The Fundamental Research Of The Pulsed-Neutron Density Logging While Drilling

In traditional density oil well logging tools, the gamma-ray source Cs-137 is normally used to measure formation density. In order to eliminate the dangers of such a chemical source, the gamma rays that are produced via inelastic interactions of fast neutrons emitted from a pulsed neutron generator are considered as an induced gamma-ray source to replace it.The density measurement method using a pulsed neutron source and its influence factors were theoretically analyzed from the neutron and gamma ray transport equations. In order to improve the accuracy and functions of MCNP-5 (Monte Carlo Neutron Photon code), which is the most commonly used software on nuclear well logging, based on a series of experiments, the source code of MCNP-5 was modified. The primary structure of pulsed neutron density logging tool and an accurate inelastic gamma spectrum stripping method were designed by using Monte Carlo simulation. The simulation responses of every detector on fifty types of common formation were obtained. And a new pulsed neutron density measurement approach based on the Monte Carlo– Library Least-Squares (MCLLS) was proposed. The influence of formation characters and LWD environment were studied, and the correction method was put forward.The results show that the D-T pulsed neutron source can be used to measure the formation density. But the density measurement is affected by the size of'induced gamma ray source'and foramtion lithology. The effects of them can be corrected by using the counting rates of epithermal detector and elemental weight fraction which obtained through MCLLS method, respectively. The experimental results proved that the accuracy and functions of the modified MCNP code is much better than the original code, it can guarantee the precision of simulation. The optimized structure of pulsed neutron density logging tool including the source spaces, detector crystal types, the material and size of shielding was studied by using modified MCNP. It is considered that the measurement of net inelastic gamma ray is easy to be mixed up with the epithermal neutron capture gamma rays, and it could be automatically stripped by using the epithermal counting rates. Not some peaks or windows of energy spectrum but total spectrum are used in MCLLS method, the accuracy is two or three times better than traditional method, so it is a good method to get the elemental weight fraction of formation. For the density measurement method based on MCLLS, the precision is much better than other methods, and it is no longer affect by oil (gas) and water reservoir characteristic in pore, formation water salinity, and shale volume. In addition, the common lithology and gas bearing formation can be identified using this method. The size of standoff between the formation and logging tool affect the pulsed neutron density measurement. Aimed at the optimized logging tool, another density measurement was calculated using the improved inelastic gamma diffusion length method. And the effects of standoff whose size is below 0.75 inch could be compensated by combined use of these two methods. As a result, this study lays a solid theoretical foundation for the design of the Pulsed-Neutron Density Logging while Drilling, data processing and interpretation method.