Question

Which of the following logging tool(s) underestimate(s) porosity in a gas-bearing formation?

• A. Neutron log
• B. Nuclear Magnetic Resonance (NMR) log
• C. Sonic log
• D. Density log

Hint:

In gas-bearing formations, tools that rely on hydrogen density (such as Neutron and NMR) tend to underestimate porosity due to the lower hydrogen content of gas compared to water. Tools like the Density log and Sonic log are less impacted.

Answer 1

The Correct Option is A, B

In gas-bearing formations, different logging tools react differently due to the presence of gas, which can lead to underestimation of porosity. Here’s a breakdown:
- Neutron log: This tool is sensitive to hydrogen content, and in gas-bearing formations, the lower hydrogen content compared to water-saturated formations can lead to an underestimation of porosity.
- Nuclear Magnetic Resonance (NMR) log: NMR logging also tends to underestimate porosity in gas-bearing formations because the hydrogen density in gas is lower compared to water, which impacts the NMR signal.
- Sonic log: The sonic log measures the travel time of compressional waves, and it is not significantly affected by the presence of gas. Hence, it does not typically underestimate porosity in gas-bearing formations.
- Density log: The density log, like the sonic log, measures bulk density and is typically less sensitive to the presence of gas in comparison to neutron and NMR logs.
Therefore, the correct answer is (A) and (B), as both Neutron and NMR logs tend to underestimate porosity in gas-bearing formations.