A New Neutron Gamma Density Algorithm Based on Drill-Collar Inelastic Gamma Information

Neutron gamma density (NGD) logging, as a safer and more environmentally friendly density logging method, has been applied in logging while drilling (LWD) for many years. However, most NGD algorithms have low density sensitivity and exhibit significant errors in the presence of gas and high-salinity water. To address the above issues, the inelastic gamma information of the drill collar is simulated by the Monte Carlo method, and its impact on NGD sensitivity and the results are studied. Using the drill-collar inelastic gamma information, a new neutron gamma density algorithm is proposed. The study shows that the drill-collar inelastic gamma information occupies a large proportion of the total inelastic gamma count recorded by the detector, which reduces the sensitivity of the NGD algorithms using the total inelastic gamma information. By eliminating the contribution of the drill collar from the total inelastic gamma count, the density sensitivity of the NGD algorithms can be significantly improved. In addition, the drill-collar inelastic gamma information is closely related to high-energy fast neutrons and can be used to correct the fast neutron distribution in NGD logging. The new NGD algorithm, utilizing the drill-collar inelastic gamma information, has excellent performance in the presence of gas and high-salinity water. However, similar to the other algorithms, the new algorithm is still affected by other environmental factors such as borehole, lithology, and shale. Furthermore, this new algorithm does not require any additional modifications to existing NGD tools and can work with other NGD algorithms.