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The concept of rock fabric has been shown to be very useful for characterization of carbonate reservoirs. This study shows that a Pickett crossplot of interparticle 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 a constant rock fabric. The slope of the straight line is related to the porosity exponent m, the water saturation exponent n, and the size of the particles forming the interparticle porosity. Different slopes are obtained for different rock fabrics. The method helps to reconcile geology to fluid flow by illustrating the important link between geology, petrophysics, and reservoir engineering.
Lines of constant rock fabric are displayed on a Pickett plot, together with water saturation, permeability, process speed k/ϕ, capillary-pressure curves, pore-throat apertures r35 and rp35, Kozeny's constant (Fsτ2), and height above the free-water table. Pattern recognition while placing all these data in a consistent form on a Pickett plot allows determination of flow units and a more rigorous characterization of carbonate reservoirs. The method is aimed at heterogeneous carbonate reservoirs, which have a limited amount of hard data.
The use of this technique is illustrated with data from the Mission Canyon Formation in the Little Knife field of North Dakota, where a significant volume of oil in place is below the structural closure and updip wells penetrate microports that provide an effective seal in this stratigraphic trap.
Roberto Aguilera is president of Servipetrol Ltd. in Calgary, Canada and an adjunct professor in the Chemical and Petroleum Engineering Department at the University of Calgary, where he concentrates in teaching about the theoretical and practical aspects of naturally fractured reservoirs. He is a petroleum engineering graduate from the Universidad de America at Bogota, Colombia, and holds a master's degree and a 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 to 1996. He has presented his course on naturally fractured reservoirs and has rendered consulting services throughout the world. He is a Distinguished Author of the Journal of Canadian Petroleum Technology (1993 and 1999), a recipient of the Outstanding Service Award from the Petroleum Society of the Canadian Institute of Mining, Metallurgy, and Petroleum Engineers (CIM) in 1994, and a Society of Petroleum Engineers Distinguished Lecturer on the subject of naturally fractured reservoirs for 2000–2001.