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Feasibility study of artifacts on the neutron logging curve near the boundary of layers

Wiącek Urszula, Woźnicka Urszula

Acta Geophysica

2019

Vol. 67, no. 6

1721--1729

The neutron borehole probe equipped with a neutron source and a system of neutron detectors is commonly used to determine porosity of geological formations. Transport of neutrons through media relies on their interaction with atomic nuclei, mainly with the process of scattering and absorption. Curves recorded by detectors for a given geological layer bordered by a medium with other neutron properties form the so-called anomalies. The course of the anomaly mainly depends on neutron properties of the bordering media and also on the measurement geometry, borehole properties, etc. The paper focuses mainly on an influence of neutron properties of bordered rock media on the course of the anomaly curve, especially near the media boundaries. In some cases of bordering media, characterized by specific neutron properties, the course of anomalies can be significantly disturbed, forming specific artifacts. The paper explains that the presence of artifacts is closely related to the properties of bordered media for both fast and thermal neutrons, i.e., the slowing down length of fast neutrons and the absorption cross section of thermal neutrons. Artifacts can be the cause of misinterpretation of the neutron rock porosity probe measurement. All considerations presented in this paper are based on results obtained from MC calculations performed using MCNP5 code (MCNP Team in X-5 Monte Carlo Team, MCNP—a general Monte Carlo N-particle transport code version 5. Los Alamos National Laboratory LA-UR-03-1987, 2008).

Neutron flux and axial moments in three-region cylindrical geometry applied for neutron log calibration. Part II: Numerical results

Drabina A.

Acta Geophysica Polonica

2000

Vol. 48, nr 4

523-529

This is the second part to the paper published in Acta Geophysica Polonica, Vol. XLVI, No. 4. The subject of both papers is the calculation of neutron flux in a three-region borehole geometry (borehole, intermediate zone ad formation). This paper presents numerical results of the theoretical solution presented in the first part. Numerical tests were performed to show how the intermediate region influences the neutron tool readings. The intermediate region was adopted in the form of mud-cakes (hematite, calcite, montmorylonite). The rock was assumed to be pure calcite. The tool readings and porosity were calculated for PKNN3 tool on the basis of general calibration curve experimentally derived at the calibration facility in Zielona Góra.