Neutron transport correction and density calculation in the neutron-gamma density logging

Highlights: • A new algorithm is derived to calculate neutron-gamma density by theoretical analysis. • The two processes of neutron transport and gamma-ray transport are not fully independent of each other. • The new algorithm of neutron-gamma density logging can provide accurate apparent density in gas-filled formations. - Abstract: Formation density is one of the most important parameters in formation evaluation. The radioisotope chemical sources are widely used in conventional nuclear logging tools. Considering security and environmental risks, the pulsed neutron generators have successfully replaced the Am–Be source in neutron porosity logging tools. However, the Cs-137 is still mainly used for bulk density measurement. Recently, there is a growing interest in measuring bulk density with the pulsed neutron generators. Although the neutron-gamma density (NGD) is also calculated by gamma-ray attenuation, the neutron transport correction is needed for neutron-induced inelastic gamma rays. A new method of NGD calculation is developed based on the coupled-field theory. This new method does not require additional neutron transport correction. Additionally, a structural model of NGD logging tool is designed by Monte Carlo method, which includes two gamma-ray detectors and two epithermal neutron detectors. According to the proposed method, the corrected inelastic gamma-ray count rate response can eliminate the effect of neutron transport and meet the law of gamma-ray attenuation. The Monte Carlo simulation results indicate that the accuracy of NGD measurement is 0.02 g/cm³, which is close to the conventional gamma-gamma density logging (0.015 g/cm³). The NGD measurement is insensitive to porosity and formation fluids in the pore space.