Difference between revisions of "Pickett plot construction"
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{| class = "wikitable" | {| class = "wikitable" | ||
|- | |- | ||
− | ! Step | + | ! Step || Action |
− | |||
|- | |- | ||
− | | 1 | + | | 1 || Plot points of matching [[porosity]] and true resistivity (R<sub>t</sub> ) on log-log paper. |
− | | Plot points of matching [[porosity]] and true resistivity (R<sub>t</sub> ) on log-log paper. | ||
|- | |- | ||
− | | 2 | + | | 2 || Plot R<sub>w</sub> point on the R<sub>t</sub> scale. |
− | | Plot R<sub>w</sub> point on the R<sub>t</sub> scale. | ||
|- | |- | ||
− | | 3 | + | | 3 || Determine ''m'' using the table of values. |
− | | Determine ''m'' using the table of values. | ||
|- | |- | ||
− | | 4 | + | | 4 || Plot the 100% S<sub>w</sub> line. |
− | | Plot the 100% S<sub>w</sub> line. | ||
|- | |- | ||
− | | 5 | + | | 5 || Plot the lines representing lower values of S<sub>w</sub> . |
− | | Plot the lines representing lower values of S<sub>w</sub> . | ||
|} | |} | ||
==Step 1: Plot points== | ==Step 1: Plot points== | ||
− | [[file:predicting-reservoir-system-quality-and-performance_fig9-36.png | + | [[file:predicting-reservoir-system-quality-and-performance_fig9-36.png|thumb|300px|{{figure number|1}}Plot points of matching porosity and true resistivity (R<sub>t</sub>) values obtained from well logs on 2×4 cycle log-log paper.]] |
Plot points of matching porosity and true resistivity (R<sub>t</sub>) values obtained from well logs on 2×4 cycle log-log paper, as shown in [[:file:predicting-reservoir-system-quality-and-performance_fig9-36.png|Figure 1]]. Use the x-axis for the resistivity (R<sub>t</sub>) scale and the y-axis for the porosity (Φ) scale. | Plot points of matching porosity and true resistivity (R<sub>t</sub>) values obtained from well logs on 2×4 cycle log-log paper, as shown in [[:file:predicting-reservoir-system-quality-and-performance_fig9-36.png|Figure 1]]. Use the x-axis for the resistivity (R<sub>t</sub>) scale and the y-axis for the porosity (Φ) scale. | ||
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==Step 2: plot r<sub>w</sub> point== | ==Step 2: plot r<sub>w</sub> point== | ||
− | [[file:predicting-reservoir-system-quality-and-performance_fig9-37.png|thumb|{{figure number|2}}Plot the R<sub>w</sub> value (resistivity of formation water) by plotting the R<sub>w</sub> point along the R<sub>t</sub> scale on the x-axis at the top of the graph grid where porosity is 100%.]] | + | [[file:predicting-reservoir-system-quality-and-performance_fig9-37.png|300px|thumb|{{figure number|2}}Plot the R<sub>w</sub> value (resistivity of formation water) by plotting the R<sub>w</sub> point along the R<sub>t</sub> scale on the x-axis at the top of the graph grid where porosity is 100%.]] |
Plot the R<sub>w</sub> value (resistivity of formation water) by plotting the R<sub>w</sub> point along the R<sub>t</sub> scale on the x-axis at the top of the graph grid where porosity is 100%, as shown in [[:file:predicting-reservoir-system-quality-and-performance_fig9-37.png|Figure 2]]. R<sub>w</sub> values are published by logging companies, or we can calculate them from the SP log. | Plot the R<sub>w</sub> value (resistivity of formation water) by plotting the R<sub>w</sub> point along the R<sub>t</sub> scale on the x-axis at the top of the graph grid where porosity is 100%, as shown in [[:file:predicting-reservoir-system-quality-and-performance_fig9-37.png|Figure 2]]. R<sub>w</sub> values are published by logging companies, or we can calculate them from the SP log. | ||
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{| class = "wikitable" | {| class = "wikitable" | ||
|- | |- | ||
− | ! Porosity type | + | ! Porosity type || Value for m |
− | |||
|- | |- | ||
− | | Sandstones with diagenetic or detrital clay in pores | + | | Sandstones with diagenetic or detrital clay in pores || 1.7–1.8 |
− | | 1.7–1.8 | ||
|- | |- | ||
− | | Formations with clean, macro- to micro-sized pore throats (Archie rocks) | + | | Formations with clean, macro- to micro-sized pore throats (Archie rocks) || 2 |
− | | 2 | ||
|- | |- | ||
