Common to geothermal areas are hard, volcanic, abrasive and heavily fractured formations prone to circulation loss, increased tortuosity, heavy dog legs, high tool wear and resulting low penetration rates. Conventional rotary drilling tools i.e. tri-cone tungsten carbide insert drill bits have proven performance to temperatures of approximately 180°C with directional drilling systems being able to resist temperatures up to 225°C (Stefánsson et al. 2018). Such conditions are much below those encountered during various high-temperature drilling campaigns in Italy, Iceland or Japan. With conventional mechanical drilling technologies, penetration rates between 1 to 6 m/h are expected. This means that rock breaking and removal should be greatly improved in order to reduce the drilling cost. Developing technologies such as metal-to-metal sealed drill bits and directional systems, hybrid bits, down-to-hole fluid hammers or non-contact and wear-free technologies i.e.: laser, plasma or electro-impulse drilling might allow for much faster and problem-free drilling for deep geothermal resources with extreme downhole temperatures.
The high temperature of drilling fluids circulating inside the borehole eventually damages to any type of elastomer part within downhole drilling or completion equipment. This eliminates the possibility of using most types of cementing, directional drilling equipment and conventional drill bit technology. During the Venelle-2 drilling campaign, at the Larderello geothermal field in Italy, a special type of PDC bit without elastomer parts was utilized. This kind of ‘all metal’ drill bit is able to operate under extreme temperatures, not typically seen in the petroleum industry. The above-mentioned drill bit technology delivered positive results in terms of drilling progress and durability (Bretani et al. 2018). In the IDDP-2 drilling campaign, in SW Iceland, high-temperature tri-cone rotary and hybrid drill bits rated for drilling fluid circulation temperatures of up to 300°C with a specially designed high-temperature grease and metal-to-metal seals were used. The drilling equipment proved to function without major problems and provided sufficient penetration rates and bit life (Stefánsson et al. 2018).
Fig. 1: Cones and bearing from high-temperature tri-cone roller cone drill bits used during drilling of the IDDP-2 well in the Reykjanes geothermal field in Iceland (Stefánsson et al. 2018)
During the IDDP-2 venture, together with temperature resistant drill bits, a prototype of an elastomer-free directional drilling system with metal-to-metal seals, intended for EGS applications with an aggressive fluid environment and high-temperature downhole conditions (up to 300°C), was implemented. The evaluation made after consecutive bit runs with the metal-to-metal directional drilling system in the IDDP-2 well, proved wear of rotor and stator. The power section was still operational and provided torque values according to the manufacturer’s specifications (Stefánsson et al. 2018). In general, the prototype of a directional system for geothermal wells up to 300°C proved to work successfully during the IDDP-2 drilling operations (Friðleifsson et al. 2017a and 2017b).
New wear-free and contact-less drilling technologies are being currently developed at various institutions around the world. The thermal drilling methods include flame, plasma, spallation, laser, and quasi-thermal electro-impulse drilling technology. Thermal energy is used to weaken the hard rock and create spalls. Subsequently, the spalls with weakened rock are being later mechanically crushed and circulated out of the hole. Efforts within different thermal drilling technologies are ongoing in research institutions in Germany, Slovakia, and Switzerland. Another group of new technologies includes fluid-assisted drilling with high-pressure water jets. This technology is currently being researched in countries such as Germany and Austria.
Fig. 2. The drill bit of the laser drill string with emerging laser water jet (source: Fraunhofer IPT)
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Article based on a study by Kruszewski and Wittig titled “Review of failure modes in supercritical geothermal drilling projects” published in Geothermal Energy (2018) 6:28 (doi.org/10.1186/s40517-018-0113-4).