The Integration of Capillary Pressures and Pickett Plots for Determination of Flow Units and Reservoir Containers

Abstract

Abstract The concept of flow (or hydraulic) units and reservoir containers has been used in the oil industry with a good deal of success during the last few years. The process or delivery speed k/f can be used in many instances to define a flow unit. Correlation of flow units between wells helps to establish reservoirs containers and to forecast reservoir performance. This study shows that a Pickett crossplot of effective porosity vs. true resistivity (in some cases apparent resistivity or true resistivity affected by a shale group) should result in parallel straight lines for intervals with constant process speed k/f. The slope of the straight lines is related to the porosity exponent m, the water saturation exponent n, and constants in the absolute permeability equation. From the straight lines it is possible to determine directly capillary pressures and pore throat apertures for each flow unit at any water saturation. Pore throats at 65% water saturation compare very well with Winland r35 values. The method has not been published previously in the literature. Building lines of constant k/ϕ allows displaying complete capillary pressure curves on the Pickett plot including regions that are and are not at irreducible water saturation. Previous empirical methods for determining absolute permeability of a given interval assume that the water saturation is at irreducible conditions. This paper presents a technique that allows estimating absolute permeability even if the interval contains moveable water. The use of this technique is illustrated with previously published data from the Morrow sandstone in the Sorrento field of Southeastern Colorado and carbonates from the Mission Canyon formation in the Little Knife field of North Dakota. It is concluded that flow units can be determined reliably from the integration within one single log-log graph of Pickett plots, capillary pressures, pore throat apertures and Winland r35 values. The correlation of the flow units between wells leads to the definition of reservoir containers.