Wettability

Contact angles of an oil droplet on water wet(left), intermediate wet (center), and oil wet (right) surfaces.

In general term, wettability is the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluid. In the petroleum context, wettability is the tendency of a reservoir rock surface to preferentially contact a particular fluid in a multiphase or two-phase fluid system.

Wettability can change the reservoir petrophysical characteristics (relative permeability, capillary pressure, etc.).Information about wettability is fundamental to understanding multiphase flow problems ranging from oil migration from source rocks through primary production mechanisms to secondary/tertiary recovery processes.

Wettability measurements

Contact angle

The conventional means of measuring the reservoir rock wetting state is by contact angle measurement of an oil droplet on the rock. Water-wet if the contact angle is less than 90; oil-wet if the contact angle is larger than 90; intermediate wet if the contact angle is ~90. The reservoir wetting state may further be divided into strongly-water-wet, weakly-water-wet, strongly-oil-wet and weakly-oil-wet.

One of the most obvious limitations of wettability characterization using contact angle measurement is the absence of a standard reference. Consequently, except at the end point wetting states, the classification of wetting state from contact angle measurement is arbitrary and subjective.

Another important limitation of the contact angle method is that the required length of equilibration time cannot be reproduced in the lab. This may lead to problems such as erroneous classification of wetting state and sometimes to reproducibility issues.

Displacement study

This includes the Amott wettability test and the United States Bureau of Mines (USBM) wettability test which are both derived, directly or indirectly, from capillary pressure phenomena. These tests reflect the ease of the wetting fluid displacing the non-wetting fluid (spontaneous imbibition).

Cautions on sample handling

As it is very difficult to measure the in-situ reservoir wetting state, reservoir wettability is typically measured in the lab through the use of core plugs or whole length cores. However, the wetting state of the core samples may be altered from their in-situ values during cutting, surfacing and handling of the core samples. Variation of the reservoir wettability from the in-situ reservoir wettability is due to a number of reasons including:

1. Flashing of the connate water present due to reduction in pressure
2. Drying
3. Invasion of drilling mud
4. Asphaltene deposition or wax precipitation from the crude oil due to temperature and pressure effects
5. Oxidation, which sometimes enhances deposition

Care must be taken in the handling of the core samples to ensure that the actual wettability is not altered.

Wettability heterogeneity

A pore level view of wetting states.

Reservoir rocks are complex structures, often comprising a variety of mineral types. Each mineral may have a different wettability, making the wetting character of the composite rock difficult to describe.

Most early analyses on the effect of wettability on oil recovery were based on the simplistic assumption of uniform/homogeneous wettability throughout the reservoir. However, research into the wetting-state of reservoir rocks suggest that wettability in a reservoir rock is indeed typically heterogeneous.

The discovery of possible non-uniform wetting condition in the reservoir opened up new vistas of understanding and research in reservoir rock wettability and led to the definition of other wetting states, besides the gamut covering strongly water-wet to strongly oil-wet reservoirs. These wettingstates include:

1. mixed-wettability;
2. fractional wettability;
3. “dalmatian” wetting;
4. speckled wetting.

A common feature of heterogeneous wettability is the presence of distinguishable zones that are respectively preferentially oil and water wet. The extent, type and distribution of the wetting heterogeneity is greatly influenced by the chemical variation of the reservoir rock mineralogy.

Effects of wettability on relative permeability

Wettability affects relative permeability because it is a major factor in the control of the location, flow, and distribution of fluids in a porous medium. Typically, as the system becomes more oil-wet, the water relative permeability increases and the oil relative permeability decreases. The more oil-wet the rock, the higher the water saturation positioned in the center of the pores competing with the oil in the most permeable pathways, reducing the relative permeability to oil, and increasing the relative permeability to water. Therefore, wettability alteration to more oil-wet due to near well-bore asphaltene deposition can hinder oil production.

Effects of wettability on oil recovery

Water flood

It has long been known that wettability is a primary determinant of waterflood recovery efficiency. Strongly oil-wet reservoirs give the least waterflood oil recovery and the best recovery appears to be the mixed-wet reservoir, particularly where surface film drainage mechanism is also observed. While most researchers are in agreement on the least waterflood oil recovery for oil-wet reservoirs, there is a lack of consensus as to the wetting condition for maximum oil recovery. The only consensus seems to be that the best oil recovery is achieved when the reservoir is at some intermediate-wetting state – not strongly oil-wet and not strongly water-wet.

Gas flood

Gas flood recovery efficiency also depends on reservoir wettability as well as the spreading coefficient. The best gas flood oil recovery was observed for oil-wet reservoirs particularly for tertiary recovery process i.e. at waterflood oil saturation. For gasfloods in secondary recovery processes, the mixed-wet and water-wet systems resulted in higher recoveries.