An alternative to the Winland R35 method for determining carbonate reservoir quality

The Winland R35 method [Log R35 = 0.732 + 0.588 (Log Kair) 0.864 (Log O)] is based on the relationship between porosity, permeability, and pore throat radius at the point of 35% mercury saturation in capillary pressure measurements and is generally reliable in rocks with only intergranular porosity (such as sandstone) where pore and pore throat geometry are related closely to rock texture. Carbonate pores are not always so; consequently, the Winland method is not as reliable for assessing reservoir quality in carbonate reservoirs. To evaluate alternatives to the conventional Winland technique, based on rock facies characteristics, samples from the Jurassic Smackover Formation in Alabama and the Permian Clearfork Formation in Texas were tested for reservoir quality with use of the Winland R35 and Pittman methods to determine if either method is more reliable in carbonate reservoir studies. Pittman's modification of the Winland method was found to be more accurate graphically. A third method for evaluating reservoir rock character is provided by Lucia. This method is based on geological rather than petrophysical characteristics, and it revealed that pore throat sizes at 35% mercury saturation may include a variety of depositional and diagenetic rock fabrics. The Winland and Pittman petrophysical evaluation techniques, as well as the Lucia geological evaluation technique - when based on depositional facies alone - do not provide reliable measures of reservoir quality. An alternative method based on genetic pore type presented by Ahr in 2005 was tested for comparison. Using a porosity-permeability plot based on the pore type, the relationship between porosity, permeability, and pore type was found to be strong and reproducible. When the ratio of permeability to porosity was used in combination with Ahr genetic pore types, the results indicate that barriers, baffles, and flow units can be reliably defined. This study demonstrates that the use of pore types in conjunction with capillary pressure measurements is a more reliable method for evaluating carbonate reservoirs than any alternative method that is based on depositional facies or rock fabrics alone.