Study On Pulse Neutron Dual Spectrum Density Logging Method Based On A D-T Neutron Source

With the development of the logging while drilling(LWD)technology and the increasing concern of nuclear radiation supervision,using controllable sources instead of chemical sources for measuring formation density has become an inevitable trend in the development of nuclear logging.Neutron gamma density(NGD),as an emerging density measurement technology with the pulsed neutron generator,avoids the damage of the chemical source to the environment and human body,and has the advantages of large penetration depth and easy operation,which is of significance for the LWD development and reservoir evaluation.However,due to the lack of NGD theory,there are still many problems in the instrument design and data processing of NGD logging,such as the empirical formula in data processing,the diversification of instrument design,and the logging results easily affected by the environment factors.To resolve the difficulties in the aspects of NGD logging theory,instrument design and data processing,a study on the pulsed neutron dual spectrum density logging method is carried out to provide a theoretical guidance for the NGD logging.Firstly,the relationships of neutron inelastic scattering cross-section,elastic scattering cross-section and capture cross section versus the neutron energy are studied,and the neutron transpotation of D-T neutron source in the formation are determined.The differences between traditional gamma-gamma density logging and neutron gamma density logging are analyzed from the aspects of gamma-ray generation,composition,distribution and attenuation.Based on the theories of neutron field distribution and gamma-ray attenuation,the neutron-gamma couple field theory is eatablished to descripe the inelastic gamma field and capture gamma field distribution in the formation.Based on the coupled field theory,the relationships between the inelastic gamma field distribution and formation parameters are quantitatively calculated,and the contributions of inelastic scattering cross section,fast-neutron scattering free path,and formation density on the field distribution are analyzed and compared,which theoretically clarifies the logging mechanism and logging responses of neutron gamma density logging.Results show that the fast neutron scattering free path and inelastic scattering cross sections are related to the generation of inelastic scattering gamma rays,and the contribution of density attenuation is opposite to that of inelastic scattering cross section and fast-neutron scattering free path.When the detector-spacing is small,the total contribution of the fast neutron scattering free path and inelastic scattering cross sections is larger than that of density attenuation,and the generation of inelastic scattering gamma rays plays a dominant role,as a result,the count rate of inelastic scattering gamma rays increases with the increasing formation density.As the detector-spacing increases,the density attenuation gradually plays a dominant role in the gamma field distribution,the count rate of inelastic scattering gamma rays decreases with the increasing formation density.Based on the coupled field theory,the basic method for determining formation density using the inelastic gamma-ray information is derived,and a new neutron dual spectrum density logging method based on the detection of fast-neutron and inelastic gamma-ray energy spectrums is develpoed.A new NGD measurement system consisting of a D-T source,one fast neutron detector and two gamma detector is designed,and the pulse emission and data acquisition timing,spacings of gamma and fast-neutron detectors,and shielding materials are optimized and designed.Research shows that the 20?s pulse width and 400?s working cycle of D-T source can effectively reduce the interference of captured gamma rays on the collection of inelastic scattering gamma spectrum.The spacing of the near gamma detector is set to 25-30 cm,the spacing of the far gamma detector is 70 cm,and fast-neutron detector is located between the near and far gamma detectors and close to the side of the near gamma detector,which can achieve the optimization of density sensitivity and accuracy.In addition,the thickness of the shield between the source and the near gamma detector must be at least 8cm to effectively shield the high-energy fast neutron and protect the detector.To further improve the sensitivity of the instrument,the total thickness of the shields between the near gamma detector and the fast neutron detector and between the fast neutron detector and the far gamma detector is preferably up to 12 cm.Using the Monte Carlo simulstion method,the neutron dual spectrum density logging model is eastablished,and the information on the inelastic gamma-ray and fast-neutron under different formation conditions is simulated.The data processing method on the neutron dual spectrum density logging is studied,and density results of the new method are well consistent with the actual formation density.Compared with the existing NGD methods,the new method has the advantages that the density results are not affected by the formation water salinity and the density algorithm is stable,while ensuring sufficient density sensitivity and density accuracy.Compare with the gamma-gamma density(GGD)logging,the depth of investigation(DOI)of the new method approaches twice that of GGD,but the density sensitivity is less than half that of GGD.Besides,the density precision and the vertical resolution of the method are also far less than those of GGD.In order to further improve the accuracy and applicability of the neutron dual spectrum density logging method,the logging responses of the new method under different lithology,wellbore size,formation water salinity and shale content conditions are studied,and the correction charts for the logging factors are eatablished.Finally,by processing the simulated model and actual well-logging data,the application results of neutron dual spectrum density logging method in different lithology,shale content and porosity conditions are presented and compared with the openhole density logging.Results show that the density results of the neutron gamma density logging are in good agreement with the openhole density.In common sandstone formation,the density errors between the two density methods are generally less than 0.05g/cm~3,which verifies the feasibility of the neutron dual spectrum density logging method.The research will provide a systematic and theoretical guidance for the NGD instrument design and data processing.