Neutron-porosity measurement is important for quantifying reservoir assets and optimizing production. The neutron-gamma porosity measurement is common for cased-hole logging. Since pulsed-neutron logging is the primary downhole technology for current residual hydrocarbon monitoring, it is cost effective to add porosity. Another benefit is the deep penetration from the 14MeV neutron, i.e., larger depth of investigation; however, its measurement sensitivity and large borehole effect are well-known limitations. These limitations may lead to large petrophysical uncertainties.
This paper introduces a new neutron-gamma porosity algorithm and test results. The new algorithm combines the inelastic count rates and capture count rates from a standard pulsed-neutron logging operation. The inelastic count rates are corrected for the capture background to optimize the measurement sensitivity. A detailed study was completed to optimize the capture count rates. Through this optimization, the capture count rate matches the net inelastic count rate for the borehole effects. The borehole-compensated ratio, between the net inelastic count rate and the total capture count rate, has been characterized for the new porosity algorithm.
The study originated from developing a comprehensive understanding by integrating a broad set of underlying nuclear data, including neutron slowing down length, scattering cross section, and activated gamma ray yields. Comprehensive understanding of the particle transport led to the new borehole compensation concept. This paper presents the detailed results to illustrate the borehole compensation improvements by more than 20%. The parameters of study covered primary environmental factors, such as borehole size, formation water salinity, and borehole fluid salinity. Depth of investigation results are derived from a large nuclear modeling database. For practical petrophysical applications, lithology curves are shown for this new porosity measurement. A detailed error analysis compares results from different porosity algorithms and concludes the significant improvement of the measurement sensitivity to be within 1.5 pu for the entire range from tight to porous rocks.
In summary, this paper introduces a new neutrongamma porosity measurement. This new measurement has been optimized by integrating nuclear tool physics and a large set of data. Error analysis was conducted for tight and porous rocks. The improved 1.5-pu measurement sensitivity and intrinsic borehole compensation leads to significant uncertainty reduction for petrophysical applications.