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
Saturation is an intensive property
Equations: 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)
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)
The mass of the fluids in the pore volume is comprised of: water, oil, and/or gas
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 (current topic)
Indirect Measurement
Capillary Pressure Measurement (previous topic)
Well Log Analysis (future topic)
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. Fluid Saturations - Core Analysis
Factors affecting fluid saturations in cores (continued)
Example: Oil zone at minimum interstitial water saturation, oil based drilling mud
Self Study: other examples in Lecture_21.pdf

10. 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
Other Applications of Core Saturations
correlation of indirect methods

11. Estimating Fluid Contact Depths from Core SaturationsSo 50
Gas
Depth
Oil
So  0 in gas zone
So > 0.15 in oil production zone
0 < So < 0.15 in water production zone
Water

12. 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

13. Application of 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 fluid-saturation 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”