Wettability and Capillarity

Question 1

What is the difference between intermediate-wetting and mixed-wetting?

Answer 1

Intermediate-wetting - lacking a strong wetting preference
Mixed-wetting - having a variety of preferences, possibly including intermediate-wetting (reservoir rocks are complex structures, often comprising a variety of mineral types. Each mineral may have different wettability)

Question 2

What is imbibition?

Answer 2

Imbibition refers to an increase in the saturation of the wetting phase, whether spontaneous or forced.

Question 3

What is drainage?

Answer 3

Drainage refers to an increase in saturation of the non-wetting phase.

Question 4

Name some wettability measurement techniques

Answer 4

• Amott-Harvey: amounts of oil and water imbibed by a sample spontaneously and by force.
• U.S. Bureau of Mines (USBM): work required to imbibe oil and water.
• Microscopic examination: microscopic examination of the interaction between the fluids and the rock matrix.
• NMR: changes in longitudinal relaxation time.
• Flotation method: the distribution of grains at water/oil or air/water interfaces.
• Glass slide method: displacement of the non-wetting fluid from a glass slide.
• Relative permeability method: shape and magnitudes of kro and krw curves.
• Resistivity logs: resistivity logs before and after injection of a reverse wetting agent.
• Dye adsorption: adsorption of a dye in an aqueous solvent.

Question 5

Why is wettability important?

Answer 5

• influences capillary pressure
• influences relative permeability
• influences Archie saturation exponent

Question 6

How is capillary pressure measured?

Answer 6

• Mercury injection (fast, cheap and many data points. Need to convert to oil-brine system (via surface tension and contact angle) and need to correct for CBW (via H-S-K).)
• Pressure equilibrium (same equipment as used to measure I-Sw)
• Centrifuge (Sw not as accurate: end face Sw vs average Sw. Only a few data points. Good for low perms (<=200mD).)

Note: Want sample to be water-wet and under stress.

Question 7

What are the corrections needed for cap curve experiments?

Answer 7

• Closure correction
• Stress correction
• CBW correction
• Wettability and interfacial tensions

Question 8

How are interfacial tensions measured?

Answer 8

• Pendant drop method
• Wilhelmy plate method
• Du Nuoy ring method