Work over the past year has concentrated on three areas: 1) to implement a concept of vertical equilibrium in geothermal modeling, 2) to improve the matrix equation solution technique for both two- and threedimensional models, and 3) to apply a vertical equilibrium, areal model to the Wairakei, New Zealand geothermal field.

At the last Stanford meeting, a concept of vertical equilibrium as applied in the petroleum industry was outlined ( for example, Coats and others, 1967). That is, vertically averaged liquid saturations are related to pressure at some reference level by employing pseudo capillary pressure and pseudo relative permeability curves. For the geothermal problem, many thermodynamic properties are strongly dependent functions of pressure and enthalpy, and an analogous approach would require many pseudo functions. Instead, the concept of vertical equilibrium is used to vertically integrate the thermodynamic properties and relate them to vertically averaged pressure and enthalpy. This results in a quasi three-dimensional model that allows a finite - difference block to become two-phase as soon as the pressure at the top of the block drops below the saturation pressure. The normal procedure for determining thermodynamic properties on the basis of pressures and enthalpies at specified reference levels in the grid block ( usually the center) can lead to significant errors for thick blocks. The implementation of this vertically averaging approach has been verified by comparing a vertical equilibrium, areal model with a three-dimensional model.

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