With increasing depth, geothermal energy offers an almost inexhaustible potential for renewable energy. The drilling costs however, rise exponentially with depth in the case of conventional rotary drilling. A thermal drilling method, which will allow for reaching greater drilling depths in a more efficient and more cost-effective way, is currently being developed at the ETH Zurich.

Tobias Rothenfluh, a doctoral student at the Institute for Process Engineering, climbs up a small ladder into the three-story pilot plant. Pipelines lead through metering and safety valves into the reactor, which is affectionately known as “Betsy”. Inch-thick plates, made of heat-resistant steel, prevent the reactor from bursting, even at a pressure of 300 bars. “In our experimental reactor we are able to ignite a flame underwater at a pressure of around 250 bars and 450 degrees Celsius” says Rothenfluh. “Thus we are able to experimentally simulate the temperature and pressure conditions prevailing in a borehole, about three kilometers below the earth’s surface.” He has constructed a first burner prototype over the last few months together with his colleagues Martin Schuler and Panagiotis Stathopoulos.

Laboratory experiments at high pressure

Heated oxygen, ethanol and water are pumped into the reactor burner through various pipelines and valves and mix under temperature and pressure conditions, which correspond to the supercritical state of water (see box). The auto-ignition of the mixture is being observed through small sapphire-glass windows by means of a camera. A newly developed sensor plate measures the heat flux from the flame to the plate and records the temperature distribution on the surface for different distances between the burner outlet and the plate.

Based on these experimental results, conclusions are drawn concerning the heat transfer from the flame to the rock. “The heat flux is the crucial parameter for the characterization of this alternative drilling method”, explains Philipp Rudolf von Rohr, professor at the Institute of Process Engineering of the ETH Zurich and supervisor of the three PhD students.

Erosion in fast motion

During the experiment, the flame reaches a maximal temperature of about 2000°C. Rapid heating of the upper rock layer induces a steep temperature gradient in it. “The heat from the flame causes the rock to crack due to the induced temperature difference and the resulting linear thermal expansion”, explains Tobias Rothenfluh. The expansion of the upper rock layer causes natural flaws, already existing in the rock, act as origin points for cracks. Disc - like rock fragments in the millimeter scale are formed in the spallation zone. These particles are transported upwards with the ascending fluid stream of the surrounding medium. “One of the main challenges of the spallation process is to prevent the rock from melting, whilst it’s being rapidly heated”, says Tobias Rothenfluh. “The lager the temperature gradient in the rock, the faster you can drill.”

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The method is particularly suitable for hard, dry rock, normally encountered at depths greater than three kilometers. In such depths conventional drilling bits wear out much faster, and their frequent replacement, renders the conventional drilling techniques uneconomic: a 10 km borehole costs around 60 million US dollars. In the case of the so-called “hydrothermal spallation drilling” method, however, the burner - drill bit wear is considerably less, because there is no mechanical contact with the rock. “It is expected that the drilling costs will rise linearly with depth in the case of spallation drilling, instead of exponentially, which is the case of the conventional methods”, says Philipp Rudolf von Rohr.

Simulation and demonstration

In order to test the flame’s behavior under different conditions, the doctoral student Martin Schuler is developing a tool for the numerical simulation of the reaction and transport processes in cooperation with the master’s student Karl Goossens. “The simulation enables us to change and optimize parameters like fuel mass flow rates, temperature and pressure, as well as the geometry of the burner”, says Martin Schuler.

The experimental results from the current test set-up are being used to design a pilot plant, on which Panagiotis Stathopoulos is working. The 1.2 million-Swiss-franc plant should demonstrate that it is actually possible to drill through rock by means of hydrothermal flames. The project is funded by the Swiss Federal Office of Energy, the industrial organization swisselectric research, ETH Zurich and the Swiss National Science Foundation.

Research in breadth and depth

The interest of federation and industry confirms the high potential of “hydrothermal spallation drilling”. Some time will pass until the method is industrially applicable, but the feasibility is undoubted so far. “It is for sure possible to speed up the project towards the industrial application”, Philipp Rudolf von Rohr realizes, “but we still want to focus on basic research at a university like the ETH Zurich”.

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After all, the Institute of Process Engineering is currently worldwide the only group investigating the heat transfer characteristics of a flame in supercritical water. “We literally want to research in both depth and breadth”, says Tobias Rothenfluh. In the future, the knowledge acquired might be useful not only for geothermal energy, but also for other applications.

Supercritical water

Above a temperature of 374.12°C and a pressure of 221.2 bars, water vapor and liquid water can no longer be distinguished from each other in terms of their density. In this supercritical state, water is less polar, has no phase boundaries any more and is a good solvent for non-polar gases like oxygen. Under these conditions fuel and oxygen can be mixed without any bubble formation and in the case of ethanol as fuel, auto-ignition occurs at approximately 450°C.