Neutron Absorption Effects On Dual-Spaced Thermal Neutron Logging Tools

The ratio response of a dual-spaced thermal neutron logging tool is primarily controlled by neutron elastic scattering with formation and borehole nuclei during the slowing down process and secondarily by neutron absorption during diffusion at thermal energy. Neutron absorption cross sections at thermal energy (sigma) are functions not only of pure matrix materials and fluids present, but also of strong absorbers such as chlorine, boron, and rare earths that may be contained in both. Even trace amounts of these materials can have a significant effect on sigma matrix and sigma fluid, and thereby on tool response. Although the dual-spaced ratio method is much less sensitive to sigma matrix and sigma fluid effects than older single detector methods, accurate porosity determination from ratio still requires correction for these effects both during ratio-porosity transform determination and during logging operations. This is true for all dual-spaced thermal neutron logging tools, but the amount of correction depends on source-to-detector spacings and the selection of shielding materials. Dual-spaced thermal neutron porosity response is normally based on ratio and porosity measurements in limestone test formations. This paper details how this process can be made more accurate by also including matrix density and sigma data. A tool model uses basic nuclear cross sections and calculations of effective neutron migration lengths in order to correct measured ratios for observed matrix density and sigma values prior to obtaining the ratio-porosity transform. This results insignificant improvement in porosity measurement accuracy at standard limestone conditions and in formations with known values of matrix density and sigma matrix. Porosity response curves for field limestones, sandstones, and dolomites are presented for typical values of sigma matrix and sigma fluid. Corrections are also presented for logging in formations with known sigma matrix and sigma fluid. New test formation and logging data using these curves are presented. Corrections for sigma matrix are particularly significant in dolomites and limestones; however, even in sandstones, such corrections can be important. Neutron absorption effects are important for porosities above about 5%. Sigma matrix corrections are most important in low salinity environments and in hydrocarbon bearing formations.