Usable porosity

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The portion of the total porosity of a reservoir (e.g. an aquifer ) that can be used for storage (or also has a storage effect) pours out the stored fluid when the pressure is reduced under gravitational forces is referred to as usable porosity , usable pore proportion or void proportion usable for storage :

With

  • the pore volume that is emptied (or refilled) when the pressure changes
  • the total rock volume considered .

Hydrogeologists also quantify this pressure change with the derived height reduction of the groundwater surface .

The usable porosity is counteracted by capillary forces in the porous medium, the proportion of so-called retained water (typically ), also defined as the pore proportion, remains filled with water (see field capacity in soil science ). This is therefore initially not usable as a reservoir in the sense of exploration .

The usable porosity is generally not identical to the open, flow-effective or effective porosity , which for stimulation of hydrocarbon reservoirs z. B. is increased by the injection of acids in order to increase the permeability of the then carbonatically cemented formation .

Basically, the term "usable porosity" is primarily used for free aquifers, because here the elastic properties of the aquifer and the compressibility of the water (these variables determine the storage properties of a confined aquifer, see storage coefficient ) are mostly negligible and the usable porosity of the water-saturated soil in corresponds to a good approximation of the effective porosity.

Total porosities and usable porosities of the grain sizes of sediments
Grain size Total porosity in% usable porosity in%
volume over 50 under 5
Silt 50 to 45 5 to 15
sand 45 to 40 15 to 35
gravel 40 to 30 20 to 35
Stones under 30 under 20

Despite their very high absolute porosity, clays have a very low usable porosity (approx. 2%), because the capillary forces do not let the pore water out of the matrix compound due to the small grain size of the clay minerals . In the case of gravel and gravel, on the other hand, there are hardly any capillary forces; here the usable porosity is identical to the effective porosity (20 to 35%).

Individual evidence

  1. ^ Bernward Hölting, Wilhelm Georg Coldewey: Hydrogeology . Introduction to General and Applied Hydrogeology. 8th edition. Springer-Verlag, Berlin / Heidelberg 2013, ISBN 978-3-8274-2353-5 , pp. 15 , doi : 10.1007 / 978-3-8274-2354-2 .
  2. ^ Bernward Hölting, Wilhelm Georg Coldewey: Hydrogeology . Introduction to General and Applied Hydrogeology. 8th edition. Springer-Verlag, Berlin / Heidelberg 2013, ISBN 978-3-8274-2353-5 , pp. 19 , doi : 10.1007 / 978-3-8274-2354-2 .
  3. Working aid for the remediation of groundwater pollution (PDF; 640 kB).

Web links

literature

  • Fetter, CW (1994): Applied Hydrogeology. Macmillan, New York
  • Castany, G. (1982): Principes et Méthodes de l'Hydrogéologie, Dunod, Paris