Difference between revisions of "Free water level determination using pressure"

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|-
 
|-
 
| 1
 
| 1
| Determine buoyancy pressure (P<sub>b</sub> ) at the depth of the measured pressure (P<sub>m</sub> ) from the measured pressure:
+
| 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>
+
:<math>P_{\rm b} = P_{\rm m} - P_{\rm hydrostatic}</math>
  
 
|-
 
|-
 
| 2
 
| 2
| Determine buoyancy pressure gradient (P<sub>bg</sub> ):
+
| 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>
+
:<math>P_{\rm bg} = P_{\rm hydrostatic\ pressure\ gradient} - P_{\rm hydrocarbon\ pressure\ gradient}</math>
  
 
|-
 
|-
 
| 3
 
| 3
| Calculate downdip length of hydrocarbon column (h):
+
| Calculate downdip length of hydrocarbon column (''h''):
  
:<math>\mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}}</math>
+
:<math>h = \frac{P_{\rm b}}{P_{\rm bg}}</math>
  
 
|}
 
|}
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As an example, let's determine the downdip length of a 30°API oil column with the following givens:
 
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>m</sub> = [[pressure::3555 psi]] at [[depth::7611 ft]]
* P<sub>hydrostatic</sub> = 3525 psi
+
* ''P''<sub>hydrostatic</sub> = 3525 psi
* P<sub>hydrostatic pressure gradient</sub> = 0.465 psi/ft
+
* ''P''<sub>hydrostatic pressure gradient</sub> = 0.465 psi/ft
* P<sub>hydrocarbon pressure gradient</sub> = 0.38 psi/ft
+
* ''P''<sub>hydrocarbon pressure gradient</sub> = 0.38 psi/ft
  
 
'''Answer''' (tied back to steps above):
 
'''Answer''' (tied back to steps above):
  
Step 1:
+
:<math>P_{\rm b} = P_{m} - P_{\rm hydrostatic} = 3555 - 3525 = 30\ \mathrm{psi}</math>
  
:<math>\mbox{P}_{\rm b} = \mbox{P}_{m} - \mbox{P}_{\rm hydrostatic} = 3555 - 3525 = 30 psi</math>
+
:<math>P_{\rm hydrostatic\ pressure\ gradient} - P_{\rm hydrocarbon\ pressure\ gradient} = 0.465 - 0.38 = 0.085 \mbox{ psi/ft}</math>
  
Step 2:
+
:<math>h = \frac{P_{\rm b}}{P_{\rm bg}} = 30 \mbox{ psi} \div 0.054 \mbox{ psi/ft} = 556 \mbox{ ft}</math>
 
 
:<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]].
 
Therefore, the free-water level is at [[depth::8167 ft]].

Revision as of 15:59, 30 January 2014

Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Formation fluid pressure and its application
Author Edward A. Beaumont, Forrest Fiedler
Link Web page
Store AAPG Store

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[edit]

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.

Figure 5-12 See text for explanation.

Procedure using a single measurement[edit]

The table below outlines the procedure for determining the free-water level using a single pressure buildup point in the reservoir.

Step Action
1 Determine buoyancy pressure (Pb ) at the depth of the measured pressure (Pm ) from the measured pressure:
2 Determine buoyancy pressure gradient (Pbg ):
3 Calculate downdip length of hydrocarbon column (h):

As an example, let's determine the downdip length of a 30°API oil column with the following givens:

Answer (tied back to steps above):

Therefore, the free-water level is at depth::8167 ft.

See also[edit]

External links[edit]

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Free water level determination using pressure
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