Fluid Saturations Introduction

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Fluid SaturationsIntroduction
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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 SoVo/Vp, SwVw/Vp, SgVg/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

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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

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Fluid Saturations

• Fundamental Relationships (continued)
• Mass of 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

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Initial Fluid Saturations in Reservoir

• Concepts typical petroleum accumulation scenario
• pores are initially saturated with water (Sw1)
• 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

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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

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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

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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

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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

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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

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Estimating Fluid Contact Depths from Core
Saturations
So
50
0
Gas
Depth
Oil
So ? 0 in gas zone So gt ?0.15 in oil production
zone 0 lt So lt ?0.15 in water production zone
Water
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Maximum Water Saturationfor Oil and Gas
Production
The trend shown here continues for even lower
permeability, with productive reservoirs existing
with Swgt0.60 for klt0.01 md
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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 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