Page 75 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 63

From a combination of Dupre's equation for wetting tension and Young's
equation [94], the adhesion tension (z,) can be given as [19,95-971:
2* = 0- - 0, = a*- cos 0, (5-73)

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

Figure 5-44. Examples of contact angles [QS].