Water saturation is the ratio of water volume to pore volume in a reservoir. It represents the fraction of pore space filled with water. There are several types of water saturation, including initial, residual, and irreducible water saturation which depend on factors like pore size. Water saturation changes over time with production and based on the reservoir's drive mechanism, such as aquifer or gas cap drive. Reservoir monitoring involves periodically logging wells to analyze changes in porosity and water saturation over time.

1. WATER SATURATION

2. • DEFINITION OF SATURATION
Saturation of any given fluid in a pore space is
the ratio of the volume of that fluid to the pore
space volume. For example, a water saturation
of 10% means that 1/10 of the pore space is
filled with water; the balance is filled with
something else (oil, gas, air, etc. - a pore cannot
be “empty”). As for porosity, saturation data is
often reported in percentage units but is always
a fraction in equations.

3. • Porosity is the capacity of the rock to hold
fluids. Saturation is the fraction of this
capacity that actually holds any particular
fluid. Porosity, hydrocarbon saturation, the
thickness of the reservoir rock and the real
extent of the reservoir determine the total
hydrocarbon volume in place. Hydrocarbon
volume, recovery factor, and production rate
establish the economic potential of the
reservoir.

4. • Irreducible water saturation (SWir) is the minimum
water saturation obtainable in a rock. Water is usually
the wetting fluid in oil or gas reservoirs, so a film of
water covers each pore surface. The surface area thus
defines the irreducible water saturation. Formations
at irreducible water saturation cannot produce water
until water encroaches into the reservoir after some
oil or gas has been withdrawn. Small pores have
larger surface area relative to their volume so the
irreducible water saturation is higher. If pores are
small enough, the irreducible water saturation may
be 1.0, leaving no room for oil or gas to accumulate.

5. • The initial water saturation (SWi) is the saturation of
an undisturbed reservoir with no prior production
from any earlier well. Usually SWir = SWi, at least
above the oil water transition zone. In the transition
zone, SWa is higher than SWir and some water would
be produced if the well was completed in this
interval.
• In a reservoir that has had some production, SWa
may be higher than SWir (and higher than SWi) so
some water may be produced with the oil.

6. • Of the total amount of oil or gas present in a
reservoir, only a fraction of it can be
produced, depending on the recovery
efficiency. This recovery factor, normally
determined by experience, is typically in the
20% to 50% range for oil, and may be as high
as 95% for gas zones, or as low as 5% in
heavy oil. Recovery factor can sometimes be
estimated from log data by observing the
moveable hydrocarbon volume.

7. • Some more definitions.
• Total water saturation (SWt) is the ratio of -
total water volume (BVW + CBW) to - total
porosity (PHIt)
• 1: SWt = (BVW + CBW) / PHItDFN 12:
• Effective water saturation (SWe) is the ratio
of: - free water volume (BVW) to - effective
porosity (PHIe)
• 2: SWe = BVW / PHIe

8. • Useful water saturation (SWuse) is the ratio
of: - useful water volume (BVW - BVI) to -
useful porosity (PHIuse)
• 3: SWuse = (BVW - BVI) / PHIuse
DFN 14:Irreducible water saturation (SWir) is
the ratio of: - immobile or irreducible water
volume (BVI) to - effective porosity (PHIe)
• 4: SWir = BVI / PHIe

9. • Residual oil saturation (Sor) is the ratio of: -
immobile oil volume (BVHr) to - effective
porosity (PHIe)
• 5: Sor = BVHr / PHIe
•
The water saturation in the flushed zone (Sxo) is
the ratio of : - free water in the flushed zone, to -
effective porosity, which is assumed to be the
same porosity as in the un-invaded zone.
• 6: Sxo = BVWflushed / PHIe

10. • The amount of free water in the invaded zone is
usually higher than in the un-invaded zone,
when oil or gas is present. Thus Sxo >= Swe. The
water saturation in the invaded zone between
the flushed and un-invaded zone is seldom used.
• All volumes defined above are in fractional
units. In tables or reports, log analysis results
are often converted to percentages by
multiplying fractional units by 100.

11. • SATURATION BASICS
Water saturation is the ratio of water volume
to pore volume. Water bound to the shale is
not included, so shale corrections must be
performed if shale is present. We calculate
water saturation from the effective porosity
and the resistivity log. Hydrocarbon
saturation is 1 (one) minus the water
saturation.

13. • Most oil and gas reservoirs are water wet; water
coats the surface of each rock grain. A few
reservoirs are oil wet, with oil on the rock
surface and water contained in the pores,
surrounded by oil. Some reservoirs are partially
oil wet. Oil wet reservoirs are very poor
producers as it is difficult to get the oil to detach
itself from the rock surface. It is fairly easy to
take a core sample, clean it and dry it, then
make the rock oil wet. However, reservoir rocks
are seldom clean and dry, so that same rock in-
situ will often be water wet.

14. • Water wet Reservoir

15. • Oil wet Reservoir

16. • When a reservoir is drilled, some of the fluids near
the wellbore are pushed away and the zone is
invaded by the drilling fluid. If hydrocarbons were
present, the water saturation after invasion will be
higher than the original reservoir conditions. A
shallow resistivity log will see the invaded zone water
saturation. A deep resistivity log should see the
original formation water saturation as long as
invasion was not too deep.
• Production of oil or gas will often change the water
saturation, but the amount of change varies with the
drive mechanism.

17. water is held in place by surface tension
surface water does not move while the oil
as is being produced.
er wet formation with hydrocarbon before invasion (lef
nd after invasion (right).
he same illustrations are used to describe a reservoir at
itial conditions (left) and after production by aquifer dri
an efficient water flood - water moves in to replace the
l that is withdrawn (right). gas is being produced.

18. • When a reservoir is drilled, some of the fluids near the wellbore
are pushed away and the zone is invaded by the drilling fluid. If
hydrocarbons were present, the water saturation after invasion
will be higher than the original reservoir conditions. A shallow
resistivity log will see the invaded zone water saturation. A deep
resistivity log should see the original formation water saturation
as long as invasion was not too deep.
• Production of oil or gas will often change the water saturation,
but the amount of change varies with the drive mechanism.
• Aquifer drive (SLIDE BELOW) pushes oil up, increases water
saturation as the oil is produced. Gas cap drive (SLIDE BELOW)
pushes oil down, but water saturation does not change until the
gas that replaced the oil is also produced. If there is no aquifer,
both situations produce only by expansion drive - in this case
water saturation does not change unless a water flood is imposed
by the field operator.

20. • Expansion drive is also called solution drive as it
is the gas in solution in the oil that pushes oil
out of the reservoir. Water saturation does not
change and oil recovery is very small (5 to 10%
depending on gas-oil ratio and oil viscosity)
unless a water flood is instituted. Gas reservoirs
can produce with reasonably high recovery from
pure expansion drive (Sw nearly constant), but
there may also be an aquifer drive component
(Sw will increase over time).SLIDE BELOW.

22. • Reservoir monitoring is used to assess the
changes in water saturation over time.
Monitoring is accomplished by periodically
running appropriate logs through casing and
analyzing the logs for porosity and water
saturation. Changes in the position of the oil-
water or gas-oil contact can lead to a workover
of the well to restrict the perforated interval to
reduce water or gas production. Modern
technology applied to older wells may even find
bypassed pay zones to find ways to improve the
economic performance of the well.