A novel framework for neutron-gamma density logging: semi-empirical modeling, directional neutron sources, and experimental benchmarking

Neutron-gamma density (NGD) logging is a vital technique in subsurface exploration, enabling accurate determination of formation density and supporting hydrocarbon reservoir characterization. This study proposes a novel NGD logging method that utilizes a directional neutron source to enhance measurement precision while reducing the number of required detectors. A semi-empirical formula was developed to calculate formation density using directional neutron flux, and detailed simulations were conducted to evaluate the method's performance under varying neutron emission angles, mudcake thicknesses, and environmental conditions. To validate the simulation results, an experimental test was designed using a scaled well model with controlled porosity levels and a calibrated Am-Be neutron source. The experimental measurements were benchmarked against the simulation results, demonstrating strong alignment and confirming the accuracy of the proposed approach. The findings highlight the advantages of steep neutron angles in achieving superior measurement accuracy, with performance comparable to NGD-TC methods and improved over NGD-FC methods. This study underscores the reliability of the proposed method and provides a framework for further optimizing NGD tools for diverse subsurface conditions, advancing hydrocarbon exploration and reservoir evaluation.