Which of the following logging tool(s) underestimate(s) porosity in a gas-bearing formation?
Show Hint
- Neutron log
- Nuclear Magnetic Resonance (NMR) log
- Sonic log
- Density log
The Correct Option is A, D
Solution and Explanation
Step 1: Neutron log.
- Neutron logs measure hydrogen concentration. - In gas zones, hydrogen index is lower compared to liquid-filled porosity. - Hence, neutron logs underestimate porosity in gas-bearing formations.
Step 2: Density log.
- Density log measures electron density, which is lower for gas compared to liquid. - This makes the log read lower bulk density, leading to underestimated porosity.
Step 3: Sonic log.
- Sonic log measures travel time of acoustic waves. - Gas affects compressibility but does not cause systematic underestimation of porosity.
Step 4: NMR log.
- NMR directly measures pore fluids via relaxation times. - It can distinguish between gas and liquid-filled pores, so it does not underestimate porosity in gas zones.
Final Answer: \[ \boxed{\text{Neutron log and Density log (A and D)}} \]
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