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Critical Water Estimates for Gulf Coast Sands
Raymond J. Granberry (1), Dare K. Keelan (2)
Hydrocarbon productive sands in the Gulf Coast area exhibit wide variances in formation water saturation (Sw). Values ranging from 10 to 70 percent pore space are common. These variances are related to the capillary properties of the sand, height above the water level, fluid interfacial tensions, and density difference of the entrained fluids. This formation water may, or may not flow in appreciable quantities, depending on the water retentive characteristics of the rock, the formation relative permeability and fluid viscosities. The combined influence of these variables results in a critical upper limit for formation water saturation, above which a significant percentage of water will be produced. This limiting water saturation exceeds the irreducible saturation value and has been called the critical interstitial water (Sciw). It can be correlated with the basic rock properties of permeability and porosity.
Presented herein is a generalized correlation of critical interstitial water saturations for Gulf Coast sands. This correlation allows a critical saturation to be estimated from permeability and porosity data given in a routine core analysis report. Higher critical water saturations are found as porosity increases in samples of constant permeability. Higher values are also noted as permeability decreases in samples of constant porosity. The critical interstitial water saturation also influences the interpretation of the quick-look computer calculated apparent water resistivity (Rwa) curve used to predict hydrocarbon productive zones. The Rwa/Rw multiple of three which is commonly used to indicate hydrocarbon production corresponds to a water saturation of 58 percent of pore space. Low permeability zones may yield hydrocarbons with a Rwa multiple of two, which corresponds to a water saturation of 70 percent. High permeability sands may require multiples of six or more to be hydrocarbon productive. The required multiple for a sample of any given permeability and porosity can be estimated from the generalized correlation of critical water values presented.
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