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The AAPG/Datapages Combined Publications Database
West Texas Geological Society
Abstract
The Application of Dielectric Logs to the Recognition of Oil Versus Water-Wet Reservoirs and in the Determination of Relative Permeabilities
Abstract
The lower Cretaceous Caballos Sandstone in the San Francisco field, Colombia has resistivities that range from <100 ohm-m to over 10,000 ohm-m. This extreme range in resistivities suggests the presence of both oil-wet and water-wet reservoirs. The Permian Glorieta-Clear Fork Dolomite in the Monahans field Ward County, Texas has resistivities that range from 5 ohm-m to 50 ohm-m which suggest a dominantly water-wet reservoir.
Plots of Archie water saturation (Sw Archie) versus dielectric water saturation (Sw dielectric), assuming a water-wet reservoir (i.e. m=n=2), shows that the majority of the Caballos reservoir plot where Sw Archie is much less than Sw dielectric, and that Sw Archie is commonly less than Swirr (20%). These very low Archie water saturations indicate the presence of oil-wet reservoirs. A histogram of the ratios Sw Archie/Sw dielectric reveal a distribution from 0.1 to 1.3 with two maximums at 0.1 (oil-wet) and 0.9 (water-wet). In contrast, the plots of Sw dielectric versus Sw Archie (m=n=2) for the Glorieta-Clear Fork reveals that the data cluster along the line where Sw dielectric = Sw Archie and that no Sw values are less than 20% (Swirr). A histogram of the ratios Sw Archie/Sw dielectric reveals a mode at 1.0 (water-wet) with symmetrical scatter about the mode from 0.3 to 1.8.
Using the Caballos Sandstone, the Jones (1945) and Gondouin and Heim (1963) equations were used to calculate relative permeabilities from both log and core data. These relative permeabilities were calculated assuming a water-wet reservoir (Sw Archie/Sw dielectric >=0.8) and assuming an oil-wet reservoir (Sw Archie/Sw, dielectric <=0.5). The calculated oil-wet relative permeability curves (Sw Archie/Sw dielectric <=0.5) compared very well to unsteady-state laboratory-derived relative permeabilities that were measured with oil as the wetting phase. However, when the unsteady-state relative permeabilities were compared to the calculated water-wet relative permeability curves (Sw Archie/Sw dielectric >=0.8) the two sets of curves were dissimilar. Thus, the Jones (1945) and Gondouin and Heim (1963) equations can be used to determine accurate relative permeability curves at reservoir condition when the correct wetting phase is identified.
Relative permeability curves were also calculated for the Glorieta-Clear Fork assuming a water-wet reservoir. Unfortunately, these results could not be compared to laboratory-derived relative permeability curves.
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