Subsidence at Wairakei field, New Zealand

This paper reports that the maximum amount of subsidence at Wairakei Field due to fluid withdrawal reached 11.6 m during 1989. The maximum subsidence rate has decreased from 450 mm/y during the 1970s, to 350 mm/y during the late 1980s. Although the subsidence has caused a pond to form along a 1 km length of the Wairakei Stream passing through the center of subsidence, there has been remarkably little damage to structures around the borefield due to associated horizontal strains. Modelling of the subsidence suggests it originates from a compaction zone of about 150 m depth. This coincides with a pumice breccia unit sandwiched between lacustrine mudstone units. The rate of compaction appears to be controlled by the rate of steam pressure decline near the top of the reservoir. Modelled compressibilities of 10 kbar-1 are consistent with measurement on pumice breccias. Hydrothermal alteration near the original outflow zone of the field may have contributed to the location of the high compressibility zone