Page 415 - Petrophysics 2E

WATER-OIL-ROCK INTERFACJAL ACTMTY 383

quartz sand, whereas sulfonic acids together with carbonyl groups and
phenols were adsorbed on kaolinite surfaces.
Crude oils contain surface-active compounds that can mode the
wettability of the reservoir by changing the chemical species at the fluid
and rock interfaces, depending on pH, salinity, and the nature of the
surface-active compounds. Depending on the immediate environmental
conditions, different types of surface-active compounds present in
the crude oil will move to the fluid and rock interfaces and govern
the wettability of the reservoir. Salinity and pH apparently control the
aqueous-mineral interfacial cation binding and acid-base reactions of
compounds. Binding of surface-active compounds present in the
crude oils and precipitated asphaltene-type molecules occur at oil-rock
interfaces [61].
All petroleum reservoirs were originally believed to be water-wet
because clean rocks of all types exhibit preferential water-wetting
tendencies. In addition, sedimentary rocks containing oil were originally
saturated with water that was displaced when oil migrated into the
geologic trap. Polar organic compounds in petroleum, however, are
expelled from the bulk phase and react chemically with clay and other
minerals in the rock to form neutral, mixed, or preferentially oil-wet
systems. The Wilcox sandstone of the Oklahoma City field, the Tensleep
sandstone in Wyoming, and the Bradford sands in Pennsylvania are well
known oil-wet reservoirs [3, 35,62,63]. Carbonate reservoirs have been
found to range in wettability from neutral to strongly oil-wet [20,64,65].
Treiber et al. used contact angle measurements to examine the
wettability of 30 silicate and 25 carbonate rocks (Table 6.3) [66]. Their
contact angle criteria were: water-wet = 0-75", intermediate-wet =
75-105', and oil-wet = 105-180". A few of the silicate rocks were
intermediate-wet, but the rest were almost equally divided between
water-wet and oil-wet. On the other hand, the carbonate rocks
were largely oil-wet. Chilingarian and Yen used contact angle measure-
ments, with different criteria for the divisions of wettability, to
measure the wettability of carbonate rocks from various parts of

TABLE 6.3
RELATIVE WETTING TENDENCIES SANDSTONES AND CARBONATES
OF
Treiber et al. [a61 Chilingarian and Yen [64]
Wettability Silicates, % Carbonates, % Carbonates, %

Water-wet 43 8 8
Intermediate-wet 7 4 12
Oil-wet 50 88 80