Study of the elastic scattering effects of a slowing-down model on D-T neutron porosity logging

摘要: measurement method used to determine the formation poros-ity.The americium-beryllium(Am-Be)source is commonlyusedforconventionalporositymeasurements.However,theypose potential risks to safety,health, and the environment [14].Researchershavestudiedsubstitutionchemicalsourceswith deuterium-deuterium (D-D) neutrons [5-7]. However,theneutronyieldofD-Dneutronsourcesisrelativelylowwhich restrictstheir application[8,9]In2005,theoilfieldservicescompanySchlumberger(nowSLB)began using a deuterium-tritium(D-T) neutron sourcefor compensated thermal neutron porosity logging[10]Compared to the chemical source, the D-T neutron sourcehas a higher neutron yield,higher neutron energy of 14 MeV,and a longer slowing-down length of the neutron; thus,D-Tneutron logging can detect more thermal neutrons and has ahigher detection accuracy.Nevertheless, neutron slowing-down is facilitated by in-elastic scattering owing to the higher energy of neutronsConsequently, the D-T neutron recording exhibits reducedsensitivitytoporositycomparedtothechemicalsource[11.12],asmeasuredby theconventionalneutronporosityinter-pretation model founded on the Am-Be source[13-15].ods have been proposed to improve the porosity sensitivityof D-T neutron logging. The near and far thermal neutroncountswere correctedusing thedensitycorrectionfunction,and the modified thermal neutron counting ratio was consid-ered tobe only associated with the hydrogen-containing in-dex[16].Further,thedensity correctionmethods were sum-marized to modify the thermal neutron counting ratio andstudied by MC simulation [17]. The relationship of slowing-downlengthofneutronsbetween theformationdensityandhydrogen index was deduced and numerically simulated[18].Basedondatafromthestandardcalibrationexperimentandactuallogging[19,20],itwas verified that thedensitycorrec-tion results were consistent with the isotopic neutron source.Inaddition,the slowing-downlengthoftheD-Tneu-tronsourcecanbeconvertedintothatoftheAm-Besource.Onemethodinvolvestherelationshipbetweentheneutronslowing-downlength,capturingofthecross-section,andden-sity to predict the counting ratio[21]. The second involves thecombination of the two-group diffusion theory with the for-mation density, reaction cross-section, and neutron slowing-down length [22].