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 (S_w Archie) versus dielectric water saturation (S_w dielectric), assuming a water-wet reservoir (i.e. m=n=2), shows that the majority of the Caballos reservoir plot where S_w Archie is much less than S_w dielectric, and that S_w Archie is commonly less than S_wirr (20%). These very low Archie water saturations indicate the presence of oil-wet reservoirs. A histogram of the ratios S_w Archie/S_w 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 S_w dielectric versus S_w Archie (m=n=2) for the Glorieta-Clear Fork reveals that the data cluster along the line where S_w dielectric = S_w Archie and that no S_w values are less than 20% (S_wirr). A histogram of the ratios S_w Archie/S_w 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 (S_w Archie/S_w dielectric >=0.8) and assuming an oil-wet reservoir (S_w Archie/S_w, dielectric <=0.5). The calculated oil-wet relative permeability curves (S_w Archie/S_w 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 (S_w Archie/S_w 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.