Neutron Log

By Luke Stoeckel with edits by Tom Sturman

Neutron logs are used to estimate the apparent porosity of a formation. It uses a radioactive source, (e.g. plutonium-beryllium or americum-beryllium) which bombard the formation with high energy neutrally charged particles (neutrons). The neutrons are about the same size as hydrogen atoms. As more hydrogen atoms encountered in the pore space more neutrons collide, thereby losing energy and becoming captured. The count rate is consequently reduced which indicates the concentration of hydrogen atoms, water and oil have roughly the same amount of hydrogen, while gas has a lower hydrogen density, hence the tool cannot distinguish between oil and water, but gas with less hydrogen can be easily detected. The neutrons are not however affected by the matrix through which they are travelling.
Some corrections are needed before the apparent porosity can be calculated from the neutron logs. The first correction is due to the salinity of the formation water. Chlorine is an excellent neutron absorber and as neutrons lose their energy they are absorbed by the chlorine atom affecting the count rate and thus the porosity calculation; the lower the count rate, the higher the porosity, the two are reciprocals of each other. Shale can also affect the porosity calculation as it contains a great deal of bound water which can also causes the count rate to decrease thusly increasing the porosity indication. Another major correction needed is for the mud and mudcake contained within the wellbore which has high hydrogen content usually a correction nomograph is used for the correction.

Figure 4. Gamma and neutron logs plotted to use correlative features.
Neutron logs are often used in conjunction with density and gamma ray logs to see if reservoir sections have hydrocarbons present and the saturation of water within the reservoir (Figure 4). Coal layers can easily be seen on neutron log due to the high hydrogen content from biological matter which is shown by the count rate decreasing to almost zero.