Which log does not work in oil-based mud?

Which log does not work in oil-based mud?

• A. Gamma Ray Log
• B. SP Logs
• C. Any Normal Electric Log
• D. Resistivity Log

Answer: (C)

In oil-based mud the drilling fluid is largely non-conductive, so there isn’t a good electrical path from the borehole tool to the formation. Logs that rely on sending current through the borehole fluids and forming a circuit with the formation—i.e., normal electric logs using contact electrodes—don’t establish a reliable current path in OBM, which makes their readings meaningless or highly inaccurate. Other tools work differently. Gamma ray logs measure natural gamma radiation emitted by the formation itself, so they don’t depend on borehole conductivity and remain usable in OBM. Spontaneous potential logs rely on ionic differences between the borehole fluid and formation water, which aren’t established when the borehole is oil-based, so they’re not reliable in OBM. Induction-based resistivity tools (the non-contact electric methods) can operate in OBM because they don’t depend on a conductive borehole fluid. So the best answer is the log type that requires a direct electrical path through the borehole fluid—normal electric logs.

In oil-based mud the drilling fluid is largely non-conductive, so there isn’t a good electrical path from the borehole tool to the formation. Logs that rely on sending current through the borehole fluids and forming a circuit with the formation—i.e., normal electric logs using contact electrodes—don’t establish a reliable current path in OBM, which makes their readings meaningless or highly inaccurate.

Other tools work differently. Gamma ray logs measure natural gamma radiation emitted by the formation itself, so they don’t depend on borehole conductivity and remain usable in OBM. Spontaneous potential logs rely on ionic differences between the borehole fluid and formation water, which aren’t established when the borehole is oil-based, so they’re not reliable in OBM. Induction-based resistivity tools (the non-contact electric methods) can operate in OBM because they don’t depend on a conductive borehole fluid.

So the best answer is the log type that requires a direct electrical path through the borehole fluid—normal electric logs.