302 Subsurface Geologic Methods 



or a fraction. The relative-permeability characteristic may be used with 

 other data from core analysis and reservoir-fluid analysis to predict the 

 performance of reservoirs under various productive mechanisms.^- The 

 relative-permeability characteristic is reflected in produced gas-oil ratios 

 as the gas saturation increases, in the drop of productivity of gas-conden- 

 sate wells when the sand face becomes saturated with condensate, in the 

 drop in potential of gas wells when water is produced through the com- 

 pletion interval, and in the production of water from zones of high connate- 

 water saturation. 



A lower limit for commercial permeability cannot be set because the 

 factors of sand thickness and reservoir pressure directly affect this limit. 

 The extremely low permeabilities of some formations in Colorado, Wyo- 

 ming, and Texas are compensated for by the great thicknesses and high 

 pressures. Actually it is not the permeability of any one foot of formation 

 that is so critical; it is the total capacity (the permeability times the thick- 

 ness) together with the reservoir pressure that controls the flow rate. 



Residual-Fluid Saturations 



The residual-liquid saturations determined by core analyses are the 

 oil or condensate and the total core water. (See figs. 123, 127, and 131.) 

 The gas volume is determined indirectly as the diff"erence between the total 

 pore volume and the liquid volume. These saturations may be determined 

 by extraction or retort methods. One of the most rapid and efficient 

 methods is the electric, water-cooled condenser-fluid stills. 



Residual Oil and Condensate 



The amount of residual oil remaining in the core sample cut from 

 a high-pressure reservoir is dependent on the following factors.^^ 



1. Reservoir-liquid saturation. 



2. Formation or reservoir pressure. 



3. Mud pressure and water loss. 



4. Reservoir-liquid viscosities. 



5. Coring time. 



6. Vertical and horizontal permeabilities. 



7. Pressure-depletion rate while pulling core. 



8. Core diameter and type of core. 



9. Method of obtaining core sample. 

 10. Solution gas-oil ratio. 



The residual hydrocarbon saturation is dependent on the flushing or 

 contamination by the drilling fluid that takes place as the formation is 

 cored. When coring with water-base mud, flushing takes place ahead of 

 the bit and radially through the core, owing to the difference between 



^- Muskat, M., and Taylor, M. O., Effect of Reservoir Fluid and Rock Characteristics on Production 

 Histories of Gas-Drive Reservoirs: Am. Inst. Min. Met. Eng. Tech. Pub. 1914, 1945 



'^ Caran, J. G., Core Analysis — Its Interpretation and Application in Reservoir Engineering : Petroleum 

 Eng., Oct. 1947. 



