✍ Scribed by Rick G. Allis; Xiaoyong Zhan
No coin nor oath required. For personal study only.
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 (H0.05±0.3 mD), compressible (15±45 kbar À1 ) 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 dierence 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.
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Chemical data obtained in 1983/84 and 1994 from groundwater monitor wells at Wairakei are reviewed and interpreted in conjunction with physical changes in the deeper system. The composition of the waters is controlled by the proportion of deep chloride, shallow meteoric and steamheated components. T