After 5 years, the Geothermal: the Next Generation programme is coming to a close.
Here, we summarise the Modelling component of the research. This research is contributing to identification of prime location targets for exploration drilling to develop supercritical resources. You can also check out the summaries for Geology research and Geophysics research.
The aim of the modelling research was to advance the modelling of supercritical fluid location above magmatic intrusions.
Key knowledge advances include:
We have demonstrated the first source-to-surface models of the central TVZ hydrothermal system in a 2D setting. These models were used to investigate the nature of the high-temperature geothermal systems and long-term fluid production and reinjection from them.
The models represent a simplified geological setting with a 20 km-wide continental rift, based on observations from New Zealand’s Taupō Rift in the central North Island. This is the target zone for future supercritical and super-hot resource developments, and this model provides fundamental information about the shallow (2 km) locations of geothermal systems and the deeper (10 km) heat sources. They represents deeper magmatic intrusion into the lower crust with a 700°C to 900°C hotplate source at 10 km depth, but considers no other shallow magma bodies. The effective heat flux at the hotplate is 0.77 W/m2. In the model, low-permeability basement-like rocks define the rift margins and basement, which is then covered with volcanic infill. The permeability within the rift decreases with depth in such a way as to match the above-mentioned temperature range, and to respect geophysical constraints, from seismicity and magnetotellurics.
The models produce unsteady, irregular rift-scale hydrothermal circulation in the upper ~5 km of the crust. Plumes of hot water are interpreted as high-temperature geothermal systems, with temperatures of ca. 300°C at 2 km depth. The shallow volcanics control the amount of cool surface water entering the rift scale hydrothermal system. Models with low permeability for the shallow volcanics produce longer lasting and higher temperature geothermal systems, and those with high permeability produce fewer and cooler geothermal systems.
Extension of these models to a 3D setting is underway, and has the potential to directly provide estimates of the size of the supercritical resource in the Central TVZ. In addition, the 3D models, once properly calibrated against rift-scale geophysical measurements, have sufficient resolution to inform both long term management and sustainability of fluid extraction from high-temperature geothermal systems.
Accessible data & publications:
By including the crystallizing magmatic heat sources in a set of numerical models, we studied the temporal evolution of the geothermal systems that form near silicic magma reservoirs.
Overall, the efficiency of the heat transfer between the heat source and the circulating hydrothermal fluids is restricted by the relatively low permeability greywacke basement and the presumed low brittle-ductile transition temperature of the silicic host rocks. Under those conditions, deep-seated magma reservoirs produce long-lived (100s kyr), but relatively cool hydrothermal systems (200-250°C at 1 km depth). Small, shallow seated (e.g. at 4 km) intrusions cool on timescales of a few thousand years, but still produce long-lived (10s kyr) hydrothermal systems which are comparably hot (250-300°C at 1 km depth), matching the conditions of the hottest geothermal systems in the TVZ currently active. These simulations also point at the potential presence of regions with supercritical geothermal fluids in the basement above the crystallized magma reservoirs (Figure 2).
The modelling also highlights the strong sensitivity of the presence of supercritical fluids at drillable depths to the brittle-ductile transition conditions. This means that future geothermal exploration should consider the mineralogical composition of the host rocks and aim at studying the loss of permeability at these depths as a function of temperature.
Simulations which include exsolution of a magmatic volatile phase from the melt showed that the addition of these fluids to the overlying geothermal systems is strongly hindered by a thick ductile shell around the magma reservoirs. Strong mixing of these magma-derived fluids with meteorically derived fluids diminished the magmatic signature, leading to only minor magmatic fluid fractions at TVZ well depths.
Accessible data & publications: