Wettability is one of the most critical concepts in petroleum engineering, directly impacting how oil, water, and gas behave in subsurface reservoirs. Whether you're optimizing waterflooding strategies or interpreting core analysis results, a strong understanding of wettability is essential for efficient oil recovery and reservoir management.
🧪 What Is Wettability?
Wettability refers to a rock's preference to be in contact with one fluid over another typically water, oil, or gas. In simple terms, it describes how fluids spread over or adhere to rock surfaces in the presence of other immiscible fluids.
In oil reservoirs, this behavior determines how fluids distribute within the porous rock and how easily they can be produced.
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| Wettability in Petroleum Engineering |
🔍 Types of Wettability
Wettability is commonly categorized into three types:
1. Water-Wet
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The rock surface prefers water.
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Water spreads across the rock surface.
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Oil is pushed into the center of the pores.
2. Oil-Wet
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The rock surface prefers oil.
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Oil coats the rock surface.
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Water is confined to the center of the pores.
3. Intermediate-Wet
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The rock has no strong preference.
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Both oil and water share the rock surface more evenly.
⚙️ Why Wettability Matters in Oil and Gas Reservoirs
Understanding wettability is crucial for:
✔️ 1. Fluid Saturation and Distribution
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In water-wet rocks, water occupies smaller pores and coats grain surfaces.
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In oil-wet systems, oil sticks to rock surfaces while water moves to larger pores.
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This impacts initial hydrocarbon saturation and flow patterns.
✔️ 2. Oil Recovery Efficiency
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Waterflooding works better in water-wet reservoirs.
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In oil-wet systems, water may bypass oil, reducing recovery.
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Enhanced oil recovery (EOR) techniques often aim to change rock wettability to water-wet.
✔️ 3. Capillary Pressure and Relative Permeability
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Wettability affects capillary pressure curves, influencing fluid movement through tiny pore throats.
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It also alters relative permeability, which defines how easily oil, water, or gas flows under certain saturations.
🧪 How Is Wettability Measured?
Several methods are used to evaluate wettability in rock samples:
📏 Contact Angle Measurement
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Measures the angle between a liquid droplet and rock surface:
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< 90° → Water-wet
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> 90° → Oil-wet
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= 90° → Neutral (intermediate)
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🧪 Amott-Harvey Index
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Measures spontaneous and forced fluid imbibition.
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Gives a numerical wettability index between -1 (strongly oil-wet) and +1 (strongly water-wet).
🔬 USBM Method
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Based on displacement energy during oil and water movement in core samples.
🔄 Can We Modify Wettability?
Yes! Altering reservoir wettability can boost oil recovery. Common methods include:
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Surfactant injection – Lowers interfacial tension and alters surface preference.
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Chemical flooding – Alters rock-fluid interaction to make the reservoir more water-wet.
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CO₂ injection – Can also shift wettability, particularly in carbonate reservoirs.
These methods are used in Enhanced Oil Recovery (EOR) projects to improve displacement efficiency.
🧠 Key Takeaways
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Wettability controls how fluids distribute and move through reservoir rocks.
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It directly impacts oil recovery, especially during secondary and tertiary recovery.
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Measuring and modifying wettability is essential for customizing reservoir management strategies.
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