| Title | What to expect when you’re not expecting - Drilling in the Tauranga Geothermal System |
|---|---|
| Authors | R. Gardner, M. Rosenberg |
| Year | 2025 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Low temperature geothermal, Tauranga Geothermal System, Drilling Standards, new geological formation |
| Abstract | Under the Resource Management Act 1991 (NZ), Bay of Plenty Regional Council (BOPRC) is responsible for managing the groundwater and geothermal resources of the region. The Council undertakes an ongoing drilling program throughout the region to improve its understanding of groundwater, low temperature and high temperature geothermal resources through its state of the environment (SOE) monitoring networks. Council uses the lithological information collected during drilling to improve 3-D geological modelling, which is key to setting aquifer allocation limits. As part of this programme, in 2024 BOPRC planned to drill in 2024, three new monitoring wells in the west of the Te Puke region. Planned drilling targets were the deeper aquifers, represented by the Aongatete, Waiteariki, and the Pokai-Chimp-Pokopoko formations, to better monitor effects from inland takes by horticulture operations. Based on the existing drilling, modelling and consented data, the targeted completion depth was approximately 480 metres for the deepest of the wells, anticipating a maximum water temperature of approximately 40–45°C, which is elsewhere typical for the Tauranga Geothermal System. During drilling of the first well, artesian flow of 8 litres per second was unexpectedly encountered at 318m below ground, with a temperature of 68.6°C. A Pressure, Temperature, Spinner (PTS) survey was subsequently undertaken to determine downhole temperature, and to locate the main flow/fractures. The highest temperature recorded was 76.34°C, with the spinner indicating flows at 310 – 312m and from below 320m (the maximum clear depth). Shut in pressure on the well was recorded as approximately 0.7 bar (10psi) and the maximum downhole pressure was 32.5 bar (471psi). Analysis of the drill chip samples (collected at 1m intervals) indicated the hot main feed zone occurs in a previously unknown lava of the Minden Rhyolite Subgroup. Council is currently working with local iwi to name the new rhyolite. |