Faculty of Engineering


Tectonic extension, in addition to causing seismic activity, is responsible for damaging and fracturing the rocks through which geothermal fluids flow.

The extent, location and reversibility of this damage, and its implications for permeability and the wider convective system are the focus of current modeling efforts.

Rock heated by geothermal fluids also expands, closing fractures in some regions, while pushing the stress state closer to failure in others, resulting in a complex interplay between fluid flow, permeability and stress.

Our simulations probe these relationships through an investigation of fluid transport to a depth of 8km over commercial (100 yr) and geologic (1 Myr) timescales, and the response of this flow to an evolving permeability distribution.

The Taupo Volcanic Zone is a dynamic and volatile region of volcanism, geothermal activity and frequent seismicity. At many locations surface geothermal activity creates colorful sinter deposits, sporadic geyser activity, boiling mud pools, and hot water springs, which are of value both culturally and to New Zealand’s tourism industry. At other locations, superheated steam and water are extracted and the heat used to produce approximately 600 MWe servicing ~10% of New Zealand’s electricity demand.

Due to their scientific and commercial value, the geothermal fields of the Taupo Volcanic Zone have been extensively surveyed, researched and modelled. Our present understanding of these complex systems is informed by various geological, geophysical and geochemical studies. More recently, advances in computing have allowed the development of computational models of groundwater flow and crustal mechanics. These can be used to explore conceptual models and plausible scenarios, particularly at depths where raw data is less accessible.

 

Key focus areas/issues


The ultimate aim of this project is to investigate and, if appropriate, quantify the link between tectonism and geothermal circulation in a Taupo Volcanic Zone context. We expect the dynamic stress state that arises during cyclical fault slip to provide some control on permeability distribution, and thus groundwater movement, through mechanisms such as:

  • Opening and closing of micro cracks as the stress state oscillates throughout the seismic cycle.
  • Development of new fractures in the damage zone as a fault grows.
  • Sealing of fractures through mineralisation and deposition.

There are two components to this project.  These two components will be integrated to produce a more complete picture of the tectonic-geothermal relationship. The cycling stress state, development of off fault plastic deformation, and evolving fault geometry will inform a permeability distribution that responds to tectonic forcing. The growth, migration and decay of convective cells hosted within this evolving crust are the final objective of modeling efforts.

cl-energy-fault-slip-chart

1. Modelling dynamic crust behaviour

A commercial finite element software package (Abaqus) is used to model the dynamic behaviour of a crust intersected by one or more normal faults, yielding deformation and stress behaviour over 1000 year timescales.

cl-energy-fehm-temp-chart

2. Modelling geothermal convection

Geothermal convection is modelled using a separate code (FEHM) capable of handling multiphase fluid flow through a permeable crust. FEHM will also model stress and subsequent permeability modification coupled to the geothermal model.

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Key people


  • Rosalind Archer
    Engineering Science
  • Julie Rowland
    School of Environment
  • David Dempsey
    PhD candidate, Engineering Science
  • Susan Ellis
    GNS Science

Contact


Rosalind Archer
Email: r.archer@auckland.ac.nz
Phone: +64 9 373 7599 ext 84517

Related publications


Ellis, S., Beavan, J., Eberhart-Phillips, D., Stockhert, B., 2006. Simplified models of the Alpine Fault seismic cycle: stress transfer in the mid-crust, Geophysical Journal International, vol. 166, pp. 386-402.

Rowland JV, Wilson CJN, Gravley DM, 2009. Spatial and temporal variations in magma-assisting rifting, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research, vol. 190, pp. 89-108.

Dempsey, D.; Ellis, S.; Rowland, J.; Archer, R.A. Extensional Faulting in the Taupo Volcanic Zone, New Zealand: Numerical Modelling of Stress/Strain Cycling and Deformation Patterns', American Geophysical Union Annual Meeting, San Francisco, 13 December 2010.

Dempsey, D; Archer, R.A; Rowland, J; Ellis, S. 'Investigation of Geothermal Convection in Dynamic Permeability Regimes Using Numerical Modelling', Geo NZ Conference, Auckland, 21 November - 24 November 2010.

Dempsey, D; Ellis, S.; Archer, R.A; Rowland, J 'Stress/Strain Dynamics on TVZ Normal Fault Systems: Insights from Numerical Modelling', Geo NZ Conference, Auckland, 21 November - 24 November 2010.

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