1 New Zealand Geothermal Workshop 201 1 Proceedings 21-23 November 2011 Auckland, New Zealand GEOTECHNICAL INVESTIGATIONS OF SUBSIDENCE IN THE WAIRAKEI- TAUHARA GEOTHERMAL FIELD Graham Ramsay 1 , Trystan Glynn-Morris 2 , Michael Pender 3 and Melvyn Griffiths 4 1 Beca Infrastructure Limited P O Box 3942 Wellington 6140, New Zealand graham.ramsay@beca.com 2 Contact Energy limited Wairakei Private Bag 2001, Taupo 3352, New Zealand trystan.glynn-morris@contactenergy.co.nz mailto: 3 University of Auckland, Engineering Department 20 Symonds St., Auckland 1142 New Zealand m.pender@auckland.ac.nz 4 Griffiths Drilling (NZ) Ltd P O Box 40422 Upper Hutt, New Zealand mel@griffithsdrilling.co.nz Keywords: Geothermal Fluid, extraction, subsidence, geotechnical, investigations, testing. ABSTRACT The extraction of geothermal fluid from the Wairakei Field commenced in the 1950’s and has resulted in the formation of a localised subsidence bowl one kilometre across with maximum subsidence of up to 15m. More recently a number of smaller subsidence features have developed above the adjoining and connected Tauhara Geothermal Field. The operator of the field, Contact Energy Limited (Contact) required geotechnical investigations, sampling, and testing to be undertaken both to satisfy conditions attached to its consents for continued extraction from the Wairakei Field and also to support consent applications for a major increase in extraction from the Tauhara field. A particular aim was to seek to identify local anomalies that would explain the subsidence bowls. The geotechnical investigations involved continuous coring and the undisturbed sampling of a variety of materials ranging from soft weak surface tephras to hard strong ignimbrites in geothermal conditions at depths of up to 800m. This paper describes the techniques and procedures used to undertake the drilling, coring and recovery of the samples for testing to determine compressibility characteristics. The procedures needed to achieve the geotechnical objectives of maximum recovery of high quality core, while maintaining well security in geothermal conditions with high temperatures and pressures. An innovative technique was used to recover undisturbed samples for compressibility testing in special triaxial cells designed and built for the project. The investigation programme involved nine geotechnical boreholes with a maximum depth of 774m and a total length of 4391m. A total of 3928m of core was recovered with 269 undisturbed samples from which 121 were selected for laboratory compressibility testing. 1. BACKGROUND 1.1 Historical Context 1.1.1 Development History The Wairakei geothermal field is near Taupo in the centre of the North Island of New Zealand. Extraction of geothermal fluid for power generation commenced in 1958 and has been ongoing, with extraction of approximately 150,000 tonnes per day of geothermal fluid as feed to the 171.5 MWe Wairakei Power Station. In 2007 Contact obtained Resource Consents for ongoing extraction and is in the process of developing new generation facilities to phase out the existing 55 year old Wairakei Power Station. Contact has also identified and investigated the adjoining Tauhara Geothermal Field as a potential geothermal power generation opportunity. Until recently there has been some limited extraction of geothermal fluid from the field for direct use in forestry industry plants. In 2010, Contact’s 23 MWe Te Huka binary plant started generating electricity. Resource consents have now been granted for a larger geothermal project (up to 240 MWe) on the Tauhara Geothermal Field. Figure 1 shows the location of the Wairakei and Tauhara geothermal fields. 1.1.2 Impacts of Extractions The geothermal fluid has been extracted from the Waiora Formation shown in Figure 2. The formation extends under both the Wairakei and Tauhara areas. In the 55 year period since extraction commenced at Wairakei the fluid pressure in the Waiora Formation has dropped by approximately 2000 kPa and that pressure drop has extended laterally under the Tauhara area. Limited readings also indicate a pressure drop in the Mid Huka Falls Formation, a permeable stratum typically 50- 100m above the Waiora Formation. The extraction at Wairakei over 55 years has resulted in widespread general subsidence in the Wairakei area in the order of 1-2m and also the formation of a localized subsidence bowl one km across and with a maximum subsidence of the order of 15m. While the local subsidence bowl has caused significant disruption to roads and drainage structures, because all these are within the borefield area owned and controlled by Contact there has been no impact on private property. More recently three smaller subsidence bowls have developed in the Tauhara area: at Spa Sights (2.9m settlement), Rakaunui Road (2.45m settlement), and Crown Road (0.88m settlement). General subsidence in the Tauhara area has varied from 0 – 1m. The locations of the subsidence features are indicated in Figure 1 and details are provided in Bromley et al. (2010).