Fluid Saturations
Introduction
Fluid Saturations
• Definition - The fluid saturation for a particular fluid is
the fraction of pore volume occupied by that fluid
– 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”
Fluid Saturations
• Fundamental Relationships
– Pore volume is occupied by fluids (water, oil, and/or gas)
Vp  Vw  Vo  Vg
1
Vw  Vo  Vg
Vp
 S w  S o  Sg
• 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
Fluid Saturations
• Fundamental Relationships (continued)
– Mass of fluids in the pore volume is comprised of: water,
oil, and/or gas
fluid mass
 m w  mo  mg
 ρ w Vw  ρ o Vo  ρ g Vg
 Vp ρ w Sw  ρ oSo  ρ gSg


• at laboratory conditions it is often assumed that gas density is
negligible
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
Initial Fluid Saturations in Reservoir
• Methods for determination of reservoir fluid saturations
– Direct Measurement
• Core Analysis (current topic)
– Indirect Measurement
• Capillary Pressure Measurement (previous topic)
• Well Log Analysis (future topic)
– electrical conductivity depends primarily on water saturation
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
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
Flushing
During
Drilling
Saturation at Surface
Compared to Reservoir
Trip to
Surface
Sw


? probably 
So



Sg
-


Fluid Saturations - Core Analysis
• Factors affecting fluid saturations in cores (continued)
– Example: Oil zone at minimum interstitial water saturation,
oil based drilling mud
Flushing
During
Drilling
Trip to
Surface
Saturation at Surface
Compared to
Reservoir
Sw
-
-
-
So
-


Sg
-


– Self Study: other examples in Lecture_21.pdf
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
Estimating Fluid Contact Depths
from Core Saturations
0
So
50
Gas
Depth
Oil
So  0 in gas zone
Water
So > 0.15 in oil production
zone
0 < So < 0.15 in water
production zone
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
Commentary on Core Saturations
• Qualitative Value, ABW (page 109, brackets added):
“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 fluidsaturation determinations from core samples.”
• Overall Value, ABW (page 110): “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”