Highlights of borehole imaging, Tauhara geothermal field drilling, New Zealand

Abstract

Assessing fracture characteristics is key to robust assessment of the contribution of structural permeability to a well’s capacity for production or injection. Contact Energy acquired eleven borehole images at the Tauhara Geothermal Field, New Zealand, during the 2011, 2019 and 2021-2022 drilling campaigns. Here, we present some key observations from borehole imaging at Tauhara. The borehole imaging technology adopted in the 2021-2022 Tauhara drilling program was the Acoustic Borehole Imager (ABI43) from ALT provided through HADES and MB Century in New Zealand. The ABI43 imaging sensor is rated to 170°C and the orientation sensor to 125°C. The ABI43 tool was run down into formation temperatures in the range of 200-300°C, with well quenching keeping temperatures generally <125°C. Borehole images provided measurements of: 1) fracture appearance, density, orientation, and apparent thickness, and 2) in-situ stress orientations. Borehole image interpretations were then combined with geological interpretation of drill cuttings and permeable (feed) zones delineated using downhole measurements of pressure, temperature and spinner velocity (PTS). The combined datasets were interpreted to evaluate the nature of
permeability and characterise the fracture network including potential fault intersections in the boreholes. The data also aided interpreting hydrothermally altered lithology types and unit boundaries.
Reservoir-scale faults were interpreted from the images and major feed zones were often found associated with these reservoir-scale faults, or with fracture clusters. Minor feed zones were usually associated with fracture clusters or fractures spread out over a larger interval. In some minor feed zones, lack of fractures suggests that fluid flow is not clearly controlled by local fractures. Integration of borehole images with geological and reservoir measurements revealed that permeability is heavily controlled by fractures at the new production and injection areas of Tauhara. This observation had implications for well targeting strategies during the drilling campaign and the geothermal system conceptual model.