Combining scanning electron microscopy and compressibility measurement to understand subsurface processes leading to subsidence at Tauhara Geothermal Field, New Zealand

• Dual methods of compressibility tests and scanning electron microscopy observations
• Insights to subsidence with scanning electron microscopy and compressibility tests
• Constrained modulus and scanning electron microscopy results on core from Tauhara
• Understanding fluid–rock interactions using scanning electron microscopy
• Quantifying subsurface rock strength using constrained modulus values

The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and has undergone subsidence in three localized areas referred to as the Crown, Rakaunui and Spa Sights bowls, with measured subsidences of 0.9, 2.4 and 2.9 m, respectively. These subsidence bowls are situated close to Taupo township and are of concern to the public and for geothermal developers. Therefore, an intensive subsidence study at the Tauhara field was undertaken to better understand and mitigate further subsidence. Tauhara and Wairakei are often referred to as one system as a shallow, low resistivity anomaly extends continuously across both fields. However, they are two individual fields with separate up-flows. Tauhara is located in the south of the system, while Wairakei is located in the northern area. The Wairakei field also has subsidence bowls that reach up to 15 m in localized areas. Extraction of fluids from the Wairakei field began in 1958 (currently 171 MWe) but did not begin at Tauhara until 2010 (currently 23 MWe). At Wairakei, initial fluid withdrawal was from the Waiora Formation which extends under both the Wairakei and Tauhara fields. Since 1958, fluid pressure in the Waiora Formation has dropped and this pressure decline extends under both the Wairakei and Tauhara fields. A pressure drop has also been detected in the Mid Huka Falls Formation which is a permeable stratigraphic unit present at shallower depth (relative to the Waiora Formation) in both fields. The Tauhara subsidence investigation included drilling, with continuous core recovery, at selected sites located inside (THM 16), outside (THM 13, THM 14) and on the periphery (THM 12) of known subsidence bowls. Cored samples representative of the seven formations encountered were analyzed to establish their stiffness by determining their constrained modulus (CM) value. On the same samples, the effect of hydrothermal alteration was established using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), petrography and X-ray diffraction (XRD). Key findings include the following: (1) CM values ranged from 20 to 1800 MPa; (2) THM 16 revealed the lowest CM values of the study (< 100 MPa) at 50 to 100 m of depth, where there has been a change in the subsurface to more acidic conditions; (3) Samples that revealed no clay minerals attached to crystal surfaces produced significantly higher CM values (THM 13, CM = 1730 MPa), than those samples where clay minerals were attached to and altering the crystals (THM 12, CM = 84 MPa); (4) Fracturing of crystals was observed in some samples from drillholes THM 12 and THM 13 located on the periphery and outside the Rakaunui subsidence bowl respectively, which may be a response to localized stress induced by subsurface compaction; (5) No crystal fracturing was observed in THM 14 located outside all subsidence bowls. The pressure drop across both the Tauhara and Wairakei geothermal fields has resulted in compaction of the Huka Falls Formation at depth with consequent subsidence at the surface. At Tauhara, compaction of the Huka Falls Formation at 130 to 400 m of depth occurs within the Rakaunui subsidence bowl. Furthermore, intense hydrothermal alteration at the Crown subsidence bowl (THM 16) has weakened a hydrothermal breccia deposit where compaction occurs at 35 to 200 m of depth. The combination of techniques used in this study proved a useful tool for unraveling complex geothermal processes altering the subsurface rocks. By establishing the hydrothermal alteration processes and coupling them with CM values we gained insights into rock stiffness and fluid–rock interactions within the Tauhara geothermal field.