The Te Mihi Steam Reservoir, Wairakei. An Exploitable Geothermal Resource

Abstract

Recent drilling in the Te Mihi Field - the western extension of Wairakei - has confirmed the presence of a 300-400 m thick steam reservoir, produced by falling aquifer pressures and trapped below an impermeable cap - the Huka Falls Formation - of lacustrine siltstone and hydrothermal eruption conglomerates. The presence of a shallow vapour-dominated reservoir was initially inferred from precise gravity changes and from direct measurement of steam pressures in the Te:Mihi 200 series wells. Investigation of the boundaries of the reservoir by shallow (600 m) wells is proceeding and wiII be followed by production drilling along known active faults. One 400 m well (WK228) already drilled has an output of 90 t/hr of dry steam (11 MWe), a substantial and cheap energy supplier to the Power Station. The strategy for Wairakei development over the next decade is likely to be changed i f the Te Mihi Steam reservoir can be successfully exploited. The cost advantage of cheap steam in the west needs to be balanced against the additional expense of extending the steam pipelines a further 1-2 km. However. the provision of extra steam supplies should allow the closing down of the Eastern Borefield - already threatened by cold water invasion - and the utilisation of existing shallow wells for reinjection. This should maintain reservoir pressures and temperatures, control ground subsidence and eliminate wasteful discharge of effluent to the Waikato River. In the long term, deep drilling in the postulated upflow regions at Wairakei, such as Te Mihi, will be necessary to maintain steam supplies to the station.