Wettability

Why one fluid clings to the rock surface while another floats in the middle of the pore - and how that preference controls recovery.

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⏱️ About 12 min

Illustrative image (AI-generated).

Two fluids share the same pore space - yet one clings to the rock and the other floats in the middle. Wettability decides which is which.

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The big idea: Wettability is a surface's preference for water over oil, set by the contact angle, and it controls where each fluid sits in the pore and how much oil is left behind.

🎯 By the end, you'll be able to

Define wettability as a surface's preference for water over oil, expressed through the contact angle
Classify rocks as water-wet, oil-wet, or intermediate/mixed-wet from the contact angle
Explain how wettability sets fluid distribution, residual saturations, and relative permeability
Name the laboratory wettability indices (Amott and USBM) and describe what they measure

What Wettability Means

When oil and water compete for the same pore space, they do not share it evenly. Wettability is the preference of a solid surface for one fluid over the other - the phase the rock 'likes' spreads across the grain surfaces, while the other is pushed into the middle of the pore. In most clastic reservoirs the rock is water-wet: water clings to the grain surfaces and oil sits in the pore centres. The strength of that preference is captured by a single angle.

The Contact Angle and the Wettability Classes

The contact angle $\theta$ is measured through the denser phase (water) at the line where oil, water, and solid meet. A small angle means water spreads easily over the solid. By convention, a rock is water-wet for contact angles below 90 degrees, oil-wet for angles above 90 degrees, and intermediate or mixed-wet near 90 degrees. Mixed-wet rocks are patchy - some pore surfaces are water-wet and some oil-wet - and they are common in carbonate reservoirs that have a long history of oil migration.

✨ Why Wettability Runs the Show

Wettability is not a curiosity - it decides how fluids are distributed and how much oil a flood can recover. The wetting phase coats the grains and occupies the small pores and corners; the non-wetting phase fills the larger pore centres. This sets the residual saturations (the oil left behind after a waterflood, or the water that cannot be produced) and the shape of the relative permeability curves. In the lab, wettability is quantified by indices such as the Amott and USBM tests - laboratory wettability indices that compare how readily a core spontaneously imbibes each fluid. No single number is perfect, but together these indices rank a core from strongly water-wet to strongly oil-wet.

📝 Worked example: Two otherwise identical sandstone cores are tested: core A is strongly water-wet and core B is strongly oil-wet. For each core, qualitatively state which fluid coats the grain surfaces and which occupies the pore centres, and which core is therefore likely to leave more oil behind (a higher residual oil saturation) after a waterflood.

Wettability decides which fluid adheres to the solid grain surfaces: the wetting phase coats the grains, and the non-wetting phase occupies the pore centres.
Core A (water-wet): water is the wetting phase, so it coats the grain surfaces as a film; oil, the non-wetting phase, sits in the pore centres.
Core B (oil-wet): oil is the wetting phase, so it coats the grain surfaces; water sits in the pore centres.
Consequence: in the oil-wet core B, oil clings to the grain surfaces and resists being flushed by injected water, so more oil is left behind - a higher residual oil saturation than in the water-wet core A.

✓ In core A (water-wet), water coats the grains and oil sits in the pore centres; in core B (oil-wet), oil coats the grains and water sits in the pore centres. The oil-wet core B typically retains more oil after a waterflood because the wetting oil clings to the grain surfaces.

Check your understanding

1. An oil-water contact angle measured through the water phase is 60 degrees. What does this indicate about the rock?

A contact angle of 60 degrees (below 90 degrees, measured through water) means water spreads on the surface, so the rock is water-wet.

2. In a strongly water-wet reservoir rock, how are water and oil distributed within each pore?

Because water is the wetting phase, it spreads as a film over the grain surfaces, while the non-wetting oil is pushed into the pore centres.

✅ Key takeaways

Wettability is a surface's preference for water over oil, quantified by the contact angle theta.
Rocks are water-wet below 90 degrees, oil-wet above 90 degrees, and intermediate/mixed-wet near 90 degrees.
The wetting phase coats the grains and the non-wetting phase fills the pore centres, setting residual saturations and relative permeability; Amott and USBM indices measure wettability in the lab.

➡️ Wettability sets the contact angle - and the contact angle drives the next idea: capillary pressure.

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