Silica has been using as one of geothermometer in the geothermal exploration. Its solubility is very sensitive due to temperature changes. When fluids rise and appear as thermal manifestation, the temperature tend to decrease. This occurs due to mixing with surface water. This mixing process use as basis to build silica-enthalpy mixing model This paper discussed condition and assumption of fluids that can be using in the mixing model. Basic assumptions that in high-temperature water, solubility of silica controlled by its quartz species, no silica deposition occurred before or after mixing, no conductive cooling occurred after mixing, and any steam that formed adiabatically did not separate from residual liquid water before mixing with the cold-water. In its application, this model be able to characterize reservoir condition validated by using measured silica and enthalpy from well. Chemically, the validation also sort water type of thermal manifestation that can be used are chloride and bicarbonate type. Chloride type represents reservoir fluids, whilst bicarbonate type point out the fluids which affected by surface water. Meantime, sulfate type cannot be used because of the tendency of formation is due to the condensation of geothermal gasses. Due to its solubility, silica mixing model is well suited to describe their process in water domination geothermal system. Interestingly, the model is also very effective to identify steam entry in geothermal well because the prediction of enthalpy from this model covers the saturation enthalpy from water. The higher measured enthalpy than the maximum enthalpy prediction indicated excess enthalpy which beneficial for power generation.

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