Hunting for heat, deep in the Earth
Hunting for heat, deep in the Earth
Ever since Jules Verne’s 1864 novel “ A Journey to the Centre of the Earth”, people have dreamt of capturing the heat of planet Earth. It exists in huge amounts, is completely renewable and emits no CO2.
“What we have done so far is no more than to scratch the surface of the Earth. The heat that many people extract from their gardens and then upgrade in heat pumps is not geothermal, but rather solar energy,” says SINTEF research scientist Alexandre Kane.
Backed by a troop of industrial and technology companies, Kane is manager of the Nextdrill research project, whose members are going ahead at full speed to develop drilling tools that will make it profitable to exploit true geothermal heat.
Stringent cost requirements
The immediate aim is to drill wells to depths of five to six kilometres, where we encounter temperatures that are high enough to allow the heat to be used for district heating and electricity generation.
- According to the International Energy Agency (IEA), the upper few kilometres of the Earth’s crust contain two hundred million times as much thermal energy as the Earth’s annual energy consumption.
- The temperature increases with depth; in Norway, a typical figure is 17 – 20 oC per kilometre.
- The heat can be supplied to district heating plants. If we drill sufficiently deep, the water will reach the surface in the form of steam that can be used in industrial processes or to generate electricity in steam turbines.
- The useful life of a five kilometre-deep well is estimated to be about 30 years, after which the bedrock will have been cooled so much by the cold water that has been pumped through it that it not longer makes sense to bring up heat.
- After a further 20 – 30 years, the heat will have built up again and the well can be brought back into service.
High speed; long working life
As in all drilling operations, the taximeter rises rapidly. If geothermal heat is to be competitive as a source of energy, the time put into drilling must be kept to a minimum. Drilling operators who want to capture this heat therefore need to be able to drill at high speed. Nor can they afford the loss of time involved if the drill-bit is always having to be brought to the surface and replaced.
The Nextdrill project is a response to this challenge. Three of its members – SINTEF, the Swedish company Sandvik and Germany’s H.C. Starck – are collaborating on the development of materials for a drill-bit with a long working life.
Another participant is the Norwegian technology company Resonator, which is in the process of developing an electric percussion rotary drill, a tool that crushes rock by dealing it hammer-like blows as the drill-bit turns. Electrical operation offers the possibility of remote control and more energy-efficient drilling systems than technology based on today’s pneumatic or hydraulic systems.
First test in August
In the course of this year the Nextdrill project will carry out its first small-scale drilling trials near Ås in Akershus County. In August and again in November, a specially designed version of Resonator’s percussion rotary drill will tackle hard rock. It will be fitted in turn with commercially available drill-bits and bits made of the highly wear-resistant materials that are being developed by the project.
These trials have two main purposes. They will provide new knowledge about how wear occurs on drill-bits when rock is crushed using an electric percussion rotary drill. The tests will also show how the number of impacts per unit time affects the speed of drilling.
“Although we will not be drilling very deep during these tests we do expect to gather important data for the next stages of our efforts to develop highly durable materials,” says Kane.
The heat that SINTEF’s French project manager wants to capture is known as geothermal energy, and is derived from two sources that lie far beneath our feet. About one third of it is heat that has been stored in the Earth’s molten core since our planet was formed. The other two-thirds have their origin in the decay of radioactive isotopes in the Earth’s crust. This process releases heat, which means that the temperature rises, metre by metre, the further we drill into the interior of the planet.
Two types of well need to be drilled to exploit this heat; one to pump cold water down, and the other to bring hot water up again (see illustration).
- Four-year competence-building project with user participation. Started in 2012. Research partners: SINTEF Materials and Chemistry, IRIS (International Research Institute of Stavanger), NTNU and the University of Stavanger.
- Financially supported by the Research Council of Norway and a consortium consisting of the industrial and technology companies Sandvik, H.C. Starck, LKAB Wassara, BP, Statoil, Resonator, Norhard and Rock Energy AS.
Laboratory trials and computer models
The drill-bit needs to be able to withstand a high level of friction, in addition to enormous amounts of mechanical abuse resulting from the high-frequency hammer impacts.
“Laboratory trials and virtual experiments performed by computer models have taught us a great deal about drilling through granite, and have enabled us to develop models that we use to find the optimal form and composition of the drill-bit. The drilling trials at Ås will give us measurements that will let us further improve the computer model,” says Kane.
The drilling equipment
- The Nextdrill project is developing materials that will give drill-bits an extremely long useful life. The project uses materials such as tungsten carbide and cobalt. Tungsten carbide is extremely hard, and will protect the drill-bit against wear. Cobalt is a hard brittle metal that absorbs energy from impacts.
- In the course of the development of new materials, the drill-bits will be tested via small-scale drilling trials in hard rock near Ås in Akershus County. The bits will be installed on a special version of an electric percussion rotary drill that is currently being developed by the Norwegian technology company Resonator AS.
- The rotary drill being developed by Resonator can be customised for drilling under different geothermal heat conditions. According to managing director Svein Hestevik, electrification of drilling operations is a prerequisite for the profitability of deep “heat wells”.