Numerical Simulation of Development of Silica Alteration Zoning in Geothermal System. Preliminary Study Based on Kinetic Reaction-Transport Model.

Dissolution and precipitation of silica polymorphs (amorphous silica, cristobalite and quartz) are coupled with geothermal reservoir simulator (simulator for hydrodynamic fluid flow in porous media) based on reaction kinetics. Reaction kinetics are simplified as follows; silica polymorphs are always precipitated from or dissolved into the solution without direct solid phase transition among them. Only inorganic processes are considered and the effect of salinity is neglected. Initial reaction surface area is assumed to be unique for both precipitation and dissolution of all polymorphs. Based on these assumptions and published experimental data of rate constants and solubilities, two dimensional numerical simulations of geothermal system development were performed. Alteration zoning of silica polymorphs was reproduced in the calculations as observed in natural geothermal systems. Sensitivity of silica alteration zoning to formation permeability and reaction surface area was also studied. The present model is preliminary, but the results show that numerical simulations incorporating chemical processes are promissing for geothermal modeling, especially for reducing the non-uniqueness of the models.