Subsurface reactive transport modeling (RTM) is an effective approach for studying the fundamental of earth science, widely applied in geo-resources development and contamination remediation, etc. In the traditional RTM, the temperature upper limit is about 300oC, which is not applicable for supercritical water conditions ( greater than 374oC, 22.1MPa). Because of lack of the thermodynamic database for high-temperature, the geochemical reactions are constrained in the aqueous phase in traditional RTM. Whereas, the chemical species can also be dissolved in the steam phase, particularly under supercritical conditions. In this study, we improved the existing reactive transport model, expanding its applicable scope towards supercritical geothermal conditions, based on the framework of TOUGHREACT. The IAPWS-IF97 formulation is implemented for calculating the properties of water in different phases. The database of Soltherm.xpt is introduced to extend the temperature upper limit, with a quaternary interpolation to consider the variation of the equilibrium constant (logK) with P and T conditions. A logK weighting method is proposed to describe the mineral dissolution/precipitation and geochemical reactions in both the steam and aqueous phase. Then, we applied the improved model to the Iceland IDDP field, to study the quartz dissolution/precipitation pattern during the formation of the supercritical geothermal reservoir, with the logK from Manning’s formula. The results show that, since the intrusion emplacement, the quartz starts to dissolve along the edge of the magma body (300~400oC isotherm). In the action of upward heat flux, the high-Si concentration area distributes more extensively above the intrusion.

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