After 5 years, the Geothermal: the Next Generation programme is coming to a close.
Here, we summarise the Experimental Geochemistry component of the research. This research is closing the knowledge gap regarding fluid-rock interactions under the physicochemical conditions which exist in supercritical reservoirs.
The aim of our experimental geochemistry research was to deliver vital geochemical data and provide thermochemical constraints for numerical models.
Key knowledge advances include:
We undertook a series of experiments at sub- and super-critical conditions to examine fluid-rock interactions, observing a range of mineral precipitation and dissolution.
We measured the thermodynamic parameters of key minerals at supercritical conditions.
We explored the stability of novel tracers under sub-critical and supercritical conditions.
1. Developing a supercritical continuous flow reactor
The Experimental Geochemistry Laboratory at GNS Science expanded its specialist capabilities into the supercritical phase. The supercritical continuous flow reactor (Figure 1) allows the experimental simulation and study of geochemical interactions between geothermal brines and NZ reservoir rocks at temperatures and pressures up to 700°C and 290 bars.
Figure 1. The supercritical continuous flow reactor.
Accessible data & publications:
B. Mountain, P. Rendel, L. Sajkowski (2024) Expanding the Limits of Experimental Water-Rock Interaction: The Supercritical Laboratory at GNS Science. Proceedings NZ Geothermal Workshop. Link.
2. Fluid-rock interactions under supercritical conditions
Key findings from our experimental work at supercritical conditions included:
Seawater - basalt interaction: Identification of distinct alteration fronts, including glass dissolution, chloritization, and anhydrite precipitation, as well as the extensive chloritization and fast rate of reaction at which magnesium is fixed by the rock (Fig. 2).
Figure 2: False colour SEM-EDS maps of unreacted basalt (left) and basalt reacted with seawater at 400oC (right). Minerals in the unreacted basalt: plagioclase (purple), clinopyroxene (turquoise), olivine, (light green), and Fe-Ti oxides (white). Minerals in the reacted basalt: chlorite (green) and Fe-Ti oxides (red).
Water – greywacke interaction: Under supercritical conditions with distilled water, the primary reaction observed was the dissolution of quartz from the rock. SEM analysis of the rock surfaces reveals significant growth of new mineral phases at 400°C and 450°C. These observations suggest significant mobility of chemical components near the rock surface, although they were not transferred into the bulk fluid. Above 500°C, the pattern of mineralisation changed significantly, with the grains being coated by a semi-continuous layer of cristobalite, suggesting that only SiO₂ was notably mobile, while other rock components were unable to mobilise into the low-density fluid.
Reinjection brine - greywacke interaction: At 350°C/300 bar, the dominant reaction observed was the dissolution of quartz, which is expected as the re-injection brine was undersaturated with respect to this mineral under these conditions. The rock surface was also coated with newly formed minerals, primarily albite. At the higher temperature, the rock is coated with newly formed phases, including quartz and at least two unidentified phases that are likely amphiboles. The implication is that the introduction of a conventional re-injection brine into a supercritical system is not advised as extensive precipitation of secondary mineral phases is expected.
Greywacke – Reinjection brine – CO2 interaction: Extensive dissolution and restructuring of rock components were observed, with the newly formed minerals including albite, anhydrite, and an unidentified phase, likely an amphibole. No carbonate phases were identified in the experiment; however, extensive mobilisation of calcium was observed, as indicated by the presence of anhydrite. This suggests that CO2 sequestration by greywacke is not an active mechanism under the tested conditions (400°C/250 bar, with added CO2 at a concentration of 2000 mg kg-1).
Figure 3: SEM images of greywacke reacted with geothermal brine at a) 350oC and b) 400oC. At 350oC the rock is covered with a coating of primarily albite (Ab) while at 400oC quartz (Qtz) and two unidentified minerals (Amph) are found.
Accessible data & publications:
P. Rendel, B. Mountain, L. Sajkowski, I. Chambefort (2022) Experimental Studies of Supercritical Fluid-Rock Interactions. Proceedings NZ Geothermal Workshop. Link.
B.Mountain, P. Rendel, L. Sajkowski (2024) Expanding the Limits of Experimental Water-Rock Interaction: The Supercritical Laboratory at GNS Science. Proceedings NZ Geothermal Workshop. Link.
3. New data on mineral solubilities
The measurement of the thermodynamic parameters of key minerals at supercritical conditions is vital for the design and implementation of supercritical power generation systems.
We conducted experimental studies on the solubility of quartz and anhydride (Figure 4) minerals under supercritical conditions. This research has provided new data and been used to refine the empirical parameters for solubility equations.
Figure 4: Experimental measured solubilities and the calibrated empirical equation results for the solubility of quartz at supercritical conditions (Rendel et al., 2023).
Accessible data & publications:
P. Rendel, B. Mountain, L. Sajkowski (2024) Solubility of anhydrite in supercritical water from 380°C to 625°C and 220 bar to 270 bar. Geochimica et Cosmochimica Acta. Link.
P. Rendel, B. Mountain (2023) Solubility of quartz in supercritical water from 375°C to 600°C and 200–270 bar. The Journal of Supercritical Fluids (196). Link.
4. Testing high-temperature tracers
Tracer testing is used to determine the connectivity between production and injection wells, and show the hydrodynamic properties of the reservoir, such as flow paths and fluid velocities. At supercritical conditions, finding a suitable tracer is even more challenging than in ‘normal’ geothermal conditions.
We conducted flow-through experiments under supercritical conditions in the presence of a NZ greywacke rock substrate to evaluate the thermal behaviour of rhenium and indium. The results show that Rhenium is potentially a suitable candidate for subcritical conditions, but not in supercritical conditions. Indium is not suitable as a geothermal tracer at any temperature, as it reacts with hydrogen sulfide.
Accessible data & publications:
L. Sajkoswki, A. Kamiya, B.W. Mountain (2022) An Experimental Study on the Potential of Rhenium and Indium as Geothermal Tracers. GNS Science Report. Link.
