtaining the formation factor are obtained by the derivation of the basic formula.By establishing a standard element logging model,the standard capture energy spectrum of the obtained elements is used to solve the relative element yield,relative sensitivity factor and the value of the formation factor.In the process of solving the least squares method of relative element yield,in order to improve the calculation accuracy,the damping matrix factor is introduced for calculation.By establishing a mixed element logging model,the mixed element energy spectrum is simulated,and the relative elemental yield,relative sensitivity factor,and specific values of the formation factor are calculated to calculate the percentage of the element.Compare this to the set percentage of elements to verify the accuracy of the deconvolution method.Through the research in this paper,it is found that among the three scintillation crystals of NaI,BGO and LaBr₃,the detection efficiency of BGO scintillation crystal is the highest,and LaBr₃ has lower energy resolution and shorter energy decay time;As the length of the scintillation crystal increases,the energy response of the detector also increases;As the source distance of the detector increases,the energy response of the detector decreases.;10cm thick tungsten-nickel alloy is best when the fast neutron shielding is selected tu blocked 90%of fast neutrons.Combined with a second layer of5 cm thick polyethylene doped with 20%boron carbide,the third layer is 2 cm thick lead,together to form a 17 cm thick three-layer structure neutron shielding body;Among the percentages of elements obtained according to the de-spectral algorithm,the percentage of H element is the largest due to the wellbore factor and the high-energy statistical fluctuation factor.However,the relative error of the percentage of all elements is in the range of-10%to 10%,which is acceptable in formation element logging.