Which log measures the natural battery effect at interfaces where drilling mud ions have entered porous zones?

Which log measures the natural battery effect at interfaces where drilling mud ions have entered porous zones?

• A. Induction log
• B. Spontaneous Potential (SP)
• C. Gamma Ray
• D. Resistivity log

Answer: (B)

Spontaneous Potential logs measure natural electrochemical potentials that exist at interfaces between drilling mud filtrate and formation water. This natural voltage, often described as a battery effect, arises when differing ionic solutions in the invaded zone contact each other and clay minerals, creating a tiny galvanic cell along permeable interfaces. The result is a voltage pattern that reflects the extent of mud-filtrate invasion, formation-water salinity contrasts, and permeability, especially in shaly or laminated formations. Because it directly records this natural potential, it uniquely captures the “natural battery” effect at the borehole wall. Other logs don’t measure this phenomenon. Induction logs rely on electromagnetic induction to gauge formation conductivity and resistivity, not the chemical potential at interfaces. Gamma ray logs detect natural radioactivity to help identify lithology and clay content, not electrical potentials. Resistivity logs inject current and measure the resulting voltage to determine resistivity, which reflects bulk conductivity rather than native electrochemical potentials at interfaces.

Spontaneous Potential logs measure natural electrochemical potentials that exist at interfaces between drilling mud filtrate and formation water. This natural voltage, often described as a battery effect, arises when differing ionic solutions in the invaded zone contact each other and clay minerals, creating a tiny galvanic cell along permeable interfaces. The result is a voltage pattern that reflects the extent of mud-filtrate invasion, formation-water salinity contrasts, and permeability, especially in shaly or laminated formations. Because it directly records this natural potential, it uniquely captures the “natural battery” effect at the borehole wall.

Other logs don’t measure this phenomenon. Induction logs rely on electromagnetic induction to gauge formation conductivity and resistivity, not the chemical potential at interfaces. Gamma ray logs detect natural radioactivity to help identify lithology and clay content, not electrical potentials. Resistivity logs inject current and measure the resulting voltage to determine resistivity, which reflects bulk conductivity rather than native electrochemical potentials at interfaces.