Modeling and simulation of neutron-gamma density logging: Impact of mudcake thickness and barite content on measurement accuracy

This study investigates the influence of mudcake thickness and barite weight fraction on Neutron-Gamma Density (NGD) logging measurements, applying both NGD-FC and NGD-TC methods to simulate the interactions of fast neutrons and gamma rays in a wellbore environment. Initially, based on fast neutron and gamma diffusion theory and incorporating the build-up factor, a mathematical model of the well logging tool was constructed. To validate this model and enable more detailed analysis, simulations were conducted using the MCNP code, which provided the response of the tool under varying conditions. Simulated responses were then compared with theoretical predictions to ensure model accuracy.

The research examines mudcake conditions with thicknesses ranging from 3 to 12 mm and barite weight fractions of 10 %–30 %, highlighting that NGD-TC is particularly sensitive to mudcake thickness, as evidenced by a Max Error increasing from 22.36 % to 44.07 %. Conversely, NGD-FC maintains lower and more stable errors (15.91–19.58) under similar conditions, though it shows higher sensitivity to barite content due to fast neutron scattering influences.

Overall, this study emphasizes the importance of selecting the appropriate NGD method to ensure reliable measurements in complex wellbore environments. By fine-tuning calibration coefficients and understanding the interaction mechanisms that influence neutron and gamma flux, the research enhances the accuracy of NGD logging, thereby supporting improved decision-making in resource extraction and subsurface evaluations.