| Abstract |
Wairakei field has passed 40 years of production with its most recent year generating more electricity than any previous year. Pressure drawdown of up to 25 bars affected production during the first decade, and lateral inflow of cool water down the original outflow zone of the reservoir became evident in part of the borefield during the subsequent two decades. This has been largely overcome by cementing up the wells with shallow downflows of cool water. Temperature and pressure are now almost stable in the western production borefield, although a gradual chloride dilution trend is still present. Production for the Eastern Borefield has been replaced by production from Te Mihi in the west of Wairakei field. Here temperatures are close to the original undisturbed temperatures of 255 ñ 260?C at 1 km depth. Since the late 1980s, replacement production has been achieved by tapping the high pressure steam zone at 300 ñ 500 m depth in Te Mihi. In the long term, deeper production from this area will sustain the power plant. Injection rates are presently over 40% of the separated water, and this will increase to 50% over the next few years. Most injection will occur near the eastern boundary of the field. Reservoir models confirm that production and injection is sustainable to beyond the year 2050, although some cooling of the reservoir is anticipated. |