| Title | Highlights of Borehole Imaging, Tauhara Geothermal Field Drilling, New Zealand |
|---|---|
| Authors | Cecile Massiot, Sarah D Milicich, Fabian Sepulveda, Matthew Sophy, Mike Rickman, Mark JF Lawrence, Angela G Griffin, Lucy B Carson, Michael D Rosenberg, Mark Simpson |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | Well targeting, borehole imaging, Tauhara Geothermal Field, New Zealand, fractures, fault, permeability |
| Abstract | Adapting well targeting to new drilling data during a drilling campaign is key to increasing the likelihood of successful geothermal well outcomes. Drill cuttings provide indirect information on potentially permeable formations. Coring, while valuable, is expensive and provides limited spatial coverage of the reservoir of fracture orientation (apparent dip magnitude only, strike not available). Borehole images provide relatively continuous and direct measurements of fracture orientation, fracture density, fracture aperture, among other parameters. Here, we present some key observations from 10 downhole images acquired at the Tauhara Geothermal Field, Aotearoa New Zealand, obtained during 2011, 2019 and 2021-2022 drilling. We also discuss implications of borehole imaging to the conceptual model of a geothermal field and how this evolving concept model was used to refine and fine-tune drilling targets specially during the 2021-2022 program. Pre-2021 wells were drilled vertical while 2021-2022 wells were all drilled directional. The borehole imaging technology adopted in the latest Tauhara drilling program was the Acoustic Borehole Imager (ABI43) from ALT and provided through HADES-MBC 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. Temperatures were generally kept <125°C through well quenching with pump rates of 50-210 tph. Image interpretations provided, to variable degree in each well: 1) fracture appearance, density, orientation, apparent thickness; and correlation of fracture zones with permeability horizons, 2) potential fault intersections in the boreholes, 3) indications of in-situ stress orientations, and 4) inference on hydrothermally altered lithologies. Where the operating limit of the ABI43’s orientation sensors was exceeded (125°C), drillpipe marks developed on the high- and low-side of the (deviated) well were used to orientate images. Image interpretations were then combined with geological interpretation of drill cuttings and permeable (feed) zones delineated using downhole measurements of pressure, temperature and spinner (PTS). Reservoir-scale faults were interpreted from the images based on breccia textures, micro-fault features, elevated fracture density, and presence of fractures seen on the travel-time image and of high apparent thickness. Major feed zones are often associated with either these reservoir-scale faults, or with fracture clusters. Minor feed zones are usually associated with a fracture cluster or with a series of fractures spread out over a larger interval. In some minor feed zones, lack of fractures suggests that fluid flow is not controlled by fractures. Downhole imaging and integration with geological and reservoir measurements helped refining, and sometimes revising the reservoir permeability targets as the drilling campaign progressed. Overall, compared to the conceptual model prior to the 2021-2022 drilling campaign, downhole images revealed that permeability is heavily controlled by fractures and this finding was used to inform, refine and fine-tune well targeting during the Tauhara drilling program. |