| Abstract |
The 2001 relevelling of benchmarks across the Wairakei - Tauhara system smoothly extends the subsidence trends established by earlier surveys. In general, subsidence rates since the early 1990s have been steadily decreasing or have been constant. At Wairakei Field, subsidence rates in the main subsidence area are now less than half the maximum rate. In the centre of the area, the subsidence rate is now one quarter the peak rate that occurred during the 1970s. In the Tauhara subsidence area, the trends suggest that maximum subsidence rates may have occurred during the 1990s and rates may now also be starting to decrease. On the western side of the Tauhara subsidence area the subsidence rate has been decreasing since the late 1970s. The only area with a systematic increase in subsidence rates during the 1990s is over the steam zones in the west of Wairakei Field. The maximum increase in subsidence rate here is from about 14 to 27 mm/y . Over a large part of the Wairakei-Tauhara system, there is a pattern of subsidence that was established between 40 and 50 years ago as a result of production of fluid for the Wairakei power station. There is no evidence that the main subsidence areas at Wairakei and Tauhara Fields have shifted with time. The long subsidence history, together with the relatively small changes in shallow reservoir pressure expected from future production, provide a sound basis for predicting subsidence for the next 25 years. Modelling of future subsidence at both fields suggests that for the next 25 years, subsidence rates will not change greatly from those occurring in the late 1990s. A map of predicted subsidence between 2001 and 2026 has been prepared and is included as part of this paper. |