Geothermal — heat beneath our feet

Drilling, water, gases, seismicity and continuous heat. Theory, formulas, worked examples and applications.

The heat is there; reaching it costs

Why there is heat beneath our feet

The Earth's interior retains an enormous amount of heat, resulting both from the residual heat of the planet's formation and from the ongoing decay of naturally occurring radioactive elements in deep rocks. That heat slowly rises toward the surface through conduction, but in certain geological zones — near tectonic plate boundaries, volcanic areas or hot spots — it accumulates at relatively accessible depths, in the form of hot rock, hot water or trapped steam in porous formations. Making useful use of that heat requires, in practice, two very different technologies depending on the depth and temperature of the resource.

Shallow versus deep geothermal: two technologies, not one

Shallow geothermal uses the ground in the first few meters below the surface as a heat source or sink for a geothermal heat pump: subsurface temperature stays reasonably stable year-round, allowing heat to be extracted in winter and dumped in summer with modest electricity consumption. Deep geothermal, on the other hand, drills wells hundreds or thousands of meters deep to directly access naturally occurring hot water or steam, and can generate both direct heat for industrial or district-heating use and electricity, if the resource temperature is high enough to drive a turbine.

Before producing anything: studies, platforms and drilling

Developing a deep geothermal project requires, before extracting anything, detailed geological studies to precisely locate the resource, followed by drilling that in many technical respects resembles the oil and gas industry: drilling rigs, steel for well casing, cement to seal it, and specialized pumps and pipes able to withstand high temperatures and, in many cases, corrosive fluids.

Once operational: continuous, compact, but not without complications

In operation, a geothermal plant can deliver heat or electricity nearly continuously, without depending on sun or wind, and with a small land footprint compared with solar or wind farms of equivalent power. However, fluids extracted from underground can contain dissolved salts and gases — sometimes corrosive or even slightly radioactive, depending on local geology —, requiring separation systems, reinjection of the used fluid back underground, and continuous corrosion monitoring in pipes and equipment.

The risk that demands active monitoring: induced seismicity

Extracting or injecting fluid under pressure underground changes the stress state of the surrounding rock, and under certain geological conditions can induce seismic activity. This risk is especially relevant in enhanced geothermal systems (EGS), which deliberately fracture low-permeability rock, with no prior natural reservoir, to create an artificial fluid-circulation system — this leaf's data tab develops a real case where this mechanism caused an earthquake with material damage and injuries, and explains why this risk is specific to that particular technology, not to all geothermal energy alike.

A finite resource at the scale of its exploitation

Contrary to the intuition of "infinite heat underground," a specific geothermal reservoir can cool down with use if heat is extracted faster than natural conduction replenishes it from depth. Wells also lose performance over the years from mineral scaling or corrosion, and the pumps needed to circulate the fluid consume electricity that must be subtracted from the net energy obtained. When closing a facility, wells must be carefully sealed to prevent underground fluids from contaminating nearby aquifers, and the surface site must be restored like any other industrial facility.

Where this technology really fits

Geothermal is especially valuable when the thermal resource is geographically close to specific heat demand — district heating, greenhouses, industrial processes — because transporting heat over long distances is far less efficient than transporting electricity. Before considering any geothermal project an obvious solution, it is worth always comparing the resource's real usable temperature, the pumping energy required, the fluid reinjection plan, local seismic risk and the reservoir's expected lifetime — confirming there is heat under the ground is not enough.

A local resource, not a universal source

In favor: dispatchable heat or electricity without depending on sun or wind, with a small surface footprint once operational. For direct thermal uses (district heating, greenhouses), it can be very efficient compared with burning fuel.

To watch: the resource is geographically limited — rock temperature, depth and permeability decide whether a site is viable, and not every country has good, accessible geothermal zones. Drilling shares much of the technical and environmental risk of the oil and gas industry: fluids with salts and gases, corrosion, and the need to reinject what is extracted.

The risk that demands active monitoring: injecting or extracting pressurized fluids underground can induce seismicity, especially in deep-rock enhanced geothermal (EGS) projects that deliberately fracture rock to create an artificial reservoir. It is not a theoretical risk: see the data tab for a real case that caused damage and injuries.

Before calling it "infinite heat": a reservoir can cool down if heat is extracted faster than the Earth replenishes it at that depth. Always compare usable temperature, pumping energy, reinjection plan, local seismic risk and expected well lifetime — not just the existence of heat underground.

The numbers, with their source

When fracturing rock goes wrong: Pohang, South Korea (2017)

An enhanced geothermal system (EGS) project injects pressurized water into deep, low-permeability rock to artificially create a network of fractures allowing hot fluid to circulate between two wells — in Pohang, at more than 4 km depth. Between January 2016 and September 2017, five successive hydraulic stimulations were carried out. Each generated minor seismicity swarms, until on 15 November 2017, two months after stimulation ceased, an earthquake of magnitude 5.4-5.5 occurred: the most damaging seismic event in South Korea since instrumental records began (1905). It caused 90 injuries and about 52 million dollars in damage. It is the largest known induced earthquake at an EGS project worldwide.

Sources: Science — Assessing whether the 2017 Mw 5.4 Pohang earthquake was an induced event; Stanford Report — Solving geothermal energy's earthquake problem.

Why this does not rule out all geothermal energy

Pohang's seismic risk is specific to enhanced geothermal systems (EGS), which deliberately fracture low-permeability rock where no natural reservoir existed. Conventional geothermal — drilling where naturally hot water or steam already exists, as in Iceland or much of shallow geothermal with heat pumps — does not share that same risk mechanism, although any pressurized fluid injection or extraction requires local seismic monitoring.

The comparative table

What this table is missing

There is not yet a consolidated global figure for geothermal-induced seismicity comparable to the mortality or emissions tables for other sources: each site has its own geology and its own risk threshold, which is why a serious assessment is always local, not a generic figure applicable to any geothermal project in the world.

Sources

• The IEA maintains a profile of geothermal energy.
• The reference study on the Pohang induced earthquake: Science — Assessing whether the 2017 Mw 5.4 Pohang earthquake was an induced event.
• Context and lessons from the South Korean case: Stanford Report — Solving geothermal energy's earthquake problem.