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Fluid Saturations Introduction

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: S o =V o /V p , S w =V w /V p , S g =V g /V p

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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 • 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) • 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 • 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. 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”

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