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Fluid Saturations Introduction

Explore fluid saturations in reservoirs through core analysis, including methods, factors affecting saturations, and applications in determining fluid contacts and production potential. Discover how core samples provide valuable insights despite limitations.

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Fluid Saturations Introduction

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  1. Fluid SaturationsIntroduction

  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 • original oil/water contact • Other Applications of Core Saturations • correlation of indirect methods

  11. Estimating Fluid Contact Depths from Core Saturations So 50 0 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 Saturationfor 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”

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