What well log is used to determine both total and effective porosity?

• A. Resistivity log
• B. Gamma ray log
• C. Neutron log
• D. Magnetic resonance imaging tool

Answer: (D)

The essential idea is that porosity comes in two flavors: total porosity, the full pore space available in the rock, and effective porosity, the portion of that space that actually allows fluid flow. Magnetic resonance logging (NMR) directly senses hydrogen in fluids inside all pores, so it can quantify the total pore volume by the overall hydrogen content. It also analyzes the relaxation times (T2 distribution) of those fluids to separate bound water from movable, or free, fluids. The signal from mobile fluids represents the portion of porosity that can contribute to flow, i.e., the effective porosity. That combination—measuring total pore space and distinguishing movable fluids—lets the NMR/magnetic resonance tool provide both total and effective porosity, something the other logs don’t do as directly.