Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Formation fluid pressure and its application

  | chapter = Formation fluid pressure and its application

  | frompg  = 5-1

  | frompg  = 5-12

  | topg    = 5-64

  | topg    = 5-15

  | author  = Edward A. Beaumont, Forrest Fiedler

  | author  = Edward A. Beaumont, Forrest Fiedler

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

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:

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:

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

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

# 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.

# Plot measured pressure from the [[aquifer]]s (100% water saturation [S_w] ) in the well. If none is available, go to step 3.

# Plot measured pressure from the aquifers in nearby wells. If none is available, go to step 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 [[Formation water density|density measured from the formation water]] or calculated from the [[Spontaneous potential logs in water resistivity calculation|water resistivity (R_w)]] . For help, see the following sections.

| 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.

| Plot measured pressure from the aquifers (100% S_w ) in the well. If none is available, go to step 3.

| Plot measured pressure from the aquifers in nearby wells. If none is available, go to step 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 R_w . For help, see the following sections.

==Calculating pressure gradients from water density==

==Calculating pressure gradients from water density==

==Rules of thumb==

==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.

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==

==Example of total dissolved solids (TDS) vs. depth==

[[file:formation-fluid-pressure-and-its-application_fig5-4.png|thumb|300px|{{figure number|1}}TDS vs. depth plot from southern Arkansas. Copyright: Dickey,<ref name=ch05r6 /> courtesy Chemical Geology.]]

[[file:formation-fluid-pressure-and-its-application_fig5-4.png|thumb|300px|{{figure number|1}}TDS vs. depth plot from southern Arkansas. Copyright: Dickey,<ref name=ch05r6 /> 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 [[:file:formation-fluid-pressure-and-its-application_fig5-4.png|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).<ref name=ch05r6>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</ref> There are exceptions to this general case.

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 [[:file:formation-fluid-pressure-and-its-application_fig5-4.png|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 brine]]s is linear and ranges from 25,000 to 100,000 mg/1 per [[depth::1000 ft]] (80 to 300 mg/1 per m).<ref name=ch05r6>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</ref> 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.

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

==See also==

==See also==

* [[Determining hydrostatic pressure gradient]]

* [[Hydrostatic pressure gradient]]

* [[Estimating formation water density]]

* [[Formation water density]]

==References==

==References==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Formation fluid pressure and its application]]

[[Category:Formation fluid pressure and its application]]