A Comprehensive Investigation of Source Effects on Neutron Porosity Response for Logging-While-Drilling Measurements

Americium-Beryllium (Am-Be) neutron sources have been used in neutron porosity logging for over 40 years. However, they now have become considerably more expensive and difficult to obtain. In addition, a recent report by the National Academy of Sciences recommended further study towards the possible replacement of Am-Be sources with either Californium-252 (Cf-252) neutron sources or neutron generators. In view of the increased interest in alternative neutron sources, a detailed investigation was made of logging-while-drilling (LWD) neutron porosity measurements with Am-Be, Cf-252 and Deuterium-Tritium (D-T) neutron sources. Monte Carlo simulations were used to investigate differences in thermal neutron porosity response due to source energy, as well as source to detector spacing, collar size, borehole plus environmental effects, and several clay minerals. Laboratory experiments were performed and a test well was logged with two identical LWD tools run together the upper tool contained Cf-252 and the lower had Am-Be. The resulting neutron logs were in good agreement. Thus, with proper tool characterizations, Cf-252 and Am-Be sources yield similar neutron porosity logs.