Page 340 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Estimation of Waterflood Residual Oil Saturation 307

Thus, the modified calculation would be

(5-248)

However, these corrections, particularly the one far bleeding factor, are only
approximate at best and considerable attention has been given to recovering
cores at reservoir pressure,

Pressure Core Barrel. Considerable development work has gone into developing
a core barrel that will bring cores to the surface without major reduction in
reservoir pressure, and thus prevent shrinkage and bleeding. The pressure core
barrel is designed to cut the core and seal it within a cylinder before retrieval.
Although the cores obtained are small in diameter (-2 Yrin.), pressure coring
has gained acceptance as one of the best methods of determining residual oil,
particularly when information on the vertical profile of the oil saturation is
wanting. Success in application of the core barrel depends to a great extent on
the skill and experience of the personnel running the operation. The main
criterion for success is retrieving cores at or close to formation pressure.
Improvements over the years have led to the present success level of about 70%
to 80% in consolidated formations [182,319,326-3311.

Sponge Cores. A recent modification [332] to a conventional core barrel is the
incorporation of a porous sponge to collect oil that bleeds from the core. The
oil saturation measured by conventional techniques is corrected for the bleeding
of oil as measured in the sponge. Oil saturations measured with this technique
have approximated the values determined by pressure coring but at a cost that
is closer to conventional coring.

Core Testlng
Laboratory tests to estimate reservoir residual oil can be performed on cores
that have been preserved at the wellsite or cores which are extracted with solvent
and subsequently restored to reservoir conditions. Cores obtained with drilling
muds that minimize wettability alteration, and that are protected at the wellsite
to prevent evaporation or oxidation are called preserved cores. Cores that are
cleaned with solvents and resaturated with reservoir fluids are called restored-
state cores or extracted cores. The restoring process is often performed on non-
preserved or weathered cores, but the same technique could apply to cores that
had been preserved.

Core Handling. Conmtimal Cow. The precautions taken in handling cores once
they have been recovered depends mainly on the measurements and tests that
are to be performed on them. If the measurements are routine and can be run
within a day or two, it is generally considered sufficient to wipe the cores, wrap
them in plastic and protect them from exposure to the sun. However, it is better
to keep the wrapped cores inside ice boxes to minimize evaporation, especially
when longer transit times are anticipated. When maintenance of wettability
conditions as they exist in the reservoir is attempted, the most widely preferred
method of preservation is to wrap the core in thin sheets of plastic followed
by aluminum foil and then isolate the wrapped core from the atmosphere by a
coating of wax or plastic. An alternative method of preservation is to store the