If the contact angle between oil and water on a core sample is less than 90°, which phase is the wetting phase?

If the contact angle between oil and water on a core sample is less than 90°, which phase is the wetting phase?

• A. Water
• B. Oil
• C. Gas
• D. Brine

Answer: (A)

Wettability is judged by the contact angle: the smaller the angle, the better that liquid wets the solid surface. When the contact angle between the oil–water interface and the rock is less than 90°, the liquid that spreads and sticks to the solid is the wetting phase. In this scenario, water is the phase that wets the core surface, so it is the wetting phase. Oil, by contrast, does not wet the surface as well and is non-wetting. Gas isn’t a liquid and doesn’t define wetting in the same way, and brine would behave similarly to water if the rock is water-wet—yet the key idea is which liquid forms the film on the solid, which here is water.

Wettability is judged by the contact angle: the smaller the angle, the better that liquid wets the solid surface. When the contact angle between the oil–water interface and the rock is less than 90°, the liquid that spreads and sticks to the solid is the wetting phase. In this scenario, water is the phase that wets the core surface, so it is the wetting phase. Oil, by contrast, does not wet the surface as well and is non-wetting. Gas isn’t a liquid and doesn’t define wetting in the same way, and brine would behave similarly to water if the rock is water-wet—yet the key idea is which liquid forms the film on the solid, which here is water.