Fluid Saturation

1. Fluid Saturations
Introduction

2. Fluid Saturations
• Definition - The fluid saturation for a particular fluid is the
fraction of pore volume occupied by that fluid; H2O, oil or gas
• Fluid saturation = total volume of the fluid phase
pore volume
– Saturation is an intensive property
– Equation Form: So=Vo/Vp, Sw=Vw/Vp, Sg=Vg/Vp
• These fluid volumes are measured under specific conditions of pressure
and temperature (e.g. reservoir, or laboratory)
– reservoir conditions are often noted as “in situ”

3. Fluid Saturations
• Fundamental Relationships
– Pore volume is occupied by fluids (water, oil, and/or gas)
= + +
S S S
w o g
= + +
V V V V
p w o g
+ +
V V V
w o g
V
p
1
=
• for the two phase case, only one of the two saturations is
independent, the other must make the sum of the saturations equal
to unity (1)
• similarly, for the three phase case, only two saturations are
independent

4. Fluid Saturations
• Fundamental Relationships (continued)
– Mass of fluids in the pore volume is comprised of: water,
oil, and/or gas
= + +
fluidmass m m m
w o g
= + +
ρ V ρ V ρ V
w w o o g g
p [ w w o o g g ]
= + +
V ρ S ρ S ρ S
• at laboratory conditions it is often assumed that gas density is
negligible

5. Initial Fluid Saturations in Reservoir
• Concepts: typical petroleum accumulation scenario
– pores are initially saturated with water (Sw=1)
– hydrocarbons migrate up dip into traps due to having
density less than water density (gravity force)
– hydrocarbons (oil and/or gas) is distributed such that
gravity and capillary forces are in equilibrium
• minimum interstitial water saturation remains in hydrocarbon zone,
even after accumulation occurs
– water wet, drainage accumulation process
» irreducible wetting phase saturation
– oil wet, imbibition accumulation process
» residual non-wetting phase saturation

6. Initial Fluid Saturations in Reservoir
• Methods for determination of reservoir fluid saturations
– Direct Measurement
• Core Analysis of samples obtained from the formation of interest in
their original state and measure saturations directly is ideal.
– Indirect Measurement
• Capillary Pressure Measurement
• Well Log Analysis
– electrical conductivity depends primarily on water saturation

7. Fluid Saturations - Core Analysis
• Factors affecting fluid saturations in cores
– flushing of core by filtrate from drilling fluids (especially for
overbalanced drilling)
• water filtrate
– water based mud
– oil emulsion mud
• oil filtrate
– oil based mud
– inverted oil emulsion mud
• gas filtrate
– air drilling
– foam drilling

8. Fluid Saturations - Core Analysis
• Factors affecting fluid saturations in cores (continued)
– Changes in pressure and temperature as core sample is
brought from bottomhole conditions to surface conditions
– Example: Oil zone at minimum interstitial water saturation,
water based drilling mud

9. Application of Core Saturations
• Application of Core Saturations: Water Based Mud
– presence of oil zone
– original oil/gas contact
– original oil/water contact
• Application of Core Saturations: Oil Based Mud
– fairly accurate minimum interstitial water saturation
– original oil/water contact
• Other Applications of Core Saturations
– correlation of indirect methods

10. Estimating Fluid Contact Depths from Core
Saturations
So
Gas
Oil
Water
0 50
Depth
So » 0 in gas zone
So > »0.15 in oil production
zone
0 < So < »0.15 in water
production zone

11. Maximum Water Saturation
for Oil and Gas Production
The trend shown here
continues for even lower
permeability, with productive
reservoirs existing with
Sw>0.60 for k<0.01 md

12. Commentary on Core Saturations
• Qualitative Value: “The saturation values obtained directly
from rock samples [cores] are usually not reliable for
determining the quantity of each fluid in the rock [reservoirs].
Other uses exist for fluid-saturation determinations from core
samples.”
• Overall Value: “Thus, in summary, it is seen that although
fluid-saturation determinations made on core samples at the
surface may not give a direct indication of the saturations
within the reservoir, they are of value and do yield very useful
and necessary information”

13. Fluid Saturation
Connate water (Swc): water entrapped in the
interstices of the rock (either sedimentary
or extrusive igneous) at the time the rock
was deposited.
Interstitial water: Water that occurs naturally
within the pores of rock. Water from fluids
introduced to a formation through drilling or
other interference, such as mud and
seawater, does not constitute interstitial
water. Interstitial water, or formation water,
might not have been the water present
when the rock originally formed. In contrast,
connate water is the water trapped in the
pores of a rock during its formation, also
called fossil water.

14. Fluid Saturation
• Irreducible water saturation (Swir): the fraction of pore volume
occupied by water in a reservoir at maximum hydrocarbon
saturation. In water-wet rock, it represents the layer of adsorbed
water coating solid surfaces and the pendular grain contacts and
at pore throats.
• The irreducible saturation of a fluid is the minimum saturation of
that fluid attainable when that fluid is displaced from a porous
medium by another fluid immiscible with the first.
• Residual oil (Sor): Oil remaining in the reservoir rock after the
flushing or invasion process, or at the end of a specific recovery
process or escape process.