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Spontaneous potential logs in water resistivity calculation (view source)
Revision as of 20:38, 31 January 2014
, 20:38, 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
}}
Water resistivity or R<sub>w</sub>, is a critical component of log analysis in calculating water saturation using the Archie equation. R<sub>w</sub> can be measured from a sample of formation water taken from the zone of interest at the well site or a nearby well, or it can be calculated using spontaneous potential (SP) log data.
==Data required==
To calculate R<sub>w</sub> from SP, we need the following data:
* Resistivity of the mud filtrate (R<sub>mf</sub>) at measured temperature, found on the log header. If only mud resistivity (R<sub>m</sub>) is given, convert it to R<sub>mf</sub> as explained below.
* Bottom-hole temperature (BHT) and total depth, found on the log header.
* SP reading from a porous zone at least [[length::20 ft]] thick. (A bed thickness correction is necessary if the zone SP is measured from is less than [[length::20 ft]] thick.)
==Converting r<sub>m</sub> to r<sub>mf</sub>==
If the log header gives R<sub>m</sub> only, then R<sub>m</sub> must be converted to R<sub>mf</sub> using this procedure:
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter R<sub>m</sub> and move across (Figure 9-32) to the appropriate mud weight.
|-
| 2
| Project to the bottom of the chart to estimate R<sub>mf</sub>
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-32.png|thumb|{{figure number|9-32}}. Copyright: Schlumberger.]]
==Steps for calculating r<sub>w</sub>==
There are five steps for calculating R<sub>w</sub> from the SP log. The table below summarizes these steps, which are detailed in the rest of this section.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Estimate formation temperature.
|-
| 2
| Convert R<sub>mf</sub> to formation temperature.
|-
| 3
| Convert R<sub>mf</sub> to R<sub>mf</sub><sub>eq</sub> .
|-
| 4
| Read SP response and estimate R<sub>we</sub> .
|-
| 5
| Convert R<sub>we</sub> to R<sub>w</sub> and NaCl at formation temperature.
|}
==Step 1: Estimate formation temperature==
Formation temperature (T<sub>f</sub>) can be estimated by using the following formula:
:<math>\mbox{T}_{\rm f} = \mbox{T}_{\rm s} + \mbox{D}_{\rm f} \frac{\mbox{BHT} - \mbox{T}_{\rm s}}{\mbox{TD}}</math>
where:
* T<sub>s</sub> = average surface temperature
* D<sub>f</sub> = depth to the formation
* BHT = bottom-hole temperature (found on log header)
* TD = total depth (make sure BHT and TD are from same log run)
==Step 2: convert r<sub>mf</sub> to r<sub>mf</sub> at formation temperature==
Follow this procedure to convert R<sub>mf</sub> (measured at surface temperature) to R<sub>mf</sub> at formation temperature.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-33 along the resistivity of solution axis and the temperature axis using the measured values for R<sub>mf</sub> and surface temperature found on the log header.
|-
| 2
| Follow the appropriate salinity line intercepted at step 1 to the appropriate formation temperature and mark on the chart.
|-
| 3
| Project down the chart from this mark to the resistivity scale and read R<sub>mf</sub> at formation temperature. Record the value of R<sub>mf</sub> at a specific temperature.
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-33.png|thumb|{{figure number|9-33}}. Copyright: Schlumberger.]]
==Step 3: convert r<sub>mf</sub> to r<sub>mf eq</sub>==
Use the R<sub>mf</sub> at formation temperature obtained above and follow the procedure below to convert R<sub>mf</sub> to equivalent mud filtrate resistivity (R<sub>mf</sub> <sub>eq</sub>).
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-34 with R<sub>mf</sub> at formation temperature on the vertical axis.
|-
| 2
| Move across the chart to the appropriate formation temperature contour, and mark this point on the figure.
|-
| 3
| Read down to R<sub>mf</sub><sub>eq</sub> . This value is used in the equation R<sub>we</sub> = R<sub>mf</sub><sub>eq</sub> (R<sub>mf</sub><sub>eq</sub> /R<sub>we</sub> value).
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-34.png|thumb|{{figure number|9-34}}. Copyright: Schlumberger.]]
==Step 4: convert SP to r<sub>we</sub>==
Follow the procedure below to convert SP from the zone of interest to equivalent formation water resistivity (R<sub>we</sub>).
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| On the log, establish the shale base line for the SP curve.
|-
| 2
| Read the maximum SP response in a zone at least [[length::20 ft]] thick.
|-
| 3
| Enter the base of Figure 9-35 with SP (SP is negative if it deflects to the left of the shale base line). Follow the SP grid line up the chart to the appropriate formation temperature. At this point, move across the chart and read the R<sub>mf eq</sub> /R<sub>we</sub> Value.
|-
| 4
| Solve for R<sub>we</sub> using the equation R<sub>we</sub> = R<sub>mf</sub><sub>eq</sub> /(R<sub>mf</sub><sub>eq</sub> /R<sub>we</sub> value).
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-35.png|thumb|{{figure number|9-35}}. Copyright: Schlumberger.]]
==Step 5: convert r<sub>we</sub> to r<sub>w</sub>==
Follow the procedure below to convert R<sub>we</sub> to R<sub>w</sub>.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-34 again with R<sub>we</sub> (along the base). Move up the chart until R<sub>we</sub> intersects the temperature slope.
|-
| 2
| Directly across from the intersection point, read R<sub>w</sub> from the vertical axis.
|}
==See also==
* [[Determining water saturation]]
* [[Calculating Sw from the Archie equation]]
* [[Determining Rt]]
* [[Constructing a Pickett plot]]
==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
}}
Water resistivity or R<sub>w</sub>, is a critical component of log analysis in calculating water saturation using the Archie equation. R<sub>w</sub> can be measured from a sample of formation water taken from the zone of interest at the well site or a nearby well, or it can be calculated using spontaneous potential (SP) log data.
