Journey to the centre of the Earth

Materials World magazine
,
1 Jun 2018

Exploiting the heat supply from the Earth’s core requires tools and materials that can withstand extreme conditions. Khai Trung Le looks at solutions from the DESCRAMBLE project.

Capitalising on the inexhaustible ambient heat of the Earth’s core is an attractive solution to the global energy crisis. But there is one problem, or specifically a few hundred of them – around 2km into the interior of the Earth, supercritical water is generated from temperatures as high as 600˚C, which means techniques, equipment, and instruments able to handle the heat need to be developed.

The recently concluded EU project, Drilling in deep, super-critical, ambients of continental Europe (DESCRAMBLE), has spent three years exploring safe drilling in high temperature and pressure and testing novel drilling techniques to control gas emissions, aggressive environments, and characterise the chemical and thermo-physical conditions.

Supercritical wells can be as efficient as 10 standard geothermal wells. But while an average geothermal well holds an ambient temperature of 350˚C, supercritical wells develop at temperatures around 400˚C at 220 bar pressure. In its supercritical phase, the water is highly corrosive to drilling and inspection equipment.

The DESCRABLE team – Enel Green Power and the National Research Council of Italy, Aachen, Kiel, and Freiberg Universities, Germany, and Stiftelsen for industriell og teknisk forskning (SINTEF), Norway – drilled a test well in Larderello, Italy, where an extensive history of geothermal energy exploitation meant there was existing infrastructure and a wealth of data for comparison. However, the DESCRAMBLE team state, ‘the expertise gained from drilling in super-critical reservoirs in Larderello is not limited to this specific location. There are deep, super-critical geothermal reservoirs in several countries in Europe and the rest of the world, at greater, but still drillable depths’.

Eclectic electric

The limiting temperature of electronics is around 250˚C, restricting the range of components available. Batteries are particularly susceptible, with the most robust battery technology available commercially operating at temperatures between 70–200˚C, and exploding at 215˚C.

Magnus Hjelstuen, Research Manager at SINTEF Harsh Environment Instrumentation, told Materials World that his group developed a wireline logging tool that measures temperature and pressure data to indicate when a drill bit has entered a zone featuring supercritical water, as well as revealing the well’s geothermal properties. ‘Our challenge has been to find a combination of existing components that can perform optimally within our limitations in terms of instrument length, weight and diameter, not least in view of the environment the equipment will encounter in the well,’ he said.

‘Since we began drilling at normal surface conditions, the temperature on a winter’s day may be as low as 0˚C initially, rising to over 400˚C at the bottom of the well. We created a reverse Thermos flask that kept equipment below 210˚C to reduce the strain on the equipment.’

The DESCRAMBLE project was able to carry out measurements at 2.18km depth, at temperatures reaching 443.6˚C. However, Hjelstuen stated they were unable to find a site where water flowed into the well. ‘We’re working on a new EU funding scheme so we can take the project to the next level. We want to drill even deeper – 3.1km – where we hope to find a water source from which we can test energy production.’

Despite the team’s limited success, Hjelstuen is optimistic about the future benefit of geothermal energy from supercritical wells. He said, ‘If we can succeed in exploiting geothermal heat, there will be enough of it to supply the entire population of the planet with energy for many generations. There are nuclear power plants where water that meets the conditions for supercritical water is fed through turbines, so we know that we can succeed in exploiting the energy once we manage to extract it.’