Pickett plot construction

A Pickett plot lets us compare water saturations of different parts of a reservoir in one or many wells. The Pickett plot is a visual representation of the Archie equation and therefore is a powerful graphic technique for estimating S_w 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 (R_t ) on log-log paper.
2. Plot R_w point on the R_t scale.
3. Determine m using the table of values.
4. Plot the 100% S_w line.
5. Plot the lines representing lower values of S_w .

Step 1: Plot points

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

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

Step 2: plot r_w point

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

Plot the R_w value (resistivity of formation water) by plotting the R_w point along the R_t scale on the x-axis at the top of the graph grid where porosity is 100%, as shown in Figure 2. R_w 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% S_w line

Figure 3 How to plot an m of 2.

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

Figure 3 shows how to plot an m of 2.

Step 5: plot s_w lines

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

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

1. Find the intercept of R_t = 1 and the 100% S_w 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 R_t of 2, 4, 6, 8, 14, and 20, draw a series of lines parallel to the 100% S_w line.
4. Points on these lines correspond to S_w of 71, 50, 41, 35, 27, and 22%. These percentages are calculated from the Archie equation using m = 2 and n = 2 at R_t of 2, 4, 6, 8, 14, and 20.

Figure 4 is an example of following this procedure.

See also

• Determining water saturation
• Archie equation
• Determining Rt
• Calculating Rw from SP logs

External links

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