Physical and mechanical depth relationships of rocks from the Rotokawa Geothermal Reservoir, Taupō Volcanic Zone, New Zealand

The Rotokawa geothermal field is in the Taupō Volcanic Zone, New Zealand. It hosts two geothermal power plants, Rotokawa and Ngā Awa Pūrua, which together have a capacity over 170 MWe. The permeability of the rock mass comprising a geothermal field controls the volume of hot fluids that can be extracted for energy production. In this research we produce a stress model to estimate in-situ rock matrix permeability for a unique set of intact rock samples obtained from depths up to 2600 m in the Rotokawa geothermal field. We show that permeability generally decreases with sample depth, both intrinsically and in response to increasing confining pressure. We explore this confining pressure effect on other petrophysical and mechanical measurements, and highlight how rock texture and composition affect how the rocks respond to confining pressure. For example, we compare two altered andesite samples: a breccia with microfractures and infilled pores that tends to experience less compaction and porosity decrease than a lava with rounded, unfilled pores. We suggest that, when developing depth-models, measurements should be conducted at relevant in-situ conditions, if possible. Finally, we explore relationships between different physical parameters and provide estimating functions for those with the clearest correlations.