Initial state and response to exploitation of Wairakei geothermal field

Abstract

All Wairakei well measurements have been reviewed and re-interpreted. In its natural state the field contained a liquid-dominated two-phase zone over a deeper liquid reservoir. The initial pressure distribution was about 10% above hydrostatic, the excess gradient driving the natural discharge. With exploitation and falling pressures, the two-phase zone expanded downwards, and within it a vapor-dominated zone developed. The changes with exploitation can be characterized by two pressures: a deep liquid pressure, reflecting mainly the reservoir mass balance, and a pressure in the stem zone, reflecting the reservoir heat balance. The heat balance indicates that most, but not all, of the recharge is hot water. Production from wells is currently declining due to falling temperatures and pressures in both zones of the field. The liquid-dominated zone pressure can be raised by reinjection. Deep peripheral injection is estimated to have a payback time of about two years.