In the geothermal energy community interest is increasingly focused on predicting how geochemistry of the rock reservoir, the chemistry of geofluid and the fluid-rock interactions affect the heat production and the longevity of the reservoir. Modeling and computer simulations are the essential tools that can be used to gain a greater understanding of geochemical processes to interpret laboratory- and field-scale experiments and to predict long term flow and transport of heat in the reservoir. Despite the urgent needs, geochemical modeling still remains a daunting task for several scientists since it calls for two, often conflicting fields: geochemistry and mathematical modeling. In this paper we will introduce and review existing geochemical models. We draw their advantages and inconveniences. We emphasize several codes that were used in national and international code comparisons. We will also address the issue of limited knowledge of the geochemical speciation and mineralogy of the hosting rock, the uncertainty in the site characterization and how these two lead to what is called either inversion or uncertainty quantification exercise. We will introduce several methods of quantification of model uncertainty and model averaging techniques often used in hydrology or climate prediction programs. Illustrations will cover calcite systems. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

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