Hydrochemistry and energy storage in aquifers - NHV.nu

The heterogeneous reactions of Si02 generally involve an activated complex; the dissolution
reactions are zero-order whereas precipitation is first-order with regard to dissolved silica
(Rimstidt and Barnes, 1980; Bird et al., 1986) (see Equations 4 - 7).
where the * indicates the activated complex.
The slower rate of precipitation as opposed to dissolution has led to the widespread use of
dissolved Si as a geothermometer (Fournier and Rowe, 1966; Arnorsson, 1975) of water
temperatures at depth in geothermal exploration. This reaction rate differential minimizes
Si02 scaling.
3.4 Temperature dependent modelling
Scaling is to be avoided in ATES systems because it results in increased maintenance costs
and decreased efficiency. Similarly, unnecessary capital costs for treatment should be
avoided. The avoidance of scaling requires that saturation indices for pertinent minerals be
calculated with a significantly greater reliability than is normally required by geochemists
and environmental chemists for interpretative studies. Therefore, a major aspect of the U.S.
contribution to Annex VI of the International Energy Agency collaborative research has
been focused on this problem. Solid solution modelling of carbonates and ATESgroundwater
mixing models have already been mentioned. The initial effort was to improve
the temperature competence of our geochemical modelling (Smith, 1988). Calculation of
saturation indices can be no more reliable than the aqueous speciation calculations on which
they are based. However, thermodynamic data are conventionally determined at 25°C and
require adjustment for use at higher temperatures. These adjustments can be estimated with
the Van 't Hoff relationship but this estimation procedure often leads to significant error at