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 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”

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

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

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

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

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

Estimating Fluid Contact Depths from Core Saturations So 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

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

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”