About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


AAPG Bulletin, V. 86, No. 4 (April 2002), P. 605-624.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Incorporating capillary pressure, pore throat aperture radii, height above free-water table, and Winland r35 values on Pickett plots

Roberto Aguilera1

1Servipetrol Ltd., 736 6th Avenue SW, Suite 1640, Calgary, Canada T2P 3T7; email: [email protected]


Roberto Aguilera is president of Servipetrol Ltd. in Calgary, Canada. He has an undergraduate degree in petroleum engineering from the Universidad de America in Bogota, Colombia, and a master's degree and Ph.D. in petroleum engineering from the Colorado School of Mines. He was an AAPG instructor on the subject of naturally fractured reservoirs from 1984 through 1996. He received the Outstanding Service Award from the Petroleum Society of the Canadian Institute of Mining, Metallurgy and Petroleum Engineers (CIM) in 1994. He is a Distinguished Author of the Journal of Canadian Petroleum Technology (1993 and 1999) and a Society of Petroleum Engineers Distinguished Lecturer on the topic "Naturally Fractured Reservoirs" for the 2000-2001 season. He has developed various methods that have been published in leading journals of the oil industry. He has authored and been a contributor to various books, including Naturally Fractured Reservoirs (PennWell, 1980 and 1995), The Technology of Artificial Lift Methods (PennWell, 1984), Horizontal Wells (Gulf Publishing, 1991), and Determination of Oil and Gas Reserves (Petroleum Society of CIM Monograph 1, 1994).


I thank Maria S. Aguilera for her help and enthusiasm during the preparation of the study. I am grateful to Dan Hartmann and Matt Pranter for their detailed review, their advice, and their contribution to the manuscript.


Methods are presented for incorporating capillary pressure, pore throat aperture radii, height above the free-water table, and Win land r35 values on Pickett plots. The techniques involve the use of log-log plots of effective porosity vs. resistivity combined with em pirical equations for calculating capillary pressure written as a func tion 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 ad vantages of the proposed methods are (1) the capillary pressure at any point on the Pickett plot is consistent with porosity, perme ability, 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 cap illary 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 em pirically derived Winland equation compare reasonably well with pore throat aperture radii (rp35) calculated from techniques pre sented 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 in dependently at different times. A mathematical relationship is de veloped 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.

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24

AAPG Member?

Please login with your Member username and password.

Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].