Logging Method And Instrument Parameter Design Of Pulsed Neutron Porosity While Drilling

Using pulse neutron generator to replace the traditional isotope neutron source for neutron porosity logging while drilling can not only avoid various radiation hazards,but also increase the reliability of rock physical measurement.However,the porosity measurement is greatly affected by the formation density.Therefore,this paper studies and establishes a new method of pulse porosity logging while drilling based on the dual epithermal neutron and double gamma structure model to eliminate the influence of formation density,so as to improve the accuracy of pulse neutron porosity measurement and promote the development of pulsed neutron porosity logging while drilling.Firstly,the effect of formation density on pulsed neutron porosity logging is studied,and a new method to eliminate the effect of formation density is derived theoretically;Then,the structure parameters of the instrument are designed;Finally,the application effect of the new logging method is verified by establishing different formation models.The results show that the porosity logging with pulse neutron source has high measurement accuracy,but the fast neutron energy released by it is high,and the influence of inelastic scattering on the formation can not be ignored,which is mainly reflected in the influence of formation density on porosity measurement.Based on neutron diffusion theory and dual neutron double gamma structure model,a new method of pulse neutron porosity logging for correcting formation density is derived.Aiming at the new method,a set of logging instrument is designed,which is mainly composed of D-T pulse neutron source,double super thermal neutron detector,double gamma detector detection system and shielding system.The combination of cadmium,plexiglass and ³He counter tube from the outside to the inside is selected as the epithermal neutron detector to detect and record the epithermal neutron flux;Lanthanum bromide?La Br₃?scintillation detector is used as gamma ray detector to detect and record inelastic gamma flux;The neutron double-layer shielding structure of tungsten nickel alloy+boron containing polyethylene and the gamma double-layer shielding structure of tungsten+lead are given.Through a large number of simulation studies in different formation models,the results show that the new method of pulse neutron porosity logging in this paper can well correct the influence of formation density on porosity measurement,and has a good application effect in simulated LWD environment.This study enriches the theory and method of PWD,provides the basis for the design of PWD tool structure,and has certain practical significance for the development of PWD in China.