Geothermal Energy

Geothermal energy is the thermal energy in the Earth's crust which originates from the formation of the planet and from radioactive decay of materials.

Geothermal energy can be exploited by directly tapping into hot surface-springs, or by drilling wells to recover heat to surface via a convective carrier (a working fluid). Conventional “open systems” produce hot formation-fluids from the subsurface pore-system of a geothermal reservoir and pass these through a heat exchanger to recover heat. Alternative to this is a “closed loop system” where a suitable heat carrier (like water or supercritical CO2) is circulated down a cased well, warmed up by conductive heat transfer (but without fluids flowing from the well into the reservoir or vice versa) and then circulated back up and passed through a surface heat exchanger. For more info on closed-loop systems follow this link.

 Harvested heat can be used for direct heating or, where conditions are favorable, for electric power generation. Technologies in use for power generation include dry steam power stations, flash steam power stations and binary cycle power stations. Geothermal electricity generation is currently used in 26 countries, while geothermal heating is in use in 70 countries.

Geological and Resource Assessment of Geothermal systems has become a main focus areas of AEGeo / Dr Arnout.

Classification of geothermal systems is typically done considering 1) subsurface temperature and mass flowrates and 2) geological setting (see picture below from Jolie et al, 2021). Geologically speaking, geothermal systems can broadly be subdivided in volcanic and non-volcanic (amagmatic) systems. In volcanic-related systems, heat transfer from deep to shallow is mostly via convection of hot fluids whilst in non-volcanic systems, heat transfer from the earth’s interior to exploitable depths is mostly via conduction. From a temperature and massflow point of view, distinction is between low, medium and high enthalpy systems. As for phase behavior, volcanic-related convective systems can be vapor-dominated, liquid-dominated or mixed whilst conductive systems are exclusively liquid.

A more detailed classification of geothermal systems and play types considering their geological setting and characteristics is shown in table below (modified after Moeck and Beardsmore, 2014). Such a detailed classification is useful because both the subsurface assessment (e.g., methodology for geothermal resource mapping) as well as the engineering challenges differ greatly for different geothermal play-types.

In volcanic-related geothermal settings, heat yield is via natural hydrothermal systems that are one way or another driven by magmatic activity. Mapping the extent and connectedness of hydrothermal systems (hot-spring sources) in the subsurface will therefore be key to resource assessment and development optimization. Estimating the thermal yield will require assessment of the balance between the rate of depletion of the hydrothermal source (due to heat withdrawal) versus the rate of natural recharge. Besides assessment of the size and temperature of subsurface hydrothermal resources, fluid phase distribution is also a key factor as the geothermal plants often use steam (dry steam or flash steam) rather than liquid (binary-cycle plants) as input.

In hot porous aquifer settings (also known as “sedheat”), wells produce formation fluids from the natural pore-system of the geothermal reservoir (usually with artificial lift and pressure-supported by reinjection of cool waste-fluids). In such systems, the ability of the reservoir to sustain large flowrates will be key to achieve meaningful energy yield (especially if the purpose is to produce electricity). Hence, resource assessment of "sedheat" systems involves mapping geothermal reservoir quality and extent (similar to what is done in oil and gas) and combining this with a good understanding of reservoir temperature. Energy yield of a well will depend on its initial flowrate, anticipated pressure depletion and anticipated timing of cool-water breakthrough between producer and injector.

For more info on the reservoir characterization and geothermal resource assessment of “hot porous aquifer” systems click here …

In EGS systems, hydraulic fracturing is applied to create connected fracture networks (either new fractures or enlargement of existing fractures) in suitably hot but generally tight rock, typically granite or metasediments. Fluids are then circulated between injector and producer wells via this fracture network and these fluids get warmed up by conductive heat-transfer from the surrounding rocks as they flow through the fractures. Thermal energy reserve of an EGS well depends on the initial flowrate, size of the fracture network involved in fluid flow and the rate of conductive heat transfer from matrix to the working fluid in the fractures. Reservoir characterization of EGS systems focuses on understanding connectedness and properties of the fracture network and estimation of recoverable thermal energy hinges on this.

For more information on applicable assessment methodologies click here …