The term for the ratio of the resistivity of a rock filled with water to the resistivity of that water is called?

The term for the ratio of the resistivity of a rock filled with water to the resistivity of that water is called?

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

Answer: (A)

The concept here is how a rock’s electrical resistivity relates to the resistivity of the water in its pores. When a rock is completely saturated with conductive water, its overall resistivity is higher than the water’s because the rock’s solid matrix adds resistance. The ratio of the rock’s resistivity to the pore-water resistivity is called the formation factor. In other words, ρ_rock = F × ρ_water, where F = ρ_rock / ρ_water. This formation factor captures how a rock’s pore geometry controls electrical flow and is influenced by porosity and the tortuosity or connectivity of the pore network. A common practical expression for clean sandstones is Archie’s law, which links F to porosity and rock texture: F ≈ a / φ^m, with φ representing porosity and m and a reflecting cementation and tortuosity. The formation factor is fundamental in interpreting formation resistivity measurements and estimating pore-water properties. The other terms don’t describe this ratio: capillary pressure relates to the movement of fluids in pores due to surface tension; while porosity and permeability describe how much void space exists and how easily fluids move, they are not themselves defined as the resistivity ratio between rock and water.

The concept here is how a rock’s electrical resistivity relates to the resistivity of the water in its pores. When a rock is completely saturated with conductive water, its overall resistivity is higher than the water’s because the rock’s solid matrix adds resistance. The ratio of the rock’s resistivity to the pore-water resistivity is called the formation factor. In other words, ρ_rock = F × ρ_water, where F = ρ_rock / ρ_water. This formation factor captures how a rock’s pore geometry controls electrical flow and is influenced by porosity and the tortuosity or connectivity of the pore network. A common practical expression for clean sandstones is Archie’s law, which links F to porosity and rock texture: F ≈ a / φ^m, with φ representing porosity and m and a reflecting cementation and tortuosity. The formation factor is fundamental in interpreting formation resistivity measurements and estimating pore-water properties.

The other terms don’t describe this ratio: capillary pressure relates to the movement of fluids in pores due to surface tension; while porosity and permeability describe how much void space exists and how easily fluids move, they are not themselves defined as the resistivity ratio between rock and water.