The Effect of Formation Absorption on the Thermal Neutron Porosity Measurement

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Abstract Compensated thermal neutron porosity devices provide reliable estimates of formation porosity in standard borehole conditions. Departure from these conditions perturb the measurement and it is necessary to apply corrections to determine the true porosity. One common perturbation is due to the salinity of the formation water. As the salinity of the formation water increases, its hydrogen density (or index) decreases. By itself, this would cause the apparent porosity reading to decrease, reflecting a lower hydrogen content. The additional chlorine, however, causes an increase in the thermal neutron capture cross section (Σ) that depresses the thermal neutron flux in the formation. This, in turn, tends to increase the apparent porosity. Empirical correction charts have been established for these two competing trends. They are sufficient for correcting porosity readings for salinity effects. However, the presence of additional thermal neutron absorbers associated with either the rock matrix or the pore fluid can result in a residual error on the porosity estimate even after salinity correction. This arises because the correlation between Σ and porosity, used to derive the salinity correction, is no longer valid. An analysis of the response of a compensated thermal neutron device has been made in terms of the slowing-down length, and diffusion length. It provides a simple method of describing salinity, absorption and lithological effects, and results in a predictive model of the tool response that separates neutron absorption effects from changes in hydrogen index. The model is shown to conform to established salinity corrections as well as to measurements made in laboratory formations with large values of Σ. It provides a means of correcting apparent neutron porosity values for variations in formation Σ not associated with salinity.