The integration of capillary pressures and Pickett plots has been shown recently to be a useful approach for determining flow units. The present study extends the method to the case of naturally fractured reservoirs by preparing Pickett plots for only the matrix. This requires calculation of matrix porosities and true resistivities for the matrix. By placing pore throat apertures, capillary pressures and heights above the free water table on Pickett plots, it is possible to generate matrix flow units and to estimate if the matrix will contribute to production. Pattern recognition is the key to success with this approach. Two examples are presented.


Pickett plots1,2 have long been recognized as very useful in log interpretation. In Pickett's method, a resistivity index, I, and water saturation, Sw, are calculated from log-log crossplots of porosity vs. true resistivity (in some cases apparent resistivity, or resistivity as affected by a shale group, Ash), as shown on Figures 1a, 1b, 1c, 1d, 1e and 1f.

The Pickett plot has been extended throughout the years to include many situations of practical importance. For example, Aguilera3,4 demonstrated that Pickett plots could be used for evaluating naturally fractured reservoirs. In these formations the value of the porosity exponent was shown to be smaller than usual (Figure 1b).

Sanyal and Ellithorpe5 and Greengold6 have shown that a Pickett plot should result in a straight line with a slope equal to (n - m) for intervals at irreducible water saturation.

Aguilera7 extended the Pickett plot to the analysis of laminar, dispersed and total shale models (Figure 1c). In this approach, the resistivity included in the plot is affected by a shale group, Ash, whose value depends on the type of shaly model being used. Aguilera showed that all equations for evaluation of shaly formations published in the literature, no matter how long they are, become Sw = Ish−1/n. He further showed that a Pickett plot for shaly formations should result in a straight line with a slope equal to (n - m) for intervals at irreducible water saturation.

Aguilera8 demonstrated that a log-log crossplot of Rt vs. effective porosity, as determined from neutron and density logs, minus free fluid porosity, as determined from a nuclear magnetic log, should result in a straight line with a negative slope equal to the water saturation exponent, n, for intervals that are at irreducible water saturation (Figure 1d). Extrapolation of the straight line to 100% porosity yields the product aRw. Gas intervals plot above the straight line. Intervals with movable water plot below the straight line.

In the same paper, Aguilera8 showed that a Pickett plot should result in a straight line for intervals of constant permeability at irreducible water saturation (Figure 1e).

The same concept has been used successfully by Doveton et al.9

More recently Aguilera10 presented techniques for incorporating capillary pressures, pore aperture radii, height above free water table, and Winland r35 values on Pickett plots (Figure 1f). He developed an equation, which compares favorably with Winland r35.

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