==Data required==
To calculate R<sub>w</sub> from SP, we need the following data:
* Resistivity of the mud filtrate (R<sub>mf</sub>) at measured temperature, found on the log header. If only mud resistivity (R<sub>m</sub>) is given, convert it to R<sub>mf</sub> as explained below.
* Bottom-hole temperature (BHT) and total depth, found on the log header.
* SP reading from a porous zone at least [[length::20 ft]] thick. (A bed thickness correction is necessary if the zone SP is measured from is less than [[length::20 ft]] thick.)
==Converting r<sub>m</sub> to r<sub>mf</sub>==
If the log header gives R<sub>m</sub> only, then R<sub>m</sub> must be converted to R<sub>mf</sub> using this procedure:
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter R<sub>m</sub> and move across (Figure 9-32) to the appropriate mud weight.
|-
| 2
| Project to the bottom of the chart to estimate R<sub>mf</sub>
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-32.png|thumb|{{figure number|9-32}}. Copyright: Schlumberger.]]
==Steps for calculating r<sub>w</sub>==
There are five steps for calculating R<sub>w</sub> from the SP log. The table below summarizes these steps, which are detailed in the rest of this section.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Estimate formation temperature.
|-
| 2
| Convert R<sub>mf</sub> to formation temperature.
|-
| 3
| Convert R<sub>mf</sub> to R<sub>mf</sub><sub>eq</sub> .
|-
| 4
| Read SP response and estimate R<sub>we</sub> .
|-
| 5
| Convert R<sub>we</sub> to R<sub>w</sub> and NaCl at formation temperature.
|}
==Step 1: Estimate formation temperature==
Formation temperature (T<sub>f</sub>) can be estimated by using the following formula:
:<math>\mbox{T}_{\rm f} = \mbox{T}_{\rm s} + \mbox{D}_{\rm f} \frac{\mbox{BHT} - \mbox{T}_{\rm s}}{\mbox{TD}}</math>
where:
* T<sub>s</sub> = average surface temperature
* D<sub>f</sub> = depth to the formation
* BHT = bottom-hole temperature (found on log header)
* TD = total depth (make sure BHT and TD are from same log run)
==Step 2: convert r<sub>mf</sub> to r<sub>mf</sub> at formation temperature==
Follow this procedure to convert R<sub>mf</sub> (measured at surface temperature) to R<sub>mf</sub> at formation temperature.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-33 along the resistivity of solution axis and the temperature axis using the measured values for R<sub>mf</sub> and surface temperature found on the log header.
|-
| 2
| Follow the appropriate salinity line intercepted at step 1 to the appropriate formation temperature and mark on the chart.
|-
| 3
| Project down the chart from this mark to the resistivity scale and read R<sub>mf</sub> at formation temperature. Record the value of R<sub>mf</sub> at a specific temperature.
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-33.png|thumb|{{figure number|9-33}}. Copyright: Schlumberger.]]
==Step 3: convert r<sub>mf</sub> to r<sub>mf eq</sub>==
Use the R<sub>mf</sub> at formation temperature obtained above and follow the procedure below to convert R<sub>mf</sub> to equivalent mud filtrate resistivity (R<sub>mf</sub> <sub>eq</sub>).
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-34 with R<sub>mf</sub> at formation temperature on the vertical axis.
|-
| 2
| Move across the chart to the appropriate formation temperature contour, and mark this point on the figure.
|-
| 3
| Read down to R<sub>mf</sub><sub>eq</sub> . This value is used in the equation R<sub>we</sub> = R<sub>mf</sub><sub>eq</sub> (R<sub>mf</sub><sub>eq</sub> /R<sub>we</sub> value).
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-34.png|thumb|{{figure number|9-34}}. Copyright: Schlumberger.]]
==Step 4: convert SP to r<sub>we</sub>==
Follow the procedure below to convert SP from the zone of interest to equivalent formation water resistivity (R<sub>we</sub>).
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| On the log, establish the shale base line for the SP curve.
|-
| 2
| Read the maximum SP response in a zone at least [[length::20 ft]] thick.
|-
| 3
| Enter the base of Figure 9-35 with SP (SP is negative if it deflects to the left of the shale base line). Follow the SP grid line up the chart to the appropriate formation temperature. At this point, move across the chart and read the R<sub>mf eq</sub> /R<sub>we</sub> Value.
|-
| 4
| Solve for R<sub>we</sub> using the equation R<sub>we</sub> = R<sub>mf</sub><sub>eq</sub> /(R<sub>mf</sub><sub>eq</sub> /R<sub>we</sub> value).
|}
[[file:predicting-reservoir-system-quality-and-performance_fig9-35.png|thumb|{{figure number|9-35}}. Copyright: Schlumberger.]]
==Step 5: convert r<sub>we</sub> to r<sub>w</sub>==
Follow the procedure below to convert R<sub>we</sub> to R<sub>w</sub>.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Enter Figure 9-34 again with R<sub>we</sub> (along the base). Move up the chart until R<sub>we</sub> intersects the temperature slope.
|-
| 2
| Directly across from the intersection point, read R<sub>w</sub> from the vertical axis.
|}
==See also==
* [[Determining water saturation]]
* [[Calculating Sw from the Archie equation]]
* [[Determining Rt]]
* [[Constructing a Pickett plot]]
==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]]