Quantitative flow modeling of fluids at elevated temperatures and pressures has generally been limited to consideration of either single-phase flow or two-phase flow at conditions below the critical point of water. In this paper, we introduce a version of the GEOTHER model that can simulate both multiphase flow and flow above the critical point, and demonstrate its capabilities by simulating flow in the vicinity of the critical point. GEOTHER2 is a multiphase, finite-difference model that simulates threedimensional flow of pure water and heat at temperatures ranging from 0" to 1,200"C and pressures ranging from 0.5 to 10,000 bars. The governing equations are expressions of mass and energy conservation that are posed in terms of pressure and enthalpy. A series of one-dimensional experiments indicates that permeability is a pivotal factor in determining pressure-enthalpy/temperature trajectories near the critical point. At low permeabilities (510-lg m2), heat transport by conduction dominates, and the trajectoq defines a constant thermal gradient. At higher permeabilities (210-l6 m2), advective heat transport dominates, and the pressure-enthalpy trajectory maintains a constant "flowing enthalpy".

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