Predicting subsidence at Wairakei and Ohaaki geothermal fields, New Zealand

A ®nite-element model coupling compaction and ¯uid ¯ow processes in porous materials has been applied to the subsidence bowls at Wairakei and Ohaaki geothermal ®elds to provide a basis for predicting subsidence. Most of the subsidence is due to slow drainage of relatively impermeable (00.05±0.3 mD), compressible (15±45 kbar ÿ1) mudstone at less than 300 m depth. Maximum subsidence rates at both ®elds peaked at close to 500 mm/year, before declining to between 200 and 300 mm/year today. However, it took over 20 years for maximum subsidence rates to start to decrease at Wairakei, compared to 8 years at Ohaaki. This di€erence is due to the relatively rapid stabilisation of pressure beneath the compacting mudstone at Ohaaki compared to that at Wairakei. Predictions of future subsidence at both ®elds are made assuming that the pressure beneath the mudstone remains constant. At Wairakei, the present total maximum subsidence of 14 m is predicted to increase to 2022 m by the year 2050. At Ohaaki, the short history of subsidence makes predictions less certain, and the present maximum subsidence of 2.5 m is predicted to be 3± 4 m by the year 2006. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.