Injection water chemistry can have a tremendous impact on the success or failure of an injection operation. The chemical interactions between host rocks and fluids, by causing the dissolution and precipitation of minerals, can either enhance or destroy permeability and porosity in the area of interest. Reactive chemical transport simulation of injection should be a useful tool aiding in decisions relating to reservoir management. To assess these interactions and their potential effects on the Coso Enhanced Geothermal Systems (EGS) experiment, modeling efforts using the program TOUGHREACT have been initiated. TOUGHREACT is a numerical simulation program for chemically reactive non-isothermal flows of multiphase fluids in porous and fractured media. The program is written in Fortran 77 and was developed by introducing reactive chemistry into the multiphase flow code TOUGH2 V2. Here we apply TOUGHREACT to one-dimensional modeling of injection into well 34-9RD2. The techniques used can be applied to any injection well of interest. Host rock and fracture mineralogies were taken directly from petrographic studies. Sensitivity studies were performed to examine the effects of changes in pH, temperature, and composition of injection fluid. Both the model and field experience suggest that amorphous silica precipitation is an important factor in the performance of a stimulation experiment. Several methods taken from the literature that attempt to quantify the rates of silica precipitation are compared to those used by TOUGHREACT.

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