Predicting Future Subsidence at Wairakei Field, New Zealand

Abstract

A releveling survey of Wairakei field duringmid 1997 has confirmed that the magnitude of subsidencerates in the main subsidence bowl continues to reduce. Peak subsidence rates are now close to 200 and are less than half the peak rates which were during the mid 1970s. The maximum total subsidence is now 14m. The greatest reduction in rates has around the eastern margin of the bowl where it coincides with the field boundary. This is attributed to lateral inflow of cooler water at shallow depth maintaining or increasing pressure near the compaction zone. Near the power station, shallow injection since the early 1990s appears to have resulted in a reduction in the subsidence rates in a localized area. This contrasts with the western half of the field where ongoing steam pressure declines have caused subsidence rates to increase since the mid 1970s. 1-Dcoupled fluid flow deformation modeling of the drainage process indicates that subsidence could continue for another years, but with subsidence rates continuing to reduce. The total maximum subsidence is predicted to be in the range 20 2 m assuming the present pattern of borefield management continues.