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Free water level determination using pressure (view source)
Revision as of 21:28, 29 January 2014
, 21:28, 29 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 = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
The free-water level occurs where buoyancy pressure is zero in the reservoir-aquifer system. It defines the downdip limits of an accumulation. Pressure data reliability affects the resolution; however, resolution improves when it is supplemented with other petrophysical information.
==Procedure: using RFT data==
An easy method for determining free-water level (FWL) is projecting RFT pressure data downward from a reservoir to the aquifer. The figure below illustrates the procedure.
[[file:formation-fluid-pressure-and-its-application_fig5-12.png|thumb|{{figure number|5-12}}See text for explanation.]]
==Procedure using a single measurement==
The table below outlines the procedure for determining the free-water level using a single pressure buildup point in the reservoir.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Determine buoyancy pressure (P<sub>b</sub> ) at the depth of the measured pressure (P<sub>m</sub> ) from the measured pressure:
:<math>\mbox{P}_{\rm b} = \mbox{P}_{\rm m} - \mbox{P}_{\rm hydrostatic}</math>
|-
| 2
| Determine buoyancy pressure gradient (P<sub>bg</sub> ):
:<math>\mbox{P}_{\rm bg} = \mbox{P}_{\rm hydrostatic\ pressure\ gradient} - \mbox{P}_{\rm hydrocarbon\ pressure\ gradient}</math>
|-
| 3
| Calculate downdip length of hydrocarbon column (h):
:<math>\mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}}</math>
|}
As an example, let's determine the downdip length of a 30°API oil column with the following givens:
* P<sub>m</sub> = 3555 psi at 7611 ft
* P<sub>hydrostatic</sub> = 3525 psi
* P<sub>hydrostatic pressure gradient</sub> = 0.465 psi/ft
* P<sub>hydrocarbon pressure gradient</sub> = 0.38 psi/ft
'''Answer''' (tied back to steps above):
Step 1:
:<math>\mbox{P}_{\rm b} = \mbox{P}_{m} - \mbox{P}_{\rm hydrostatic} = 3555 - 3525 = 30 psi</math>
Step 2:
:<math>\mbox{P}_{\rm hydrostatic\ pressure\ gradient} - \mbox{P}_{\rm hydrocarbon\ pressure\ gradient} = 0.465 - 0.38 = 0.085 \mbox{ psi/ft}</math>
Step 3:
:<math>\mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}} = \mbox{30 \mbox{ psi}}{0.054 \mbox{ psi/ft}} = 556 \mbox{ ft}</math>
Therefore, the free-water level is at [[depth::8167 ft]].
==See also==
* [[Static hydrocarbon pressure gradients]]
* [[Estimating static oil pressure gradients]]
* [[Estimating static gas pressure gradients]]
* [[Plotting the hydrocarbon pressure gradient]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
The free-water level occurs where buoyancy pressure is zero in the reservoir-aquifer system. It defines the downdip limits of an accumulation. Pressure data reliability affects the resolution; however, resolution improves when it is supplemented with other petrophysical information.
==Procedure: using RFT data==
An easy method for determining free-water level (FWL) is projecting RFT pressure data downward from a reservoir to the aquifer. The figure below illustrates the procedure.
[[file:formation-fluid-pressure-and-its-application_fig5-12.png|thumb|{{figure number|5-12}}See text for explanation.]]
==Procedure using a single measurement==
The table below outlines the procedure for determining the free-water level using a single pressure buildup point in the reservoir.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Determine buoyancy pressure (P<sub>b</sub> ) at the depth of the measured pressure (P<sub>m</sub> ) from the measured pressure:
:<math>\mbox{P}_{\rm b} = \mbox{P}_{\rm m} - \mbox{P}_{\rm hydrostatic}</math>
|-
| 2
| Determine buoyancy pressure gradient (P<sub>bg</sub> ):
:<math>\mbox{P}_{\rm bg} = \mbox{P}_{\rm hydrostatic\ pressure\ gradient} - \mbox{P}_{\rm hydrocarbon\ pressure\ gradient}</math>
|-
| 3
| Calculate downdip length of hydrocarbon column (h):
:<math>\mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}}</math>
|}
As an example, let's determine the downdip length of a 30°API oil column with the following givens:
* P<sub>m</sub> = 3555 psi at 7611 ft
* P<sub>hydrostatic</sub> = 3525 psi
* P<sub>hydrostatic pressure gradient</sub> = 0.465 psi/ft
* P<sub>hydrocarbon pressure gradient</sub> = 0.38 psi/ft
'''Answer''' (tied back to steps above):
Step 1:
:<math>\mbox{P}_{\rm b} = \mbox{P}_{m} - \mbox{P}_{\rm hydrostatic} = 3555 - 3525 = 30 psi</math>
Step 2:
:<math>\mbox{P}_{\rm hydrostatic\ pressure\ gradient} - \mbox{P}_{\rm hydrocarbon\ pressure\ gradient} = 0.465 - 0.38 = 0.085 \mbox{ psi/ft}</math>
Step 3:
:<math>\mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}} = \mbox{30 \mbox{ psi}}{0.054 \mbox{ psi/ft}} = 556 \mbox{ ft}</math>
Therefore, the free-water level is at [[depth::8167 ft]].
==See also==
* [[Static hydrocarbon pressure gradients]]
* [[Estimating static oil pressure gradients]]
* [[Estimating static gas pressure gradients]]
* [[Plotting the hydrocarbon pressure gradient]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]