Wulf Brandt, Ali Saadat, Lothar Wohlgemuth, Ernst Huenges, Heinz-Gerd Holl, Inga Moeck

1. Drilling a geothermal well into a deep sedimentary geothermal reservoir – case study on the Groß Schoenebeck wells Wulf Brandt, Ali Saadat, Lothar Wohlgemuth, Ernst Huenges, Heinz-Gerd Holl, Inga Moeck

2. Geothermal Technology Development • The Rotliegend of the North German Basin has been investigated for the generation of geothermal electricity from low-enthalpy reservoirs in the in situ laboratory Groß Schoenebeck

3. Successfull Stimulation of Well Groß Schoenebeck 3 Enhancing the flow conditions within the reservoir is the key issue for the geothermal utilization of the low permeable Rotliegend formation. The results of • Reopening of the abandoned gas exploration well • Production tests • Comprehensive stimulation experiments justified the drilling of a second well.

4. Continued development of the in situ laboratory Drilling the second well Stimulation and test of the new well Completion of the first well GrSk 3/90 as an injector Testing the communication of the doublet … Demonstration of power generation

5. Borehole Design • The well design based on • Deep dynamic water table • The option for an additional protective casing string in the top hole region • The required minimum distance between the two wells in the target horizon • A drilling mud concept with a minimum damage potential • The increase of the flow potential by the inclined well as well as by (possibly) multiple fracs • The knowlegde about the geological conditions from the analysis of adjacent boreholes

6. Borehole Design of the Research Well GrSk 4

7. Drilling Rig VDD 370.2 (Streicher AG) Drilling contractor: Drilltec GUT GmbH, Deggendorf Lifting capacity 370 t Top drive up to 160 rpm Super single joints (13,5 m) Diesel-electro-hydraulically driven 5 mud pumps (max. 4000 l/min) Automatic pipe handling system with storage on ground Very low noise emission

8. Drilling Rig Particularities: Rigid guidance of the top drive requires a straight vertical starting hole (conductor pipe) Difficult handling of large diameters (18 5/8“ casing) and of joints with more than 13,5 m. Trip velocity is limited by the pipe handling system and requires a high level of the driller´s experience for high trip velocities (330 m/h have been reached)

9. Drilling Performance Planned performance based on the results of the first well could not be reached – the significantly larger diameter affects ROP much more than expected. Insufficient pumping capacity in the top hole region as well as improper bit selection reduced ROP in the mesozoic section (16“). 16“ PDC-bit after drilling of 7m unexpectedly abrasive sandstone

10. Drilling problems • Total fluid loss occurred during cementation of the combined casing 16“ x 13 3/8“ despite of a slurry density of 1450 kg/m³ To prevent casing damage due to thermally induced stress in the future of the production well, the annulus had to be cemented to the the surface. For this purpose, a squeeze cementation down from the well-head was designed and successfully performed.

11. Drilling problems • 9 5/8“ liner collapsed during drilling into the • target formation after reduction of the mud • density from 2000 kg/m³ to 1060 kg/m³ (Heavy • Deformation between 3880…3200 m) Ovality 8 mm • Causes not yet clear: • Design – according to the rules with an overburden pressure gradient of 2,3 • Casing material – certified quality • Anisotropic stress due to well inclination of about 20° in connection with anhydrite content of the salt?

12. Remedy for the collapse • Replacement of the collapsed 9 5/8“ liner by a 7“x7 5/8“ liner • after sidetracking Repeated attempts (4 times) to set the mechanical anchor of the whipstock required the modification of the anchor for a reliable operation in mud with 40% baryte content

13. Remedy for the collaps The loss of one casing dimension required the adjustment of the borehole design – drilling of the Rotliegend section with 5 7/8“ and running and cementing of a combined 5“ liner with an uncemented section of preperforated pipes at the bottom.

14. Drilling problems • HC - gas with H2S - content was encountered just below the 7 5/8“ casing shoe within the fissured Zechsteinkalk with a pressure gradient exceeding 1,2. The appropriate safety measures had to be taken first. • Thus the concept for drilling into the reservoir with minimum damage potential had to be adjusted: • The target of nearly balanced drilling with a reduced solid content could not be met • With a formation pressure gradient of 1,06 in the Rotliegend there was a permanent threat of differential sticking with a mud density of 1200…1160 kg/m³ while drilling and running the 5“ liner • Despite intensive treatment of the marble flour based mud (OptiBrigdeTM) we could not avoid small fluid losses while drilling the coarse sandstones of the Rotliegend formation.

15. Lessons learned • 1st. Well: • Reopening of an abandoned well turned out to be a cost saving option • Due to formation damage this option could be used for creating an injection well only • Various enhancing treatments were performed resulting in a significant increase of injectivity • 2nd. Well: • Reliable planning requires the input of all available neighbouring wells with respect to • Drilling experience • Geological and reservoir information

16. Lessons learned • Discussion about the necessity of large diameters seems to be necessary again taking into account all related problems during the lifetime of the well • Directional drilling proved to be a standard • The unexpected casing collapse within the salt formation requires further investigation for a better understanding – with the remedial liner we considered a safety factor against collapse of 1.8. • The mud controlled the bore hole even under complicated conditions • No sign of unstable borehole conditions occurred during the 20 days lasting rig repair without circulation • The high mud density in the Zechstein-formation protected a stable borehole • The drilling fluid succesfully formed the filter cake in the Rotliegend sandstones despite of the required overbalancing of the mud

17. Lessons learned • The problems arising from the gas indications could be solved successfully taking into consideration gas inflow prevention vs. danger of differential sticking • Drilling such challenging geothermal wells requires the union of sufficient experience, the appropriate idea to remedy unexpected situations as well as sufficient financial resources.