A comparative study on the neutron-gamma density and gamma-gamma density logging

Abstract

With the increasing demand for the radioisotope-free operations, using pulsed neutron gamma density (NGD) instead of gamma-gamma density (GGD) has become increasingly important for the development of logging while drilling (LWD) technology. However, due to the different logging mechanisms, there are huge differences between GGD and NGD in the instrument designs, measurement methods, and instrument specifications. To further promote the NGD development, a comparative study on GGD and NGD was carried out from theoretical and simulation aspects. Based on the theoretical methods, the differences between GGD and NGD in the gamma sources, field distribution, methods for density measurement and instrument specifications were systematically elaborated. Then, using the Monte Carlo simulation, the GGD and NGD models were built to quantitatively verify the theoretical results. Last, the detailed performances of NGD and GGD methods in different logging environments were compared. Results showed that the theoretical results are highly consistent with the simulation results. The density sensitivity of NGD is less than half that of GGD, but the depth of investigation (DOI) approaches twice that of GGD. The density precision and vertical resolution of NGD are less than those of GGD. Additionally, compared to GGD, NGD also has an excellent performance in different pore fluid and lithology formations, but it is easily affected by borehole factors. The research can provide quantitative performance evaluations for the NGD replacing GGD in LWD logging.