Difference between revisions of "Hydrostatic pressure-depth plot construction"

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| Calculate and plot hydrostatic pressures for a depth above and a depth below the zone of interest. Use the rule-of-thumb pressure gradient for that zone. If it is not available, calculate the gradient from the water density using [[Formation water density|density measured from the formation water]] or calculated from R<sub>w</sub> . For help, see the following sections.
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| Calculate and plot hydrostatic pressures for a depth above and a depth below the zone of interest. Use the rule-of-thumb pressure gradient for that zone. If it is not available, calculate the gradient from the water density using [[Formation water density|density measured from the formation water]] or calculated from the formation water resistivity {R<sub>w</sub>} . For help, see the following sections.
 
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Revision as of 21:08, 14 July 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 goal of constructing a hydrostatic pressure–depth plot is to identify pressures greater than the hydrostatic gradient that may correspond to a hydrocarbon-bearing zone. A hydrostatic pressure–depth plot can be constructed from any of the following:

  • Measured pressures
  • Regional rule-of-thumb pressure gradients
  • Pressures calculated from water density

Calculated pressures are much less accurate than measured pressures but can be used with some effectiveness when they are supplemented with other petrophysical data.

Procedure: constructing a plot

The table below outlines a procedure for constructing a hydrostatic pressure-depth plot for a single well.

Step Action
1 Using graph paper with a linear grid, label the X-axis as pressure and the Y-axis as depth. Use as large a scale as possible. Also, make the Y-axis on at least one plot the same scale as on the well logs to aid in interpretation.
2 Plot measured pressure from the aquifers (100% water saturation [Sw] ) in the well. If none is available, go to step 3.
3 Plot measured pressure from the aquifers in nearby wells. If none is available, go to step 4.
4 Calculate and plot hydrostatic pressures for a depth above and a depth below the zone of interest. Use the rule-of-thumb pressure gradient for that zone. If it is not available, calculate the gradient from the water density using density measured from the formation water or calculated from the formation water resistivity {Rw} . For help, see the following sections.

Calculating pressure gradients from water density

The formation fluid pressure at any depth in a well is a function of the average formation water density (ave. ρw) above that depth, not the density of the formation water at any particular depth. Formation water generally becomes more dense with increasing depth.

To calculate water pressure gradient (Pgrad), use the following formula:

where:

  • ρw = water density

For example, given ave. ρ = 1.13, the equation works as follows:

Table of water pressure gradients

The table below lists hydrostatic pressure gradients, water density, and salinity in weight percent total dissolved solids (TDS).

Gradient (psi/ft) Density (g/cc) TDS (ppm) TDS (wt %)
04.33 1.000 0
0.437 1.010 13,500 13.5
0.441 1.020 27,500 27.5
0.444 1.029 37,000 37.0
0.445 1.030 41,400 41.4
0.451 1.040 55,400 55.4
0.454 1.050 69,400 69.4
0.459 1.060 83,700 83.7
0.463 1.070 98,400 98.4
0.465 1.075 100,000 100.0
0.467 1.080 113,200 113.2
0.471 1.090 128,300 128.3
0.476 1.100 143,500 143.5
0.480 1.110 159,500 159.5
0.485 1.120 175,800 175.8
0.489 1.130 192,400 192.4
0.491 1.135 200,000 200.0
0.493 1.137 210,000 210.0
0.500 1.153 230,000 230.0
0.510 1.176 260,000 260.0

Rules of thumb

Most sedimentary basins have a rule of thumb for average hydrostatic water pressure gradients. For the Gulf Coast basin, it is 0.465 psi/ft. For Rocky Mountain basins, it is 0.45 psi/ft. For fresh water, it is 0.433 psi/ft. If measured hydrostatic pressure is not available for a well, find out the accepted rule-of-thumb average hydrostatic pressure gradient for the depth of the zone of interest where the well is located.

Example of total dissolved solids (TDS) vs. depth

Figure 1 TDS vs. depth plot from southern Arkansas. Copyright: Dickey,[1] courtesy Chemical Geology.

Water density is a function of its TDS concentration. The hydrostatic pressure at any depth is a function of TDS concentration from the surface to that point. The plot in Figure 1 of TDS vs. depth is from southern Arkansas. It shows a gradual increase in TDS from the surface to about depth::2000 ft, probably due to meteoric effects, and then a linear, more rapid increase in TDS from 2000 to depth::10,000 ft. Generally, below the depth of meteoric water influence, the increase in TDS in connate brines is linear and ranges from 25,000 to 100,000 mg/1 per depth::1000 ft (80 to 300 mg/1 per m).[1] There are exceptions to this general case.

Such consistent salinity increase with depth is not unique to the East Texas basin but is characteristic of most basins.

See also

References

  1. 1.0 1.1 Dickey, P., A., 1969, Increasing concentration of subsurface brines with depth: Chemical Geology, vol. 4, p. 361–370., 10., 1016/0009-2541(69)90055-2

External links

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Hydrostatic pressure-depth plot construction
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