Formation density is one of the most important parameters in formation evaluation. Radioisotope chemical sources are widely used in the conventional gamma-gamma density (GGD) logging. Considering security and environmental risks, there has been a growing interest with the pulsed neutron generators (PNGs) in place of the radioactive chemical source in using bulk-density measurement. However, there is still high requirement of the accuracy of the neutron-gamma density (NGD) calculation. Pair production is one of the factors influencing the accuracy of the results, which should be considered. We propose a method, based on the difference between inelastic gamma-ray response of high- and low-energy windows, to reduce the impact of pair production upon calculating bulk density. A new density estimation algorithm is derived based on the coupled-field theory and gamma-ray attenuation law in the NGD logging. We analyze the NGD measurement accuracy with different mineral type, porosity, and pore fluid and discuss the influence of the borehole environment on the NGD logging. The Monte Carlo simulation results indicate that the improved processing algorithm limits the influence of mineral type, porosity or pore fluid. The NGD measurement accuracy is ±0.025 g/cm3 in shale-free formations, which is close to the GGD measurement (±0.015 g/cm3). The results also show that the borehole environment has a significant impact on NGD measurement. So, it is necessary to take the influence of the borehole parameters into account in the NGD measurement. Combined with the Monte Carlo simulation cases, the application results of the new density estimation algorithm in various model wells are presented.
In this study, we used a two-dimensional fluid model to investigate the argon discharge in Penning ion gauge (PIG) ion source influenced by magnetic field and the structure of ion source. Under fixed anode voltage U and pressure P, the relationships between the axial magnetic field, the structure parameter K, which is the ratio of the distance of two cathodes and the diameter of the anode tube, and the electron density are obtained. For fixed magnetic field B=0.01 T and pressure P=0.5 Torr, the electron density along the central axis has the maximal value when K=0.8. With fixed K=0.8 and P=1 Torr, the maximal electron density was found when B=0.1 T. Our results imply that there are optimal values for magnetic field and K which induce the maximal plasma density.
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