Page 124 - Petrophysics

POROSITY 97

Equally important the relative extent to which the pores of the rock
are filled with specific fluids. This property is called fluid saturation
and is expressed as the fraction, or percent, of the total pore volume
occupied by the oil, gas, or water. Thus, for instance, the oil saturation
So is equal to:

Volume of oil in the rock, V,
so =
Total pore volume of the rock, Vp

Similar expressions can be written for gas and water. It is evident that:

so + s, + sw = 1 (3.3)

and:

vo+v!g+vw 'VP (3.4)

Ideally, because of the difference in fluid densities, a petroleum reservoir
is formed in such a way that, from top to bottom of the sand bed
there will be gas, oil and water. Connate water, however, is nearly
always found throughout the petroleum reservoir. Connate water is
the seawater trapped in porous spaces of the sediments during their
deposition and lithification, long before the oil migrated into the reservoir
rock. In addition to density, wettability and interfacial tension combine
to alter the manner in which the three fluids are distributed in the
reservoir.
The amount of connate water present in the porous space varies
from 100% below the oil zone to theoretically zero at heights above
the free water level. However, in practical cases a nearly constant
content of irreducible connate water (Si,) exists above the transition
zone. The magnitude of Si, and height of the transition zone depend
on the pore size and texture. High Siw values are indicative of small
pore sizes. The transition zone corresponds to the zone of varying
water saturation. Wells completed within this zone will produce
hydrocarbons and water, and wells completed above this zone, i.e.,
within the zone of irreducible water saturation, will produce only
hydrocarbons [ 111.

QUANTITATIVE USE OF POROSITY

One of the simplest methods of calculating reservoir oil content is
called the volumetric method. The mathematical expression for the