Abstract

Methods are presented for incorporating capillary pressure, pore throat aperture radii, height above the free-water table, and Winland r35 values on Pickett plots. The techniques involve the use of log-log plots of effective porosity vs. resistivity combined with empirical equations for calculating capillary pressure written as a function of permeability, porosity, and water saturation.

I show that a crossplot of porosity vs. true resistivity (in some cases apparent resistivity or true resistivity affected by a shale group) should result in a straight line for intervals with constant capillary pressure and constant pore throat aperture radii. Key advantages of the proposed methods are (1) the capillary pressure at any point on the Pickett plot is consistent with porosity, permeability, and water saturation at that particular point; (2) the value of Rw does not have to be known in advance, provided that the reservoir contains some water-bearing intervals; and (3) core data are not essential, although it is strongly recommended to have cores to properly calibrate the equations presented in this article. If capillary pressures from cores are available, it is possible to estimate the value of Rw even if there are not water-bearing intervals in the reservoir.

Pore throat aperture radii (r35) values computed using the empirically derived Winland equation compare reasonably well with pore throat aperture radii (rp35) calculated from techniques presented in this article. This is significant because the data sets used to establish these empirical equations come from different areas, different reservoirs, and different lithologies and were evaluated independently at different times. A mathematical relationship is developed between Winland r35 values and the pore throat aperture rp35 presented in this article.

The methods are illustrated using two case histories. The first one is a Gulf Coast high-porosity sand-shale sequence. The second is a limestone oil reservoir from the Lansing Kansas City formation.

The integration of permeability, capillary pressures, pore-size classes, and geometry of the pores on a log-log graph of porosity vs. resistivity makes the Pickett plot one of the most formidable formation evaluation tools yet devised.

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