Thomas M. Burnett, Craig A. Koopersmith, and Ronald L. Spross
Sperry-Sun Drilling Services, Houston, TX
ABSTRACT
Monte Carlo modelling of the MWD neutron porosity configuration reveals that the thick steel wall of the drill collar has a significant influence on the relative population of epithermal neutrons, thermal neutrons, and capture gamma radiation. The drill collar provides a channel for neutrons to scatter up through the tool and into the borehole and formation. This effect produces a markedly different neutron distribution than that of a typical wireline neutron porosity tool. The iron in the collar also has a relatively large thermal neutron capture cross section and radiates multiple gamma rays per capture, resulting in high capture gamma count rates. The combination of these interactions results in environmental sensitivities for each of the three neutron porosity determination methods that are significantly different from their wireline counterparts.
Analysis of the three known methods for MWD neutron porosity measurements reveals the following advantages and disadvantages:
— Epithermal measurements are sensitive to borehole washouts and tool eccentering, have a shallow depth of investigation, very low count rates, but are insensitive to trace elements such as boron, and to salinity changes.
— Thermal neutron measurements ate less sensitive to borehole washouts and eccentering, have higher count rates (for typical steel thickness) and deeper depth of investigation than epithermal measurements, but are sensitive to boron and salinity effects.
— Capture gamma ray measurements have the least borehole sensitivity to washouts and eccentering, highest count rates, deepest depth of investigation, but are also sensitive to trace elements like boron. Capture gamma ray measurements are less sensitive to formation salinity than purely thermal sensors due to offsetting effects of gamma radiation from the formation, borehole, and thermal neutron capture in the steel drill collar.
— The results indicate that in an MWD environment a capture gamma ray tool may offer distinct advantages over thermal or epithermal tools.