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 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.

Figure 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.

Step	Action
1	Determine buoyancy pressure (Pb ) at the depth of the measured pressure (Pm ) from the measured pressure: ${\mbox{P}}_{\rm {b}}={\mbox{P}}_{\rm {m}}-{\mbox{P}}_{\rm {hydrostatic}}$
2	Determine buoyancy pressure gradient (Pbg ): ${\mbox{P}}_{\rm {bg}}={\mbox{P}}_{\rm {hydrostatic\ pressure\ gradient}}-{\mbox{P}}_{\rm {hydrocarbon\ pressure\ gradient}}$
3	Calculate downdip length of hydrocarbon column (h): ${\mbox{h}}={\frac {{\mbox{P}}_{\rm {b}}}{{\mbox{P}}_{\rm {bg}}}}$

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

• P_m = 3555 psi at 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):

Step 1:

${\mbox{P}}_{\rm {b}}={\mbox{P}}_{m}-{\mbox{P}}_{\rm {hydrostatic}}=3555-3525=30psi$

Step 2:

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

Step 3:

Failed to parse (syntax error): {\displaystyle \mbox{h} = \frac{\mbox{P}_{\rm b}}{\mbox{P}_{\rm bg}} = \mbox{30 \mbox{ psi}}{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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