Page 425 - Petrophysics 2E

ALTERATION OF WETTABILITY

cationic surfactants were attributed to ionic reaction of the cations with
adsorbed organic carboxylates from the oil.
Water-wet silica cores are produced by successively cleaning the rock
with toluene to remove organic compounds, steam to remove the
residual toluene and heavy crude oil components or humic acids (in the
case of outcrop sandstone), followed by treatment with hydrochloric
acid. This sequence will produce a water-wet core whose wettability
depends on the mineralogy of the rock and the composition of the fluids.
Excellent reproducibility is therefore possible under carefully controlled
conditions.
The surfaces of carbonate rocks may be made more oil-wet by
treatment with naphthenic acids, which react with the calcium carbonate
to produce a stable oil-wet surface [56]. Another way to control the
wettability of clean cores is to add surface-active compounds to the
fluids. Owens and Archer used barium dinonyl sulfonate dissolved in
oil to achieve an extreme oil-wet condition with a contact angle of
180' [99]. Mungan used hexylamine and n-octylamine dissolved in water
to change the advancing contact angle on a silica surface from 60"
(slightly water-wet) to 120° (slightly oil-wet) [69]. Kowaleswki et al.
changed the wettability of sandstone cores from water-wet to neutral
by adding hexadecylamine to n-decane [93]. The degree of wettability
change was controlled by the concentration of the amine dissolved in
the oil. Grattoni et al. altered wettability with oil-soluble tetramethyl
orthosilicate, which reacts with water in the pores to form a silicate
gel [loo]. The gel initially produces a water-wet system that changes
with respect to time to a neutral or oil-wet system.
The wettability of reservoir cores may be altered by penetration of
drilling fluids containing surface-active compounds or possessing a pH
which is either acidic or basic. Other aspects that must be controlled for
proper evaluation of oilfield cores are the packaging at the wellhead,
the length of storage prior to use, and laboratory core-cutting and
handling procedures. As the core is extracted to the surface, the decrease
of pressure results in expansion and loss of low molecular weight
components. This loss of lighter components can result in precipitation
of paraffins and asphaltenes that can alter the wettability toward a more
oil-wet condition. Several investigators have succeeded in preserving
long-term (years) wettability of oiLfield cores by placing the core in a
glass jar containing oil from the formation as soon as the core is available
from the driller; the cores are then transported as soon as possible and
kept in storage at about 5°C [101, 1021. Wrapping the cores in foil
and coating with plastic (polyethylene, polyvinylidene) or paraffin is
adequate for about six months. The problem with this method is that the
light components slowly diffuse through the coating, leaving the high
molecular weight compounds to gradually precipitate, which causes a