For the drilling and the completion of shallow geothermal wells (to 400 m) a lot of technical improvements were developed over the last 20 years. But wells are drilled with 2 m short pipes and casings still, which takes a lot of time and logistic efforts.

Using a Coil Tubing Rig with a 500 m long coil the drilling will be much faster and will deliver a better borehole integrity (hole geometry), because flushing the hole while drilling is not interrupted when drill pipes have to be added. The non- productive time will be much shorter because the BHA can be pulled from 500 m depth to surface in less than 40 minutes.

The transport dimensions of such a Coil Tubing Unit are 2.7 m width and 3.1 m height with a length of 6 m only. The tracks of the rig are 60 cm wide and 4.00 m long. By these dimensions and a perfectly balanced centre of gravity a good stability during moves and drilling operation is guaranteed even on slightly inclined drill pads. Drill pad size required is much smaller than for conventional rigs and doesn’t need a lot of improvement neither levelling, because the rig can correct up to 10 degrees inclination and can operate on uneven grounds.

Hydraulic downhole hammer drilling technologies have been widely used in the mining as well as O&G industry to speed up especially hard rock drilling operations. Typically, any DTH hammer principle is based on an axially reciprocating piston, powered via intensified fluids. Subsequently, the rather dynamic forces are being transferred onto a drill bit to crush the rock ahead. All of today’s hydraulic DTH percussion systems have a certain water quality limit, needing rather clean fluids for smooth operation, whereby the fluid quality heavily impacts the service and tool life. Also, wellbore control and cuttings transport are more of a challenge with current, water powered DTH tools compared to standard mud rotary drilling technologies. Therefore, an all-new DTH hammer concept for deep, hard rock drilling applications has been designed and developed at IEG based on a new control switch for the percussion mechanism instead of a mechanical system. The final percussion mechanism and DTH hammer works with only one single, main moving part inside and without the need for overly accurate tolerances and, therefore, does tolerate much better low-quality water / fluids and drill mud without excessive wear. The development of this prototype percussion hammer system was realized via iterative design, experimentally as well as numerical simulation work. A final, 4 inch DTH hammer percussion unit has successfully been validated and tested at the Fraunhofer IEG drill site in Bochum, Germany.

Due to accumulations of minerals in deep, geothermal type waters or brines, massive precipitation and deposits, so called scaling, repeatedly do occur during thermal water production out of geothermal wells as pressures and temperatures are changing. Especially during thermal production from carbonate type reservoirs, large amounts of e.g. calcium carbonate are deposited inside casings and other production equipment. As a result, pipe´s cross sections and thus, flow rates, are being reduced, decreasing overall thermal output and efficiency of such wells and power plants. Today’s methods for scaling removal, having been mainly used and developed in the oil and gas industry, are rather costly and time consuming, and, moreover, may involve large amounts of water / drilling fluid and environmental pollution. Thus, the occurrence of scaling and its required removal does pose some major challenges for operators of many geothermal power plants.

Therefore, developing a for-purpose scaling removal workover procedure especially for geothermal type wells, only requiring the use of ambient well water encountered in the well is rather desirable. Such an environmentally sound scaling removal, mechanical drilling process has been developed and tested at Fraunhofer IEG together with the geothermal Power Plant in Traunreut, Bavaria, Germany, back then still operated by Grünwald Equity. The developed, mechanical scaling removal process is based on multiphase, underbalanced drilling conditions (UBD). Results of the successful developments and full scale field operation in Traunreut in 2021 are being presented here.

The European funded GEOTHERMICA GRE-GEO project rallies a multinational consortium of geothermal experts to develop a new glass fiber reinforced epoxy casing system for geothermal wells. Such a system would solve the corrosion and scaling challenges of conventional steel based well designs.

Glass Reinforced Epoxy (GRE) tubular are already being used for decades in highly corrosive oil wells (e.g. H2S, CO2, Sulfide Reducing Bacteria based corrosion). However, the industry-standards describing such tubular are missing and design envelopes representing their load capacities have not been verified. This presently limits the down-hole use of GRE tubular.

One of the objectives of the project is the development of product qualification procedures to provide the basis for the construction and verification of a final product suitable for installation in the conventionally used well designs.

In order to provide these procedures, a Full-scale performance-based test program on GRE down-hole tubular is presently being developed by the GRE-GEO consortium. This includes the tests carried out under external pressure in combination with axial loads, which are novel for fiberglass casings and have created new insights in the collapse failure modes and have stimulated the development of dedicated test methodologies and test instrumentation.

Another critical subject is the definition of the threshold (condition, under which the first damage occurs to the pipe) for the different loads, which must be in line with ISO14692. But also, to try to combine this design methodology with the traditional ISO13679 used for qualification of casing connections.

Cementing is one of the most critical steps during the drilling process of geothermal wells. Here, we employ many techniques and technologies well-known in the oil & gas industry. However, weak formations and CO₂-containing formation water may entail the use of specially customized recipes. Thus, a dedicated design based upon extensive lab research and thorough engineering is crucial.

This paper presents our dream team for geothermal projects in the Netherlands. Here, to counteract losses into weak formations, the use of lightweight slurries is essential. The HOZlite consists of blast-furnace slag cement and contains hollow spheres providing low densities with a high compressive strength after hardening. This system provides excellent mechanical properties as determined via tri-axial tests. The HMR⁺ Blend, on the other hand, is chemically and physically optimized ensuring durability of the resulting sheath, even in the presence of CO₂. The gas-tightness of the hardened system was confirmed via lab experiments with H₂.

Recently, we employed the well-established combination of HOZlite (lead @ 1.35 kg/L) and HMR⁺ Blend (tail @ 1.88 kg/L) in the 20″, as well as in the 16″ casing section with great success. Prior to the actual cementation, our abrasive spacer effectively removed residual mud and its filter cake facilitating premium cement bonding. Thus, laboratory and field results impressively proved the premium properties of our new technologies for cementing geothermal wells.