Optimization Calculation Of Pulsed Neutron Porosity Logging

Traditional epithermal neutron or thermal neutron porosity logging results are often affected by lithology,fluid,well diameter,salinity and other borehole and formation environment,so the measurement accuracy is not high.Especially in gas-bearing low-porosity formation,it is impossible to obtain accurate measurement value,so it is necessary to carry out research on high-precision neutron porosity logging method.The thermal neutron cross section,a new physical parameter of rock nuclear logging,is sensitive to natural gas in formation pores and is not affected by gas mining effect,and the thermal neutron cross section of gas and matrix is quite different,this provides an innovative solution for high-precision porosity measurement.In this thesis,monte Carlo numerical simulation is used to study the thermal neutron cross section calculation method based on capture gamma counting ratio and thermal neutron counting ratio.On this basis,thermal neutron cross section logging responses of different lithology,fluid,well diameter,standoff,mud density,formation salinity,formation pressure,formation temperature and mud content are studied.By comparing the porosity of thermal neutron cross section logging with that of traditional thermal neutron logging,the adaptability of thermal neutron cross section logging is studied from the influence of lithology,borehole diameter,borehole gap and salinity.The results show that the thermal neutron cross section can be accurately obtained by both the capture gamma counting ratio and thermal neutron counting ratio calculation methods.The porosity sensitivity of thermal neutron cross section logging is high,which is not affected by gas mining effect and is almost not affected by borehole diameter,temperature and pressure.The measured thermal neutron cross section increases when the formation mud content or hole clearance increases.As the fluid density increases,the thermal neutron cross section decreases.In the influence of different lithology fluid,caliper,hole standoff,mud density,formation water salinity,formation temperature,formation pressure and shale content,porosity thermal cross section than traditional thermal neutron porosity are closer to the real formation porosity formation,this shows that thermal neutron cross section logging is less affected by environment,has better applicability and higher measurement accuracy.