Page 414 - Petrophysics 2E

382 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

displacement pressure is required for both fluids (for water-displacing-oil
from Si,, or oil-displacing-water from S,,,), which is the basis for
determination of neutral wettability. If, however, a small amount of
water will imbibe at Si, and an almost equal amount of oil will imbibe at
Sworr the system is at a condition of fractional or mixed wettability. The
distinction between these can be made only by microscopic observations
of thin sections.
If the system is oil-wet, these conditions for the water-wet case are
reversed: A1 is small and A2 is large. Oil will spontaneously imbibe into
the system, displacing water. Water must be forced into the system and,
therefore, A2 is a large value.
The work required for displacement of oil by water is the theoretical
work required for a waterflood and is one of the economic factors of
oil production. For example: if the reserve estimates, from field and
laboratory analyses of a small field, indicate that 1.6 x 105m3 (one million
barrels of oil) will be recovered from a waterflood and the work required
for displacement of the oil (from the current field saturation to So,) is
10kJ/m3 (1.5 BTU/bbl), then 1.6 mJ (1.5 million BTU) of energy, in
addition to friction losses in pumps and tubing, will be required for
completion of the waterflood.

WATER~OIL-ROCK INTERFACIAL
ACTIVITY

Surfactant-type compounds in crude oils, which are partially soluble
in water, have been found to pass rapidly through the thin water film on
water-wet surfaces and adsorb strongly on the rock [ 1 1, 351. Asphaltenes
(high molecular weight, polynuclear aromatic compounds containing
nitrogen, sulfur, and oxygen in ring structures) penetrate the aqueous
film to produce oil-wet surfaces in the rock. Thus, rocks containing
asphaltic oils will exhibit oil-wetting tendencies.
The silicate-water interface is acidic. Acidic compounds in crude oils
(those containing carboxylic and phenol groups) do not adsorb on silicate
surfaces, but basic constituents (nitrogen-containing compounds such
as amines and amides) adsorb readily, rendering the surface oil-wet. In
contrast, the carbonate-water surface is basic and the acid compounds
adsorb, whereas the basic compounds are repelled [28, 56-59]. Since
crude oils generally contain polar compounds that are acidic, the wetting
tendencies of brine-crude oil, -rock systems is for silicate rocks to
be neutral to water-wet and for carbonates to be neutral to oil-wet.
Akhlaq treated quartz and kaolinite samples with crude oils and then
characterized the adsorbed compounds with infi-ared spectroscopy [GO].
Basic nitrogen compounds and organic esters were found adsorbed to