Formation porosity measurement using epithermal neutron lifetime

A logging tool for in-situ measurement of earth formation porosity using epithermal neutron lifetime has been constructed and tested. A 14-MeV neutron generator in the tool emits pulses of neutrons and a Gd-covered ³He counter detects neutrons returning from the formation as a function of time following the emission pulse. Counts are accumulated in 200 1-μs-wide channels for repeated pulses of the source. Tests of the logging tool in formation models of known porosity and in oil wells indicate superior porosity sensitivity and decreased lithology (rock type) dependence compared to methods using steady-state sources. Computer modeling using the time-dependent Monte Carlo Neutron Photon (MCNP) code has provided lifetimes which support and extend measurements made with the tool. MCNP results from a three-dimensional model of the logging tool in a borehole have yielded excellent agreement with actual tool measurements