Managing decline: Optimising generation by prediction of two-phase well productivities

Economic optimisation of the Ohaaki Geothermal Field dual-flash system indicated the requirement to program for sliding High Pressure turbine inlet pressures and the de-rating of individual wells to Intermediate Pressure. A wellbore simulator was used to generate output curves up to 5 years into the future to enable 'what-if' modelling for maximum electrical generation under different scenarios. The key to predicting future output curves as a function of wellhead pressure was predicting two-phase well productivities as a function of field pressure and enthalpy trends. Using a wellbore simulator to generate inflow pressure curves from output test data and matching measured downhole data showed that the Duns and Ros flow correlation produced a linear response with a consistent relationship to static pressures for most wells. This was used to generate predicted output characteristic curves up to 1998, enabling the modelling of varying turbine inlet pressures