Alteration zonation of silica minerals in a geoghermal system - a numerical simulation based on reaction-transport model

N. TAKENO
Geological Survey of Japan
T. ISHIDO
Geological Survey of Japan
J.W. PRITCHETT
Maxwell Technologies, Inc., U.S.A.

SUMMARY - Dissolution and precipitation of silica polymorphs (amorphous silica, cristobalite and quartz) were incorporated into a geothermal reservoir simulator based on reaction kinetics. Reaction kinetics are simplified as follows; silica minerals are always precipitated from or dissolved into the solution without a direct solid phase transition among them. Only inorganic processes are considered and the effect of salinity is neglected. Modeling of reaction surface area was devised for both precipitation and dissolution of all silica minerals. 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 minerals was reproduced in the simulations as observed in natural geothermal systems. Sensitivity of silica alteration zoning to formation permeability and reaction surface area was also examined. The present model is preliminary, but the results show that numerical simulations incorporating chemical processes are promising for geothermal modeling, especially for reducing the non-uniqueness of the models.