Ro/Rw is also known as which term?

Ro/Rw is also known as which term?

• A. Capillary pressure
• B. Porosity factor
• C. Formation factor
• D. Wettability factor

Answer: (C)

This ratio Ro over Rw is the formation factor, a fundamental term in Archie's law. Ro is the electrical resistivity of the rock when it is fully saturated with formation water (brine) at a given salinity, and Rw is the resistivity of that same formation water itself. The formation factor measures how much the rock’s pore structure and connectivity impede electrical current compared with the pore water alone. A rock with lower porosity or poorer pore connectivity causes higher resistivity and thus a higher formation factor. In practice, this factor links to porosity and rock texture: as porosity increases and pore pathways become more continuous, the formation factor tends to decrease, meaning the rock conducts electricity more readily. This concept is central to estimating water saturation and hydrocarbon content from resistivity measurements. Other terms don’t fit because capillary pressure concerns the pressure needed to move fluids through pore throats and is governed by wettability and pore geometry, not a simple resistivity ratio. Wettability and the so-called porosity factor aren’t standard descriptors for Ro/Rw. The formation factor is the established term for this specific ratio of rock to water resistivity.

This ratio Ro over Rw is the formation factor, a fundamental term in Archie's law. Ro is the electrical resistivity of the rock when it is fully saturated with formation water (brine) at a given salinity, and Rw is the resistivity of that same formation water itself. The formation factor measures how much the rock’s pore structure and connectivity impede electrical current compared with the pore water alone. A rock with lower porosity or poorer pore connectivity causes higher resistivity and thus a higher formation factor.

In practice, this factor links to porosity and rock texture: as porosity increases and pore pathways become more continuous, the formation factor tends to decrease, meaning the rock conducts electricity more readily. This concept is central to estimating water saturation and hydrocarbon content from resistivity measurements.

Other terms don’t fit because capillary pressure concerns the pressure needed to move fluids through pore throats and is governed by wettability and pore geometry, not a simple resistivity ratio. Wettability and the so-called porosity factor aren’t standard descriptors for Ro/Rw. The formation factor is the established term for this specific ratio of rock to water resistivity.