The greywacke basement rocks of the North Island, New Zealand host a number of hot geothermal reservoirs. These have been utilised at a number of locations for energy production, most notably Kawerau and Ohaaki-Broadlands in the Taupo Volcanic Zone (TVZ) and the Ngawha Geothermal Field in Northland. Additionally this basement rock is a potential target of future deep geothermal exploration and utilisation in New Zealand. Understanding how the greywacke basement terrain rocks behave mechanically and obtaining information on relevant rock properties are vital to its future management and increased utilisation as an energy source. To this end we present here initial findings of rock property testing of examples of both greywacke terrains known to host geothermal reservoirs; the Waipapa and Torlesse terrains. Testing included uniaxial compressive strength and Brazilian strength tests, high confining pressure tri-axial strength tests, acoustic velocity tests, argon gas - thermally stimulated porosity tests and nuclear magnetic resonance (NMR) rock core analysis. Tests were performed on greywacke rock samples obtained from appropriate quarry exposures, Waipapa from the Waotu Quarry and Torlesse from the Blue Rock Quarry. UCS testing shows that both greywacke terrains have high strengths but that greywacke from the Waotu Quarry is systematically stronger than that of the Blue Rock Quarry. Additionally Waotu Quarry greywacke has higher densities and a higher Poisson’s Ratio (more incompressible) than the Blue Rock Quarry greywacke. Brazilian testing shows that the Waotu Quarry greywacke also has high tensile strength. Additional Brazilian testing of quartz veins in this greywacke reveals they have lower tensile strengths than intact rock and that as the fracture aperture widens, tensile strength decreases. Permeability tests on Blue Rock Quarry greywacke reveal very low porosity as does NMR core analysis of Waotu Quarry greywacke. Tri-axial testing of both greywacke terrains up to confining pressures of 75 MPa shows high strengths (cohesion = ~50 MPa, phi = ~45 degrees) though displays no significant difference between the two terrains they represent. Very high rock strengths and low porosity / intrinsic permeability supports the standing hypothesis that geothermal fluid flow in these basement greywacke hosted reservoirs is dominated by structure (faults and fractures). Initial testing indicates that fluid flow is likely constrained to existing structural pathways (reactivation) as this requires less stress than that needed to generate new fractures in these rocks. Importantly most testing indicates a systematic mechanical difference between greywacke from the Waipapa and Torlesse terrains. This will be an important consideration for individually managing and utilising basement hosted reservoirs in each terrain and potentially impact the decision for initial exploration of deeper (>3 km) geothermal exploration in New Zealand.

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