IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_1E-P13

Date/Time: July 21 Poster

Stress Distribution in the Rotokawa Geothermal Field, New Zealand and implications for
fluid flow through brittle crust
Jonathan R J Davidson1 , Paul Siratovich1 , Irene Wallis2 , Darren Gravley1 , David McNamara3
1

University of Canterbury, New Zealand, 2 Mighty River Power, New Zealand, 3 GNS Science, NewZealandE-mail: jrjdavidson@gmail.com

Quantifying the stress field is a vital tool for mapping and determining how to maintain the pathways thathydrothermal fluids follow in a naturally fractured geothermal system. Permeability in the Rotokawa Geothermal
Reservoir, located in the Taupo Volcanic Zone (TVZ), is primarily fracture hosted and is influenced by the activityof large faults. Thus permeability preservation in the reservoir relies on cyclical reactivation of these structures.
The magnitude and orientation of the principal stress axes will influence fault mechanics and how fracture-hosted
permeability operates in geothermal reservoirs. Slip on faults and fractures can depend heavily on their orientation
relative to that of the stress field and the relative magnitude of the principal stresses.
Leak-off Tests (LOTs), density models and borehole images are used to constrain stress magnitudes at depth in theRotokawa Geothermal Reservoir. In the extensional tectonic setting of the TVZ, the vertical stress axis coincides
with the maximum principal stress. As this is dependent on the weight of the overburden, we employed extensive
lab-based density measurements that have been performed on core from Rotokawa and other geothermal fields toderive a vertical stress model. Using this data, the vertical stress across the Rotokawa Field has been modeledtaking into account the variable lateral thicknesses of different lithologies. Data from LOTs and the observationof drilling induced tensile fractures (DITFs) on borehole acoustic images are used to quantify the minimum andmaximum horizontal stresses respectively.
The differential between the vertical and minimum horizontal stresses corresponds to a crust in frictional equilibrium.
Results of this study indicate that spatial variation in the vertical stress magnitude due to lateral variation in thethickness of the overlying stratigraphy may be an important factor in maintaining permeable fractures. For example,
the presence of a young andesitic cone in the strata causes a local increase in the vertical stress. This may have a
local effect on the permeability of the underlying rocks.
Another key outcome of this study is that a portion of the fractures imaged in boreholes is exploiting weaknesses
in an otherwise strong rock. The range of fractures observed suggest that the fractures were activated undercompressional shear conditions, yet the strength of the rock measured suggest that is difficult to initiate suchfractures. It is likely that the rock is weaker due to the presence of primary fractures (i.e., created when the rockwas emplaced) or fractures due to subsequent volcanic activity. It is therefore crucial to have a good grasp on thevolcano-tectonic history of the region in order to understand the pathways that fluids take to reach the surface.

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