In hydrothermal systems, transport phenomena are complicated by coupled fluid, energy and chemical transport processes. Substantial quantities of energy and dissolved chemical species are carried by and also interact with the flow field; the potential field of energy and chemical species can also influence the flow field. processes often exhibit mn-linear behavior, thus requiring numerical methods for solution. Many problems arise during the exploitation of geothermal systems that involve complicated transport processes. These coupled One example is quartz precipitation near wells at the Cerro Prieto geothermal field, Mexico, caused by localized aquifer boiling (Truesdell et al., 1984). Boiling causes a gradual temperature decline and a consequent decrease in quartz solubility. Also separation o f stem from the produced fluids can increase quartz concentrations in the residual water. Under such circunstances, quartz w i l l precipit a t e after the concentration reaches a high degree of supersaturation, which results in reduction o f permeability and mass flow rate. Another example is s i l i c a scaling during reinjection operations. Reinjection is often employed in order t o prevent chemical contamination of the envirornnent by surface disposal and to enhance the energy recovery from the system. However, the spent brine often gets cooled below the saturation temperature so that deposition occurs in the surface pipelines, the disposal well or within the reservoir rock i t s e l f . around the wellbore, permeabilities w i l l be reduced and the injectivity of the well w i l l decrease.

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