Case history of semi-empirical neutron tool calibration (user's guide)

The actual state of art of the semi-empirical way of calibration of the neutron porosity tools is presented. First a short description of the physics of the method itself is given with the emphasis on the sequence of calculation programs which has to be used in the whole calibration procedure. The principles of the correlation of the tool readings with different combinations of apparent neutron parameters of the calibration blocks are presented both for thermal and epithermal neutron signals. The physical, chemical and neutron properties of the calibration blocks at the Zielona Gora calibration facility are given. Special attention is paid on the uncertainty with which the crucial block parameters such as hydrogen content per cubic centimeter and the absorption cross section for thermal neutrons, are known. What was found during the work was the inadequacy in the knowledge of the bound water content in the rock matrices of some calibration blocks (Biala Marianna, Mucharz and Brenna) and the necessity to continue the research in the determination of the thermal neutron absorption cross section for rock matrices of calibration blocks, especially the carbonate ones. The method of establishing the basic calibration curve versus porosity is shown. The theoretical calculations of the porosity corrections for the variable borehole diameter and for the variable lithology are shown. For the single detector the influence of the rock matrix absorption cross section on the porosity determination has been found very significant. For the dual thermal detector readings the latter influence seems to be much less significant. The short instruction of the semi empirical way of neutron tool calibration is also presented. The project of further investigation is given. 18 figs., 16 tabs. 14 refs. (author)