| Title | Long term production in TVZ geothermal systems using a 2D source to surface model |
|---|---|
| Authors | W. Kissling, S. Ellis |
| Year | 2023 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Source-to-surface models, TVZ, mass and heat flow, geothermal systems, sustainability |
| Abstract | A 2-D ‘Source to Surface’ regional-scale heat and fluid transport model of the central TVZ is described and used to investigate the nature of the high-temperature geothermal systems and long-term fluid production and reinjection from them. The model represents a 10 km-deep NW-SE cross-section across the TVZ with simplified geology including a 20 km wide ‘rift’. At the base of the model a ‘hotplate’ heat source has a temperature of ca. 700oC and produces a heat flux of 0.7 Wm-2. The (horizontal) permeability of the rifted basement is the key factor in the model. This is described as a function of depth, and the free parameters in this function allow a) the hotplate temperature to be set independently of the prescribed heat flux, b) sufficient shallow permeability to support widespread convection in the upper few km of the model, and c) control of the temperature distribution within the rift so that high-temperature geothermal systems form and temperature proxies for rift-scale geophysical constraints are satisfied. Another essential feature of the model is that the geological units outside of the rift must have low permeability (~10-16 m2) to prevent excessive cooling from cool fluid flowing into the rift. The model suggests that the high-temperature geothermal systems in the TVZ are transient surface expressions of an unsteady rift-scale convective hydrothermal system. Temperatures in the geothermal systems can reach ca. 300oC at 2 km depth. The models respect constraints from geophysics, with the depth of the shallowest melt close to 10 km and the maximum depth of seismicity at 9 +/- 1 km. The model describes the complete hydrothermal system underlying the geothermal systems and its principal geological controls. For this reason, it can be used to investigate the effects of fluid production and reinjection on timescales longer than the normal plant life of 30-50 years. We give some simple examples of this. |