Optimized Numerical Modeling of Production from the Poihipi Dry Steam Zone: Wairakei Geothermal System

Abstract

The Poihipi power station utilizes dry steam from a shallow zone near the margin of the Wairakei geothermal reservoir. The station has an installed capacity of 55 MWe, however resource consents limit the average daily production to approximately 25 MWe. The plant has been operated for several years with daily variations in the production rate. The corresponding pressure history provides a good database for testing models of the reservoir.

In the first stage of modeling, a model for each well was set up, assuming that the reservoir is a single layer and that the flow near a well is approximately radially symmetric. Three different types of model (uniform porous medium, fractured medium and fractional dimension) were calibrated using data from a three day period. The best match to the pressure data was achieved with the fractional dimension model with a dimension of approximately 2.5 (i.e. midway between 2D and 3D flow).

Then the fractional dimension models were embedded into a 2D reservoir model including all the wells and some of the surrounding area. The new composite model was calibrated with production history for a seventy-six day period. This composite small-scale reservoir model should be useful for making medium term predictions.