WETTABILITY
Wettability is the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids. Wettability refers to the interaction between fluid and solid phases. In a reservoir rock the liquid phase can be water or oil or gas, and the solid phase is the rock mineral assemblage.

Wettability is defined by the contact angle of the fluid with the solid phase.

Interfacial (boundary) tension (SIGMA) is the energy per unit area (force per unit distance) at the surface between phases. It is commonly expressed in milli-Newtons/meter (also, dynes/cm).


 



Adhesion tension (AT) is expressed as the difference between two solid-fluid interfacial tensions.
A negative adhesion tension indicates that the denser phase (water) preferentially wets the solid surface (and vice versa). An adhesion tension of 0.0 indicates that both phases have equal affinity for the solid surface.
 
When 2 or more fluids are present, there are at least 3 sets of forces acting on the fluids and affecting hydrocarbon  recovery.
 
DEFINITIONS
   A
T = adhesion tension
   THETA = contact angle between the oil/water/solid interface measured through the water
   SIGMAos = interfacial energy between the oil and solid
   SIGMAws = interfacial energy between the water and solid
   SIGMAow = interfacial energy (interfacial tension) between the oil and water

Wetting phase fluid preferentially wets the solid rock surface. Attractive forces between rock and fluid draw the wetting phase into small pores. Wetting phase fluid often has low mobility.

Attractive forces determine the lower limit to irreducible wetting phase saturation. Many hydrocarbon reservoirs are either totally or partially water-wet.

Reservoir rock is water wet if water preferentially wets the rock surfaces. The rock is water wet under the following conditions:
         SIGMAws >= SIGMAos
         AT < 0 (i.e., the adhesion tension is negative)
         0° <= THETA <= 90°

When THETA is close to 0
°, the rock is considered to be “strongly water wet”.

Nonwetting phase does not preferentially wet the solid rock surface. Repulsive forces between rock and fluid cause nonwetting phase to occupy largest pores. Nonwetting phase fluid is often the most mobile fluid, especially at large nonwetting phase saturations. Natural gas is never the wetting phase in hydrocarbon reservoirs.


Reservoir rock is oil-wet if oil preferentially wets the rock surfaces.
The rock is oil wet under the following conditions:
       SIGMAos >= SIGMAws
       A

T > 0 (i.e., the adhesion tension is positive)
       90
° <= THETA <= 180°
When THETA is close to 180
°, the rock is considered to be “strongly oil wet”


Comparison of water wet and oil wet rocks


Wettability is classified by its variations;
    Strongly oil or water wetting.
    Neutral wettability – no preferential wettability to either water or oil in the pores.
    Fractional wettability – reservoir that has local areas that are strongly oil-wet, whereas most
          of the reservoir is strongly water-wet - occurs where reservoir rock has variable
          mineral composition and surface chemistry.
    Mixed wettability – smaller pores are water wet and filled with water, whereas larger pores are oil wet
          and filled with oil. Residual oil saturation is low - occurs where oil with polar organic compounds
          invades a water-wet rock saturated with brine.

Wettability varies with surface roughness, so the wettability of a rock will vary with grain shape, size, and rounding.


Roughness or angularity of grains affects wettability.

The current measuring method uses a modern contact angle goniometer and is called the static sessile drop method. The contact angle goniometer is an optical subsystem which capture the profile of a pure liquid on a solid substrate. The angle formed between the liquid/solid interface and the liquid/vapor interface is the contact angle. Older systems used a microscope optical system with a back light. Current-generation systems employ high resolution cameras and software to capture and analyze the contact angle.

The dynamic sessile drop method is similar to the static sessile drop but requires the drop to be modified. A common type of dynamic sessile drop study determines the largest contact angle possible without increasing its solid/liquid interfacial area by adding volume dynamically. This maximum angle is the advancing angle. Volume is removed to produce the smallest possible angle, the receding angle. The difference between the advancing and receding angle is the contact angle hysteresis. 

 

IMBIBITION and DRAINAGE
Imbibition is a fluid flow process in which the saturation of the wetting phase increases and the nonwetting phase saturation decreases. (e.g., waterflood of an oil reservoir that is water-wet). Mobility of wetting phase increases as wetting phase saturation increases. Mobility is the fraction of total flow capacity for a particular phase

If a water-wet rock saturated with oil is placed in water, it will imbibe water into the smallest pores, displacing oil. If an oil-wet rock saturated with water is placed in oil, it will imbibe oil into the smallest pores, displacing water.

Drainage is a fluid flow process in which the saturation of the nonwetting phase increases
. Mobility of nonwetting fluid phase increases as nonwetting phase saturation increases e.g., waterflood of an oil reservoir that is oil wet  Gas injection in an oil or water wet reservoir. Pressure maintenance or gas cycling by gas injection in a retrograde condensate reservoir. A water-wet reservoir that accumulation of oil or gas in a trap does so by drainage.
 
Primary and waterflood oil recovery is affected by the wettability of the system. A water-wet system will exhibit greater primary oil recovery.
 
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