Page 78 - Petrophysics
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52 PETROPHYSICS: RESERVOIR ROCK PROPERTIES
Figure 2.9. Displacement of an oil droplet through a pore throat in a water-wet rock
The two forces in a reservoir that are most likely to be operating on
the droplet are buoyancy and hydrodynamic pressure, neither of which
are normally sufficient to dislodge an isolated droplet of oil.
The dispfacement pressure due to buoyancy is expressed as:
where: Z, = height of the oil column.
gc = gravitational constant, 9.81 m/s2.
pw = water density, kg/m3.
po = oil density, kg/m3.
Pd = displacement pressure, Pa.
Since the combined buoyant and hydrodynamic pressure acting on
an isolated droplet are insufficient to exceed the displacement pressure
required by the capillary forces, isolated drops of oil cannot migrate
under the influence of these forces alone [ 141.
As the oil leaves the source rock under the forces of compaction, large
saturations develop at the entry to the reservoir rock. The oil then begins
to migrate upward as a continuous phase in long filaments within the
pores. Under these circumstances, sufficient buoyant and hydrodynamic
forces can develop to cause migration of the oil.
It also has been suggested that oil migration may occur by molecular
solution of oil in water that is in motion, or by colloidal soIution brought
about by surfactants present in petroleum. Both theories have been
challenged because the solubility of oil molecules in water is extremely
low and the actual concentration of surfactant-type molecules in crude
oils is very small [9, 171. Leaching of sand containing discrete droplets
