Wettability and Contact Angle

The contact angle (ec), existing between two fluids in contact with a solid and measured through the more dense phase, is a measure of the relative wetting or spreading by a fluid on a solid. A contact angle of zero indicates complete wetting by the more dense phase, an angle of 180" indicates complete wetting of the less dense phase, and an angle of 90' means that neither fluid preferentially wets the solid.

where is the interfacial tension between the solid and the less dense fluid phase, 0, is the interfacial tension between the solid and the more dense phase, and 0, is the interfacial tension between the fluids of interest. With gas-oil systems, oil is the more dense phase and is always the wetting phase [SS]. With oil-water systems water is almost always the more dense phase, but either can be the wetting phase. For oil and water, a positive adhesion tension (ec e 90") indicates a preferentially water-wet surface, whereas a negative adhesion tension (0, > 90") indicates a preferentially oil-wet surface. For a contact angle of go", an adhesion tension of zero indicates that neither fluid preferentially wets the solid, Examples of various contact angles are depicted in Figure 5-44 [96].

The importance of wettability on crude oil recovery has been recognized for many years. This subject is discussed in a subsequent section of this chapter. Although Nutting E981 observed that some producing formations were oil-wet, many early workers considered most oil reservoirs to be water-wet (e.g., References 23, 99, and 100; discussion and comments in Reference 96). From a thermodynamic standpoint, it was felt that pure, clean silica must be wetted by water in preference to any hydrocarbon.

In one study [loll, no crude oils were tested that had a greater adhesion than pure water. Other results [lo21 tended to support this contention: capillary pressure tests suggested that all cores tested were water-wet with contact angles ranging from 31" to SO". However, there are two reasons why these results were obtained [103]: (1) the cores were extracted with chloroform priar to the tests which could have affected the natural wettability, and (2) only receding (decrease in wetting phase saturation) contact angles were .measured during the capillary pressure tests. As with capillary pressures, there is a hysteresis in the receding and advancing (increase in wetting

phase saturation) contact angles; receding angles are smaller than advancing angles [97]. Bartell and coworkers [95-971 were among some of the first investigators to measure contact angles with crude oil systems that suggested the possibility that oil reservoirs may not be water-wet. Furthermore, they concluded that spontaneous displacement of oil by water should occur only when both advancing and receding angles are less than go", and no spontaneous imbibition should occur if the two angles are on opposite sides of 90" [97].