Pickett plot construction

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Predicting reservoir system quality and performance
Author Dan J. Hartmann, Edward A. Beaumont
Link Web page
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A Pickett plot lets us compare water saturations of different parts of a reservoir in one or many wells. The Pickett plot[1] is a visual representation of the Archie equation[2] and therefore is a powerful graphic technique for estimating Sw ranges within a reservoir. All that is needed to make a Pickett plot is a set of porosities and corresponding resistivities taken from well logs and 2×4 cycle log-log paper. The procedure for making a Pickett plot consists of five steps, detailed below.

  1. Plot points of matching porosity and true resistivity (Rt ) on log-log paper.
  2. Plot Rw point on the Rt scale.
  3. Determine m using the table of values.
  4. Plot the 100% Sw line.
  5. Plot the lines representing lower values of Sw .

Step 1: Plot points

Figure 1 Plot points of matching porosity and true resistivity (Rt) values obtained from well logs on 2×4 cycle log-log paper.

Plot points of matching porosity and true resistivity (Rt) values obtained from well logs on 2×4 cycle log-log paper, as shown in Figure 1. Use the x-axis for the resistivity (Rt) scale and the y-axis for the porosity (Φ) scale.

Step 2: plot rw point

Figure 2 Plot the Rw value (resistivity of formation water) by plotting the Rw point along the Rt scale on the x-axis at the top of the graph grid where porosity is 100%.

Plot the Rw value (resistivity of formation water) by plotting the Rw point along the Rt scale on the x-axis at the top of the graph grid where porosity is 100%, as shown in Figure 2. Rw values are published by logging companies, or we can calculate them from the SP log.

Step 3: Determine m

Estimate m (cementation factor) using the table below. Laboratory analysis is necessary for a precise determination of m. However, by knowing what the expected porosity type is, we can estimate the value. If you are unsure of the porosity type, use an m of 2.

Porosity type Value for m
Sandstones with diagenetic or detrital clay in pores 1.7–1.8
Formations with clean, macro- to micro-sized pore throats (Archie rocks) 2
Formations with vuggy porosity (touching to non touching) 2.2–3.0

Step 4: Plot the 100% Sw line

Figure 3 How to plot an m of 2.

On a Pickett plot, the value of m determines the slope of the Sw lines. The first Sw line plotted on a Pickett plot is the 100% Sw line. To plot this line, draw a line with a negative slope equal to m that begins at the Rw point. Use a linear scale to measure the slope; for example, go down 1 in.0.0254 m
0.0833 ft
and over 2 in.

Figure 3 shows how to plot an m of 2.

Step 5: plot sw lines

Figure 4 An example of plotting the lower percentages of Sw.

After plotting the 100% Sw line, plot the lines representing lower percentages of Sw using this procedure:

  1. Find the intercept of Rt = 1 and the 100% Sw line (made in the last procedure).
  2. From this intercept, draw a line parallel to the x-axis across the plot. Any point on this line has the same porosity.
  3. Where this line passes through Rt of 2, 4, 6, 8, 14, and 20, draw a series of lines parallel to the 100% Sw line.
  4. Points on these lines correspond to Sw of 71, 50, 41, 35, 27, and 22%. These percentages are calculated from the Archie equation using m = 2 and n = 2 at Rt of 2, 4, 6, 8, 14, and 20.

Figure 4 is an example of following this procedure.

See also

References

  1. Pickett, G., R., 1973, Pattern recognition as a means of formation evaluation: The Log Analyst, vol. 14, no. 4, p. 3–11.
  2. Archie, G. E., 1942, The electrical resistivity log as an aid in determining some reservoir characteristics: Petroleum Transactions of AIME 146: 54–62.

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