Page 355 - Petrophysics 2E

SEMIPERMEABLE DISK MEASUREMENT 323

Hydrocarbon reservoirs were initially saturated with water, which
was displaced by migrating hydrocarbons. The water accumulated in
the geologic structure and formed a trap for the oil, thus producing a
petroleum reservoir. This process can be repeated in the laboratory by
displacing water from a core with a gas or oil. The pressure required
for the equilibrium displacement of the wetting phase (water) with the
non-wetting gas or oil is the water drainage capillary pressure, which is
recorded as a function of the water saturation.
A core is saturated with water containing salts (NaCl, CaC12, or KCl)
to stabilize the clay minerals, which tend to swell and dislodge when in
contact with freshwater. The saturated core is then placed on a porous
disk, which also is saturated with water (Figure 5.7). The porous disk
has finer pores than does the rock sample. (The permeability of the disk
should be at least 10 times lower than the permeability of the core.)
The pore sizes of the porous disk should be small enough to prevent
penetration of the displacing fluid until the water saturation in the core
has reached its irreducible value.

Figure 5.7. Porous disk method for measurement of capillary pressure using a
manometer f4].