− | | Formations with vuggy porosity (touching to | + | | Formations with vuggy porosity (touching to non touching) || 2.2–3.0 |
− | | 2.2–3.0 | ||
|} | |} | ||
==Step 4: Plot the 100% S<sub>w</sub> line== | ==Step 4: Plot the 100% S<sub>w</sub> line== | ||
− | [[file:predicting-reservoir-system-quality-and-performance_fig9-38.png|thumb|{{figure number|3}}How to plot an ''m'' of 2.]] | + | [[file:predicting-reservoir-system-quality-and-performance_fig9-38.png|300px|thumb|{{figure number|3}}How to plot an ''m'' of 2.]] |
On a Pickett plot, the value of ''m'' determines the slope of the S<sub>w</sub> lines. The first S<sub>w</sub> line plotted on a Pickett plot is the 100% S<sub>w</sub> line. To plot this line, draw a line with a negative slope equal to ''m'' that begins at the R<sub>w</sub> point. Use a linear scale to measure the slope; for example, go down [[length::1 in.]] and over 2 in. | On a Pickett plot, the value of ''m'' determines the slope of the S<sub>w</sub> lines. The first S<sub>w</sub> line plotted on a Pickett plot is the 100% S<sub>w</sub> line. To plot this line, draw a line with a negative slope equal to ''m'' that begins at the R<sub>w</sub> point. Use a linear scale to measure the slope; for example, go down [[length::1 in.]] and over 2 in. | ||
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==Step 5: plot s<sub>w</sub> lines== | ==Step 5: plot s<sub>w</sub> lines== | ||
− | [[file:predicting-reservoir-system-quality-and-performance_fig9-39.png|thumb|{{figure number|4}}An example of plotting the lower percentages of S<sub>w</sub>.]] | + | [[file:predicting-reservoir-system-quality-and-performance_fig9-39.png|300px|thumb|{{figure number|4}}An example of plotting the lower percentages of S<sub>w</sub>.]] |
After plotting the 100% S<sub>w</sub> line, plot the lines representing lower percentages of S<sub>w</sub> using this procedure. | After plotting the 100% S<sub>w</sub> line, plot the lines representing lower percentages of S<sub>w</sub> using this procedure. | ||
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{| class = "wikitable" | {| class = "wikitable" | ||
|- | |- | ||
− | ! Step | + | ! Step || Action |
− | |||
|- | |- | ||
− | | 1 | + | | 1 || Find the intercept of R<sub>t</sub> = 1 and the 100% S<sub>w</sub> line (made in the last procedure). |
− | | Find the intercept of R<sub>t</sub> = 1 and the 100% S<sub>w</sub> line (made in the last procedure). | ||
|- | |- | ||
− | | 2 | + | | 2 || From this intercept, draw a line parallel to the x-axis across the plot. Any point on this line has the same porosity. |
− | | From this intercept, draw a line parallel to the x-axis across the plot. Any point on this line has the same porosity. | ||
|- | |- | ||
− | | 3 | + | | 3 || Where this line passes through R<sub>t</sub> of 2, 4, 6, 8, 14, and 20, draw a series of lines parallel to the 100% S<sub>w</sub> line. |
− | | Where this line passes through R<sub>t</sub> of 2, 4, 6, 8, 14, and 20, draw a series of lines parallel to the 100% S<sub>w</sub> line. | ||
|- | |- | ||
− | | 4 | + | | 4 || Points on these lines correspond to S<sub>w</sub> of 71, 50, 41, 35, 27, and 22%. These percentages are calculated from the Archie equation using ''m'' = 2 and ''n'' = 2 at R<sub>t</sub> of 2, 4, 6, 8, 14, and 20. |
− | | Points on these lines correspond to S<sub>w</sub> of 71, 50, 41, 35, 27, and 22%. These percentages are calculated from the Archie equation using ''m'' = 2 and ''n'' = 2 at R<sub>t</sub> of 2, 4, 6, 8, 14, and 20. | ||
|} | |} | ||
Revision as of 14:16, 20 May 2014
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 |
Store | AAPG Store |
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.
Step | Action |
---|---|
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
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
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
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 length::1 in. and over 2 in.
Figure 3 shows how to plot an m of 2.
Step 5: plot sw lines
After plotting the 100% Sw line, plot the lines representing lower percentages of Sw using this procedure.
Step | Action |
---|---|
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.