Which log type is used to measure porosity by neutron interactions?

Which log type is used to measure porosity by neutron interactions?

• A. Sonic log
• B. Density log
• C. Neutron log
• D. Gamma ray log

Answer: (C)

The key idea is that porosity is about how much pore space is available to hold fluids, and the neutron log is designed to sense hydrogen, which lives in those fluids. Fast neutrons emitted by the tool collide with nuclei in the rock, and when they hit hydrogen, they slow down and become slow neutrons. The detector then counts these slow neutrons. More slow neutrons indicate more hydrogen per unit volume, which means more pore space filled with fluids. Since most hydrogen is in the pore fluids (water, oil, or gas) rather than in the rock matrix, the neutron log provides a direct proxy for porosity. In contrast, other logs measure different properties: sonic logs assess how fast sound travels through the formation (linked to pore structure and rock stiffness), density logs gauge bulk density (a combination of rock matrix and fluids), and gamma ray logs track natural radioactivity (helping to identify lithology like shale). So the neutron log is the one specifically used to infer porosity by monitoring neutron interactions with hydrogen.

The key idea is that porosity is about how much pore space is available to hold fluids, and the neutron log is designed to sense hydrogen, which lives in those fluids. Fast neutrons emitted by the tool collide with nuclei in the rock, and when they hit hydrogen, they slow down and become slow neutrons. The detector then counts these slow neutrons. More slow neutrons indicate more hydrogen per unit volume, which means more pore space filled with fluids. Since most hydrogen is in the pore fluids (water, oil, or gas) rather than in the rock matrix, the neutron log provides a direct proxy for porosity.

In contrast, other logs measure different properties: sonic logs assess how fast sound travels through the formation (linked to pore structure and rock stiffness), density logs gauge bulk density (a combination of rock matrix and fluids), and gamma ray logs track natural radioactivity (helping to identify lithology like shale). So the neutron log is the one specifically used to infer porosity by monitoring neutron interactions with hydrogen.