Physical and mechanical controls of matrix permeability on rocks from Rotokawa Geothermal Field, Taupo Volcanic Zone, New Zealand (2016)
AuthorsJones, Timothy Peter Careyshow all
The Taupo Volcanic Zone (TVZ), situated in the central North Island of New Zealand, is the host of an active volcanic region with an enormous heat ux (4200±500 MW). Twenty-three geothermal fields lie within TVZ, seven of which have been developed for geothermal power generation, including the Rotokawa Geothermal Field (RGF), where 3% of New Zealand's power is generated. Relationships between key mechanical properties, such as porosity and permeability, can be complex. Developing an understanding of such relationships in the subsurface helps with the optimisation and maintenance of geothermal resources. In this thesis, a systematic physical and mechanical property study was carried out in laboratory conditions on rocks from the RGF, including rhyolite, ignimbrite, andesite and basement greywacke. Physical properties investigated were porosity, density, matrix permeability, ultrasonic wave velocities, triaxial strength and microstructure analysis. It was determined that there is a distinct relationship between increasing porosity and increasing permeability represented by a single power-law function. However, the contrast between two andesite samples highlights how complex this relationship is. The two samples have similar porosities, yet due to differing microstructure, permeabilities are nearly two orders of magnitude different. The behaviour during triaxial testing was also significantly different between these samples, where the presence of pre-existing microfractures in one resulted in more brittle behaviour than the other. Increasing confining stress decreases matrix permeability for the majority of sam- ples. The decrease in permeability is more pronounced for samples with pre-existing microfractures, compared to those with no microfractures. Relationships between ultra- sonic wave velocities and other parameters are weak. There is no distinct relationship between in situ permeability and depth. The significance of microstructure and the variability in the mechanical relationships described in this thesis demonstrate the importance of thorough investigation of localised samples, especially when hosted in a hydrothermally active system.