Page 426 - Petrophysics

394 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

gradual change to a more oil-wet condition. Any method of storage that
allows even a small amount of evaporation will result in alteration of
wettability .

AGING THE OIL-BRINE-ROCK SYSTEM

Reservoir rocks that have been cleaned, and outcrop rocks that
have not been in contact with oil, generally exhibit a water-wet
condition, especially if refined oils are selected for experiments. Cleaned
carbonate rocks, however, have neutral to slightly oil-wet tendencies.
A small change toward more oil-wet is observed if the core is first
saturated with water, and then the water is displaced to the point of
irreducible water saturation with the oil. Immersion in the crude oil at
an elevated temperature (60"-90OC) will change the wettability to an
oil-wet system. Stable core wettability is usually obtained after the core
is aged in crude oil at an elevated temperature for a least 100 hours
(Figure 6.12). Surface-active heterogeneous nitrogen, sulfur, and oxygen
(NSO) compounds in the crude oil slowly migrate to the oil-rock interface
and are adsorbed strongly on rock mineral surfaces. The oil should
be centrifuged to remove suspended sedimentary particles and high
molecular weight compounds that have been precipitated by changes
of temperature, pressure, and storage time after production from the
reservoirs. Some oils have compounds that react with atmospheric
oxygen to form cross-linked compounds that precipitate from the
oil. Micelles holding asphaltenes in suspension can be broken by air
oxidation, causing precipitation of the asphaltenes. When an oil is
encountered that is sensitive to air oxidation, even after repeated
filtration, it must be collected in the field under a blanket of nitrogen
and maintained under nitrogen for all transfers between containers when
used in the laboratory [ 1031.
Emery et al. investigated the effects of aging cores with water and
Singleton crude oil for varying lengths of time [8]. Being saturated with
oil, the system behaved like a water-wet system. Most of the production
occurred shortly after breakthrough, and the practical residual oil
saturation was attained just after one pore volume of water was injected.
As the aging time was increased, water breakthrough occurred sooner
and there was a considerable amount of subsequent production, with the
So, occurring after two or three pore volumes had been injected. These
results are similar to those obtained by Donaldson et al. [48], as shown in
Figure 6.9, and they show that one must equilibrate a core and its fluids
before running waterflood tests to determine the amount of production,
relative permeability curves, or wettability.