Free water level determination using pressure

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

External links

find literature about Free water level determination using pressure

• Original content in Datapages
• Find the book in the AAPG Store

feet from west line

American Petroleum Institute