The Neutron Log

A Hydrogen Counter

The neutron tool sprays fast neutrons into the rock. A neutron loses the most energy when it hits something its own size, and the proton in a hydrogen atom is almost exactly a neutron mass, so hydrogen is far and away the best moderator in the formation. The tool counts how many neutrons are slowed and returned, which makes it a hydrogen counter. We summarise a material by its hydrogen index (HI), the hydrogen density relative to fresh water. Water and most oils have $HI \approx 1$, so the hydrogen sits in the pore fluid and the neutron porosity $\phi_N$ tracks the true porosity: in clean liquid-filled rock, $\phi_N \approx \phi$.

Calibrated in Limestone

There is a catch, the mirror image of the density log. The neutron is calibrated so that a limestone reads its true porosity, so limestone falls on the 1:1 line. The matrices moderate neutrons a little differently, so the same pore volume reads about two porosity units low in sandstone and several units high in dolomite. In the figure, one true porosity gives three different neutron readings. The neutron alone, like the density alone, cannot tell you the porosity until you know the lithology.

What the Neutron Needs

So the neutron carries the same ambiguity as the density, but it leans the opposite way under two important fluids: gas, with its low hydrogen index, drives the neutron porosity down, while shale, full of clay-bound water, drives it up. Those opposite leans against the density are exactly what makes the two logs together so powerful, the subject of the next sections. Read alone, the neutron is a hydrogen log; read with the density, it becomes a porosity-and-lithology tool.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.