1,705
edits
Changes
Jump to navigation
Jump to search
Pickett plot construction (view source)
Revision as of 20:44, 31 January 2014
, 20:44, 31 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Predicting reservoir system quality and performance
| frompg = 9-1
| topg = 9-156
| author = Dan J. Hartmann, Edward A. Beaumont
| link = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
A Pickett plot lets us compare water saturations of different parts of a reservoir in one or many wells. The Pickett plot<ref name=ch09r45>Pickett, G., R., 1973, Pattern recognition as a means of formation evaluation: The Log Analyst, vol. 14, no. 4, p. 3–11.</ref> is a visual representation of the Archie equation and therefore is a powerful graphic technique for estimating S<sub>w</sub> 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.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Plot points of matching [[porosity]] and true resistivity (R<sub>t</sub> ) on log-log paper.
|-
| 2
| Plot R<sub>w</sub> point on the R<sub>t</sub> scale.
|-
| 3
| Determine ''m'' using the table of values.
|-
| 4
| Plot the 100% S<sub>w</sub> line.
|-
| 5
| Plot the lines representing lower values of S<sub>w</sub> .
|}
==Step 1: Plot points==
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 below. Use the x-axis for the resistivity (R<sub>t</sub>) scale and the y-axis for the porosity (Φ) scale.
[[file:predicting-reservoir-system-quality-and-performance_fig9-36.png|thumb|{{figure number|9-36}}See text for explanation.]]
==Step 2: plot r<sub>w</sub> point==
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 below. R<sub>w</sub> values are published by logging companies, or we can calculate them from the SP log.
[[file:predicting-reservoir-system-quality-and-performance_fig9-37.png|thumb|{{figure number|9-37}}See text for explanation.]]
==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.
{| class = "wikitable"
|-
! [[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 nontouching)
| 2.2–3.0
|}
==Step 4: Plot the 100% S<sub>w</sub> line==
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.
[[file:predicting-reservoir-system-quality-and-performance_fig9-38.png|thumb|{{figure number|9-38}}See text for explanation.]]
The example below shows how to plot an ''m'' of 2.
==Step 5: plot s<sub>w</sub> lines==
After plotting the 100% S<sub>w</sub> line, plot the lines representing lower percentages of S<sub>w</sub> using this procedure.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Find the intercept of R<sub>t</sub> = 1 and the 100% S<sub>w</sub> 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<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
| 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.
|}
The figure below is an example of following this procedure.
[[file:predicting-reservoir-system-quality-and-performance_fig9-39.png|thumb|{{figure number|9-39}}See text for explanation.]]
==See also==
* [[Determining water saturation]]
* [[Calculating Sw from the Archie equation]]
* [[Determining Rt]]
* [[Calculating Rw from SP logs]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Predicting reservoir system quality and performance]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Predicting reservoir system quality and performance
| frompg = 9-1
| topg = 9-156
| author = Dan J. Hartmann, Edward A. Beaumont
| link = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
A Pickett plot lets us compare water saturations of different parts of a reservoir in one or many wells. The Pickett plot<ref name=ch09r45>Pickett, G., R., 1973, Pattern recognition as a means of formation evaluation: The Log Analyst, vol. 14, no. 4, p. 3–11.</ref> is a visual representation of the Archie equation and therefore is a powerful graphic technique for estimating S<sub>w</sub> 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.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Plot points of matching [[porosity]] and true resistivity (R<sub>t</sub> ) on log-log paper.
|-
| 2
| Plot R<sub>w</sub> point on the R<sub>t</sub> scale.
|-
| 3
| Determine ''m'' using the table of values.
|-
| 4
| Plot the 100% S<sub>w</sub> line.
|-
| 5
| Plot the lines representing lower values of S<sub>w</sub> .
|}
==Step 1: Plot points==
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 below. Use the x-axis for the resistivity (R<sub>t</sub>) scale and the y-axis for the porosity (Φ) scale.
[[file:predicting-reservoir-system-quality-and-performance_fig9-36.png|thumb|{{figure number|9-36}}See text for explanation.]]
==Step 2: plot r<sub>w</sub> point==
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 below. R<sub>w</sub> values are published by logging companies, or we can calculate them from the SP log.
[[file:predicting-reservoir-system-quality-and-performance_fig9-37.png|thumb|{{figure number|9-37}}See text for explanation.]]
==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.
{| class = "wikitable"
|-
! [[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 nontouching)
| 2.2–3.0
|}
==Step 4: Plot the 100% S<sub>w</sub> line==
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.
[[file:predicting-reservoir-system-quality-and-performance_fig9-38.png|thumb|{{figure number|9-38}}See text for explanation.]]
The example below shows how to plot an ''m'' of 2.
==Step 5: plot s<sub>w</sub> lines==
After plotting the 100% S<sub>w</sub> line, plot the lines representing lower percentages of S<sub>w</sub> using this procedure.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Find the intercept of R<sub>t</sub> = 1 and the 100% S<sub>w</sub> 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<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
| 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.
|}
The figure below is an example of following this procedure.
[[file:predicting-reservoir-system-quality-and-performance_fig9-39.png|thumb|{{figure number|9-39}}See text for explanation.]]
==See also==
* [[Determining water saturation]]
* [[Calculating Sw from the Archie equation]]
* [[Determining Rt]]
* [[Calculating Rw from SP logs]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Predicting reservoir system quality and performance]]