Predicting subsidence at Wairakei and Ohaaki geothermal fields, New Zealand

Abstract

A finite-element model coupling compaction and fluid flow processes in porous materials has been applied to the subsidence bowls at Wairakei and Ohaaki geothermal fields to provide a basis for predicting subsidence. Most of the subsidence is due to slow drainage of relatively impermeable (∼0.05–0.3 mD), compressible (15–45 kbar −1) mudstone at less than 300 m depth. Maximum subsidence rates at both fields 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 difference 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 fields 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 20 ± 2 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.