The total quantity of the earth’s geothermal energy is estimated as 3.5×1015 TWh. In other words, there is sufficient to satisfy the current global consumption of energy for the next 500 000 years. Our main challenge is develop efficient means of accessing that energy for exploitation. It is common for most thermogeologists to distinguish between shallow and deep geothermal energy:
- Deep geothermal energy is heat from depths greater than 300 meters. This is replenished by energy from the radioactive disintegration of naturally-occurring radionuclides in the bedrock. The disintegration process creates heat that is stored in the bedrock.
- Shallow geothermal energy, also called ground source heat, refers to the energy stored in the upper 300 meters of the ground. Once this heat is extracted, the main source from which it is replenished is absorption of solar or atmospheric energy by the ground surface. Exploitation of ground source heat is a known and well-functioning technology.
Data reported from 72 countries in connection with the World Geothermal Congress in 2005 show that the production of geothermal energy in 2004 was 76 and 57 TWh for heat and electricity production, respectively.
In the text that follows here, we will consider only deep geothermal energy (and refer to the shallow variant simply as “ground source heat”):
Two kinds of reservoirs
Schematic illustration of a Hot Dry Rock (HDR) system. Illustration: Earth and Environmental Sciences, Los Alamos National Laboratory, USA There are, simply speaking, two kinds of geothermal reservoirs, Hot Dry Rock (HDR) and Hot Wet Rock (HWR).
A HDR-system targets deep, hot, low permeability (“dry”) bedrock. In order to extract heat, channels have to be created in the bedrock through which a heat transfer fluid, such as water, can be circulated.
These channels may simply be bored holes, but are most commonly artificially created fractures (produced by a technique called hydraulic fracturing, involving injection of water at very high pressure).
A HWR-system utilizes deep, warm, permeable bedrock aquifers or regional fracture systems and the heat transfer fluid is natural groundwater. Production- and reinjection wells are typically established in the geothermal aquifer. The bedrock in Norway is generally of very low permeability and is better suited to HDR-plants. However, permeable rock units or fracture systems that may be exploitable as HWR-systems can be found at certain locations.
No plants in Norway
Unfortunately, Norway is a cold country when it comes to geothermal energy as well as surface climate. Geothermal temperature gradients vary between 10 and 30°C/km. Although both Sweden and Denmark have district heating systems based on geothermal energy, no deep geothermal plant has hitherto been established in Norway. In 1999, a pilot project was initiated at Rikshospitalet (the National Hospital) in Oslo. The ambition was to exploit 2 MW heat by circulating water in a closed system down to 5400 meters depth. The project was run by Geovarme AS, who developed mathematical models for the optimization of drilling patterns and energy extraction. A special drilling technique was also developed in the project. Due to unforeseen geological challenges at 1600 meters depth, the expenses became too large, and the project had to be halted before its completion.
Heat production in Norwegian bedrockNGU has drilled several boreholes down to 800 meters during the past few years in order to enhance our knowledge of the temperature and geothermal properties of the ground at such depths. See information on heat flow.
The Danish geothermal pilot plant at Margretheholm. The plant is a HWR- system. Illustration: Dong EnergyThere are obvious challenges related to the drilling of deep boreholes, but detailed geological and geophysical pre-investigations will reduce this risk. Currently, we look to the offshore oil industry to assist us with the equipment and technical know-how to drill deep boreholes onshore.
Drilling costs typically represent the largest up-front cost in the development of a new geothermal plant and any innovative, cost-reducing technology is welcome. In an early phase of the planning of a geothermal plant, one of the first tasks will be to find an appropriate geological location: an ideal site would comprise a hot reservoir rock at depth, with a high radionuclide content, overlain by softer, homogeneous, easily drillable rock cover that will keep the rate of bit wear and the risk of engineering problems to a minimum. Such cover rocks would also ideally be of a low thermal conductivity, in order to “insulate” the target reservoir. By the use of heat pumps, relatively low temperature heat resources (40°C or less) could be exploited - this only requires comparatively shallow drilled depths.