
Kansas Geological Survey Openfile Report 200138 
“Irreducible” Water SaturationWith finer pores in the matrix surrounding large oomolds it is important to understand capillary pressure relationships since high porosity may not be directly associated with effective oil porosity. Correlations of “Irreducible” water saturations (measured at pressures equivalent to 60120 feet above free water level) indicate that Swi increases with decreasing permeability as exhibited by many rocks. Differences between fields are largely attributable to the samples/wells analyzed and may not reflect true field differences. Saturation increases with decreasing permeability following the relation:

Capillary Pressure CurvesLansingKansas City oomoldic limestone exhibit a near loglinear trend between wetting phase saturation (assumed to be water in the reservoir) and oilbrine height above free water level. This would also translate to a loglinear relationship between wettingphase saturation and reservoir oilbrine capillary pressure. Comparison between samples of different permeability indicates that capillary pressures decrease with increasing permeability at any given saturation. This is a typical trend for most rocks. Analyzing the relationship between the change in capillary pressure and permeability, an equation was constructed that provides approximate capillary pressure curves for any given permeability: Pc = 10^{(A Sw + B)} (rho_{water}rho_{oil}) Where Pc is reservoir oilbrine capillary pressure (psia), Sw is water saturation (fraction),rho_{water} and rho_{oil} are water and oil density (g/cc), and A and B are constants that vary with permeability. These constants can be predicted from permeability using: A = 0.1663 log_{10}Permeability (md)1.5186 These equations provide generalized capillary pressure curves that approximate the relationships shown by the samples studied. 

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Last updated December 2001
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