Homuth, S. ; Hornich, W. ; Bär, K. (2016)
Down-the-hole water powered hammer drilling for medium deep geothermal energy systems.
European Geothermal Congress. Strasbourg (20.-22.09.2016)
Konferenzveröffentlichung, Bibliographie

Kurzbeschreibung (Abstract)

The main part of the overall energy demand in the northern European Union is due to building heating purposes. Therefore, groundbreakingground-breaking techniques are needed to save energy and reduce greenhouse gas emissions especially in this low exergy sector. The technique of using water instead of air as an energy carrier in down-the-hole (DTH) drill hammers has been known for years in the mining and geotechnical industry. Recent technical developments in terms of tooling and drilling fluid circulation systems enables the utilization of this drilling method for medium deep (400 - 1000 m) geothermal energy applications. Especially geothermal heat storage in moderate depths is an innovative and yet barely tested concept. In difference to shallow heat storage systems, this approach upgrades the naturally available geothermal energy in the subsurface by means of external heat input and stores the heat below the general depth of aquifers used for drinking water production. This is done in summer when no space heating is required or at times when surplus energy from nearby sources is available. In winter when other sources of energy are not sufficiently and cheaply available, the thermal energy from the geothermal storage is used for heating purposes. The system of a medium deep borehole thermal energy storage (MD-BTES) consists of multiple boreholes with depths of 400 m – 1,000 m b.g.s completed as coaxial borehole heat exchangers. The surrounding rock is utilized as heat storage, the cementation and borehole wall function as heat exchanger. Typically, water is used as heat carrier fluid. An important factor for the economic feasibility of MD-BTES systems are the drilling costs. A competitive and innovative drilling method is a prerequisite. The technique of using water instead of air as an energy carrier in down-the-hole (DTH) drill hammers has been known for years in the mining and geotechnical industry. Recent technical developments in terms of tooling and drilling fluid circulation systems enables the utilization of this drilling method for medium deep (400 - 1000 m) geothermal energy applications. Improved cutting transport, fast penetration rates, increased hole stability and enhanced deviation control (less than 5 to 10 % vertical deviation angle compared to 10 to 40 % with pneumatic hammers) are reasons for the water powered hammer drilling application. Especially a minimized deviation from the vertical is a crucial prerequisite in borehole heat exchanger (BHE) fields, where usually less than 10 m spacing between single BHEs is applied. Additionally, a reduction of CO2 emissions reduction can be achieved due to the fact that transmitting energy by water hydraulics instead of compressed air can reduce diesel consumption to only 1/3 or less. Dust is eliminated and the drilling fluid is oil free and without grease residues. Also no fluid additives like bentonite are necessary which allows the application of this drilling method in water protection and zones. The water hammer technique has high requirements in terms of water quality. This motivates the use of a drilling fluid cleaning system for the re-use of the water. A modular drilling fluid recycling system adaptable to specific rock conditions is designed to recirculate the drilling fluid and to improve the ecological and economical footprint of medium deep drilling projects. The water powered DTH hammer drilling method can be utilized with small truck-based drill rigs and corresponding small site requirements which results in lower compatible price ranges for medium deep drilling operations compared to conventional rotary mud drilling methods.

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