Dynamics of scaling and corrosion in the natural environment: the long-term sustainability of geothermal energy technologies through exp

Background
Harnessing high enthalpy geothermal energy, which is energy stored in the Earth's subsurface, will be critical for decarbonising our economy. For example, energy may be extracted from hot water contained or injected deep into the earth where temperatures reach in excess of 150 degrees C. However, without a fundamental understanding of the thermo-mechanical-chemical perturbations caused by these decarbonising technologies, as well as their effect on the properties of the deep rocks, it is difficult to assess if a geothermal plant is economically viable. Here, it is imperative that the long-term functionality of the system can be predicted; key components are the rate of fluid flow and thermal equilibration and linked energy extraction, which in turn are highly dependent on permeability of the rock and the nature of fluid pathways such as fractures or matrix porosity. Long-term permeability of the subsurface is therefore pivotal for geothermal energy extraction to remain sustainable. Permeability is continuously changing as fluid-and rock are interacting, closing pore space and fractures through scaling, opening through dissolution and renewed fracturing. Hence, knowledge of permeability changes and its dynamics (physics and chemistry) is urgently needed.

Permeability in rocks is facilitated by open fractures and pore porosity. In open-loop geothermal systems, changes in chemical composition and temperature of the reinjection water may induce a series of interactions between the reservoir rock, residing fluids and the reinjected water due to local chemical disequilibrium, which may impact the porosity and permeability of a reservoir through scaling within open fractures and pores, and/or dissolution at fluid-rock interfaces.

The mineralogy of the rock and solubility of the minerals will determine the order in which minerals dissolve and precipitate. Some minerals become more supersaturated on cooling and hence are at risk of precipitating during fluid convection. In a reservoir where carbonate rocks are the hosts to the geothermal energy, the mineral calcite is omnipresent. Calcite is one of the most chemically reactive minerals, thus calcite reactions are the most common in carbonate-rich reservoirs- (e.g., Liu and Zhao, 2000). Furthermore, prominent other phases to consider are sulphate minerals, barite, and gypsum (e.g., Burton and Walter, 1987; Arnórsson, 1989; Brehme et al., 2019).

Objectives
This project aims to achieve a new level of understanding and quantification of the underlying principles governing permeability changes through time in carbonate rocks. Four main areas will be addressed:
1) Processes and Rates: What physiochemical processes occur at different conditions? How do these physical and chemical processes interact with each other? What are their rates?
2) Effect: How do these processes effect the permeability of a natural rock, as well as its rheology such as resistance to fracturing?
3) Prediction: Based on the resolution of the two questions above, what are our possibilities to (a) predict material behaviour to allow for assessment of natural resources for deep geothermal energy extraction and to (b) develop solutions to ensure long-term sustainability of such resources.
4) Identification of optimum window of sustainable operation: Based on (1) - (3) develop an appropriate methodology to identify whether a particular system/chemistry will operate successfully for prolonged periods.