Free water level determination using pressure

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 repeat formation tester (RFT) data

Figure 1 Easy method for determining free-water level (FWL) by projecting RFT pressure data downward from a reservoir to the aquifer.

An easy method for determining free water level (FWL) is projecting RFT pressure data downward from a reservoir to the aquifer. Figure 1 illustrates the procedure.

Procedure using a single measurement

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

1. Determine buoyancy pressure (P_b ) at the depth of the measured pressure (P_m ) from the measured pressure:

   • $P_{\rm {b}}=P_{\rm {m}}-P_{\rm {hydrostatic}}$

2. Determine buoyancy pressure gradient (P_bg ):

   • $P_{\rm {bg}}=P_{\rm {hydrostatic\ pressure\ gradient}}-P_{\rm {hydrocarbon\ pressure\ gradient}}$

3. Calculate downdip length of hydrocarbon column (h):

   • $h={\frac {P_{\rm {b}}}{P_{\rm {bg}}}}$

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

• P_m = pressure::3555 psi at depth::7611 ft
• P_hydrostatic = 3525 psi
• P_{hydrostatic pressure gradient} = 0.465 psi/ft
• P_{hydrocarbon pressure gradient} = 0.38 psi/ft

Answer (tied back to steps above):

$P_{\rm {b}}=P_{m}-P_{\rm {hydrostatic}}=3555-3525=30\ \mathrm {psi}$

$P_{\rm {hydrostatic\ pressure\ gradient}}-P_{\rm {hydrocarbon\ pressure\ gradient}}=0.465-0.38=0.085{\mbox{ psi/ft}}$

$h={\frac {P_{\rm {b}}}{P_{\rm {bg}}}}=30{\mbox{ psi}}\div 0.054{\mbox{ psi/ft}}=556{\mbox{ ft}}$

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

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