Hydrodynamic traps: mapping

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

Mapping hydrodynamic traps

Formation fluid pressure data are necessary for mapping prospective hydrodynamic traps. Combining formation fluid pressures with other data, such as density and subsea elevation, produces maps that help outline areas where hydrodynamic gradients may have created, destroyed, or modified traps.

Procedure: mapping hydrodynamic traps

Follow the steps listed in the table below to map the location of possible hydrodynamic traps.

Step	Action
1	Look for permeability barriers or hydrocarbon-free water levels using pressure-depth plots.
2	Calculate potentiometric values for each well using the formula Hw = Z + (P/grad P). (See “Calculating Hw ” below)
3	Make a water isopotential map with respect to oil or gas, using the formula for V, ${\mbox{V}}=\left({\frac {\rho _{\rm {w}}}{\rho _{\rm {w}}-\rho _{\rm {hc}}}}\right){\mbox{H}}_{\rm {w}}$ <break> </break> Note: Use gas density where gas is expected and oil density where oil is expected.
4	Make a gas or oil isopotential contour map using the formula for U where U = V – Z. The values for Z, the subsea elevation of the target aquifer, can be determined by simply overlaying a structure contour map on the water isopotential map and subtracting (see “Example” below).
5	Locate possible accumulations of hydrocarbons in hydrodynamic traps by locating the areas of low gas or oil potential. Locations should make sense in context with all other geologic data.

Calculating h_w

A critical step in mapping hydrodynamic traps in a target formation is making a water isopotential map (step 3 above). Before that can be done, however, one must calculate a potentiometric value for each well.

Calculate H_w for each well using the formula

${\mbox{H}}_{\rm {w}}={\mbox{Z}}+{\frac {\mbox{P}}{\mbox{grad P}}}$

where:

• Z = structural elevation of target formation
• P = measured fluid pressure in the formation
• grad P = static formation water pressure gradient

For grad P, use 0.435 psi/ft for areas where regional water is probably fresh and 0.465 psi/ft for areas where regional water is probably highly saline.

Example: u₀ contours

Figure 1 Map showing oil in a hydrodynamic trap (shaded area). Copyright: Dahlberg; courtesy Springer-Verlag.

The map in Figure 1 shows oil in a hydrodynamic trap (shaded area). No oil could be trapped in the structure, represented by solid contours, without the aid of hydrodynamics. The bold contours with long dashes represent the isopotential of oil (labeled U₀). To make U_o contours, follow this procedure.

Step	Action
1	Subtract the structure of the target formation (Z) from the isopotential of water (Vo ).
2	Plot the remainder at the intersection of the two sets of contours.
3	Draw contours of the values.

See also

• Hydrodynamics
• Hydrodynamic influence on trapping

External links

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