1,705
edits
Changes
Jump to navigation
Jump to search
Geostatic and lithostatic pressure (view source)
Revision as of 21:14, 29 January 2014
, 21:14, 29 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==Definitions==
The geostatic pressure at a given depth is the vertical pressure due to the weight of a column of rock and the fluids contained in the rock above that depth. Lithostatic pressure is the vertical pressure due to the weight of the rock only.
==Geostatic variables==
Three variables determine geostatic pressure:
* Densities of formation waters as related to salinities
* Net thickness of different lithologies, e.g., sandstone, shale, limestone
* Porosities of different lithologies
==Calculating geostatic pressure==
We can calculate geostatic pressure using the formula below:
:<math>\mbox{P}_{\rm G} = [\rho_{\rm m} \times (1 - \phi) \times \mbox{d}] + [\rho_{\rm w} \times \phi \times \mbox{d}]</math>
where:
* P<sub>G</sub> = geostatic pressure (psi)
* ρ<sub>m</sub> = weighted average of grain (mineral) density (sandstone and shale = 2.65 g/cm<sup>3</sup>, limestone = 2.71 g/cm<sup>3</sup>)
* ρ<sub>w</sub> = weighted average of pore-water density (g/cm<sup>3</sup>)
* φ = weighted average of rock [[porosity]]
* d = depth (ft)
To calculate weighted averages, use 1000-ft (300-m) increments.
==Geostatic gradient==
Geostatic gradient is the rate of change of geostatic pressure with depth. A geostatic gradient of 1 psi/ft results from an average density of 2.3 g/cm<sup>3</sup>.
==How geostatic gradient varies==
How geostatic Geostatic gradients vary with depth and location. The gradient increases with depth for gradient varies two reasons:
* Rock bulk density increases with increasing compaction.
* Formation water density increases because the amount of total dissolved solids (TDS) in the water increases with depth.
For example, in the Cenozoic of Louisiana, the geostatic gradient is 0.85 psi/ft at [[depth::1000 ft]] and 0.95 psi/ft at [[depth::14,000 ft]].
==See also==
* [[Pressure regimes]]
* [[Normal hydrostatic pressure]]
* [[Normal hydrostatic pressure gradients]]
* [[Abnormal hydrostatic pressure]]
* [[Causes of overpressure]]
* [[Causes of underpressure]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==Definitions==
The geostatic pressure at a given depth is the vertical pressure due to the weight of a column of rock and the fluids contained in the rock above that depth. Lithostatic pressure is the vertical pressure due to the weight of the rock only.
==Geostatic variables==
Three variables determine geostatic pressure:
* Densities of formation waters as related to salinities
* Net thickness of different lithologies, e.g., sandstone, shale, limestone
* Porosities of different lithologies
==Calculating geostatic pressure==
We can calculate geostatic pressure using the formula below:
:<math>\mbox{P}_{\rm G} = [\rho_{\rm m} \times (1 - \phi) \times \mbox{d}] + [\rho_{\rm w} \times \phi \times \mbox{d}]</math>
where:
* P<sub>G</sub> = geostatic pressure (psi)
* ρ<sub>m</sub> = weighted average of grain (mineral) density (sandstone and shale = 2.65 g/cm<sup>3</sup>, limestone = 2.71 g/cm<sup>3</sup>)
* ρ<sub>w</sub> = weighted average of pore-water density (g/cm<sup>3</sup>)
* φ = weighted average of rock [[porosity]]
* d = depth (ft)
To calculate weighted averages, use 1000-ft (300-m) increments.
==Geostatic gradient==
Geostatic gradient is the rate of change of geostatic pressure with depth. A geostatic gradient of 1 psi/ft results from an average density of 2.3 g/cm<sup>3</sup>.
==How geostatic gradient varies==
How geostatic Geostatic gradients vary with depth and location. The gradient increases with depth for gradient varies two reasons:
* Rock bulk density increases with increasing compaction.
* Formation water density increases because the amount of total dissolved solids (TDS) in the water increases with depth.
For example, in the Cenozoic of Louisiana, the geostatic gradient is 0.85 psi/ft at [[depth::1000 ft]] and 0.95 psi/ft at [[depth::14,000 ft]].
==See also==
* [[Pressure regimes]]
* [[Normal hydrostatic pressure]]
* [[Normal hydrostatic pressure gradients]]
* [[Abnormal hydrostatic pressure]]
* [[Causes of overpressure]]
* [[Causes of underpressure]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]