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Fundamentals of Wettability

Wael Abdallah Edmonton, Alberta, Canada Jill S. Buckley New Mexico Petroleum Recovery Research Center Socorro, New Mexico, USA Andrew Carnegie Kuala Lumpur, Malaysia John Edwards Bernd Herold Muscat, Oman Edmund Fordham Cambridge, England Arne Graue University of Bergen Bergen, Norway Tarek Habashy Nikita Seleznev Claude Signer Boston, Massachusetts, USA HassanHussain Petroleum Development Oman Muscat, Oman Bernard Montaron Dubai, UAE Murtaza Ziauddin Abu Dhabi, UAE
For help in preparation of this article, thanks to Austin Boyd, Gabriela Leu and Romain Prioul, Boston; Ray Kennedy, Edmonton; Patrice Ligneul, Dhahran, Saudi Arabia; John McCullagh, Sugar Land, Texas, USA; Guillemette Picard, Clamart, France; Raghu Ramamoorthy, Abu Dhabi, UAE; and Alan Sibbit,Moscow. Thanks also to the participants of the May 2007 Schlumberger Wettability Workshop, Bahrain. ECLIPSE, RSTPro (Reservoir Saturation Tool) and WFL (Water Flow Log) are marks of Schlumberger.

Understanding formation wettability is crucial for optimizing oil recovery. The oilversus-water wetting preference influences many aspects of reservoir performance, particularly in waterflooding andenhanced oil recovery techniques. Making the assumption that a reservoir is water-wet, when it is not, can lead to irreversible reservoir damage.

Wetting forces are in play all around us. They have practical applications, such as making rain bead up on a freshly waxed car so it is protected from rust. And they provide whimsy: wetting forces bind sand grains to hold the shape of a child’s sandcastle. Forces of wetting influence hydrocarbon reservoir behavior in many ways, including saturation, multiphase flow and certain log interpretation parameters. However, before getting into these details, it is best to first establish what wettability is. Wettability describes the preference of a solid to be in contact with one fluid rather than another. Although the term “preference” may seem oddwhen describing an inanimate object, it aptly describes the balance of surface and interfacial forces. A drop of a preferentially wetting fluid will displace another fluid; at the extreme it will spread over the entire surface. Conversely, if a nonwetting fluid is dropped onto a surface already covered by the wetting fluid, it

will bead up, minimizing its contact with the solid. If thecondition is neither strongly waterwetting nor strongly oil-wetting, the balance of forces in the oil/water/solid system will result in a contact angle, θ , between the fluids at the solid surface (below). In many oilfield applications, wettability is treated as a binary switch—the rock is either water-wet or oil-wet. This extreme simplification masks the complexity of wetting physics in reservoir rock.In a homogeneous, porous material saturated with oil and water, “strongly water-wetting” describes one end member of a continuum in which the surface strongly prefers contact with water. A strongly oil-wetting surface prefers contact with oil.1 Degrees of wetting apply along the continuum, and if the solid does not have a marked preference for one fluid over the other, its condition is termedintermediatewetting or neutral-wetting. Parameters that influence where on the continuum a system lies are discussed later.

θ θ ~ 0°

γow γso γso = γsw + γow cos θ γsw

θ

θ ~ 180°

> Contact angle. An oil drop (green) surrounded by water (blue) on a water-wet surface (left) forms a bead. The contact angle θ is approximately zero. On an oil-wet surface (right), the drop spreads, resulting ina contact angle of about 180°. An intermediate-wet surface (center) also forms a bead, but the contact angle comes from a force balance among the interfacial tension terms, which are γso and γsw for the surface-oil and surface-water terms, respectively, and γow for the oil-water term.

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Oilfield Review

Reservoir rocks are complex structures, often comprising a variety of mineral...
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