Gravity Changes at Tauhara Geothermal Field

Abstract

Microgravity measurements in the northern part of the Tauhara field (Wairakei-Tauhara geothermal system) began in 1972, and repeat surveys over the whole of the Tauhara field were made in 1985, 1996 and 2002. In the northern part of the field there were large gravity decreases prior to 1985, associated with pressure drawdown from production at Wairakei. Since then there have been gravity increases here of up to +175 pgal,, corresponding to a mass increase of about 20 Mt, centred near deep well TH4. It is suggested these gravity changes result fiom a downflow of water in the well (via a known casing break at 24 mRL, 393 m depth) from a confined groundwater aquifer, which has resulted in resaturation of steam and/or 2-phase conditions in the upper part of the geothermal reservoir. Gravity changes in the central part of the field since 1985 have been less than -70 pgal indicating little net mass loss (a Mt), and hence little effect in this area of production from Wairakei.