| Abstract |
After 50 years of production, Wairakei power plant is still producing at capacity, and there are plans for a significant expansion of generation in the Wairakei-Tauhara geothermal system. During the 1970’s there was a high rate of subsidence (up to 450 mm/y) in a 1 km2 area adjacent to the original Geyser Valley. However, the subsidence rate here has now decreased to less than 60 mm/y, and is not expected to increase with future production. A maximum of 15 ± 0.5 m of subsidence occurred, and depending on the production-injection scenario, future subsidence is predicted to be a maximum of 2 – 4 m. Details of the subsidence history of the Wairakei-Tauhara geothermal system have just been published in a special issue of Geothermics focusing on Wairakei’s 50-year production history (Allis et al., 2009). The purpose of this paper is to review the present understanding of the cause of the subsidence. The locations of the main subsidence bowls in the Wairakei- Tauhara system are close to the original liquid outflow zones of the system. Two factors have contributed to the large amount of subsidence. In these areas, a pressure decline due to production has propagated to shallow depth and the boiling water was replaced initially by steam, and subsequently by a lateral invasion of cold ground water. The most important factor, however, appears to be the presence of hydrothermally altered lake sediments and tuff. Casing deformation within the adjacent Wairakei borefield, and the results of modeling vertical and horizontal movement suggest the highest compressibility sediments are at 150 – 200 m depth. The cause of the large compressibility is inferred to be higher clay content due to hydrothermal alteration on a geologic time scale close to the natural fluid discharge areas. There appear to be many similarities between the Wairakei subsidence and the mechanism of subsidence at Mexico City where many decades of ground water extraction from tuffaceous lake sediments have caused up to 12 m of subsidence. Subsidence and adjacent surface fissuring in lake sediments in some Basin and Range ground water or geothermal systems may also have similarities, suggesting that relatively non-compacted lake sediments in geothermal fields may cause significant subsidence during the early development years. |