Rocks sourced from active geothermal systems can
have unique responses to deformation, due to unique
alteration mineralogy and complex microstructure.
The current state of understanding of mechanical
behaviour of rocks under varying stress conditions is
well established on suites of rocks with simple
mineralogy and microstructure. Brittle failure can
increase porosity and permeability and generate
seismicity, whereas inelastic deformation in the
ductile regime will decrease porosity and will likely
decrease permeability, and generate no or distinct low
frequency seismicity. Many studies have focused on
the behaviour of siliclastic and carbonate rocks to
establish the transition form brittle to ductile
behaviour. The geothermal systems in New Zealand,
and many other areas, are hosted in mainly volcanic
rocks, limiting the applicability of current data and
knowledge to these systems.
We present results from laboratory triaxial
deformation and strength testing of drill core sampled
from a deep geothermal reservoir. We have used our
findings to construct failure criteria based on our
investigations and compared them to the in-situ and
induced stress conditions that may lead to
macroscopically brittle or ductile deformation of the
host rock. Our results show that under the current
stress conditions at the Rotokawa geothermal field the
host rock behaves in a brittle, rather than compactive,
fashion. Under these in-situ stress conditions brittle
fracture generation dominates over cataclastic pore
collapse, resulting in a rock mass with suitable macroscale
permeability for fluid extraction. Our results also
show that the rock strength is typically too high for the
induced stresses during drilling to initiate borehole
breakout. This is supported by borehole observations
revealing very little borehole damage in the host rock
