| Title | Productivity Enhancement by Hydraulic Fracturing and Circulation of Thermal Water at the Test Site Horstberg in the North German Basin |
|---|---|
| Authors | Torsten TISCHNER, Alireza HASSANZADEGAN, Stefanie KRUG, André STECHERN, Kalliopi TZOUFKA, Reinhard JUNG |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | hydraulic test, hydraulic stimulation, fracture, North German Basin, Horstberg |
| Abstract | The well Horstberg in the North German Basin is a geothermal research site of the Federal Institute for Geosciences and Natural Resources (BGR) in Germany. The deep well is completed in an alternating sequence of clayey and sandy Triasic sediments (Buntsandstone) with a final depth of 4.120 m. The experiments in Horstberg aim towards the development of geothermal utilisation concepts in low permeable sedimentary rock. The investigated key method is hydraulic stimulation. In 2003 first hydraulic stimulation experiments were conducted in the very low permeable sandstone of the Volpriehausen sequence at 3.920 m depth. Access to the formation was achieved by a special perforation technique, by a propellant stimulation. A surprisingly high wellhead pressure of more than 400 bar had to be applied for fracturing and limited the execution of the stimulation operations. In the largest stimulation test about 600 m3 of freshwater were injected at a pressure of about 460 bar. Succeeding venting tests revealed a poor fracture performance and no significant productivity enhancement. In contrast, in the Detfurth sandstone some 130 m above the Volpriehausen, a successful hydraulic stimulation was conducted. By the injection of about 20.000 m3 freshwater without proppants or additives a large fracture area was created. Based on pressure decline analysis and on fracture propagation modelling the created fracture area could be estimated to about 350.000 m2. After the stimulations several hydraulic tests showed, that a significant part of the originally created fracture remained highly conductive even at a pressure below the undisturbed fluid pore pressure. An extended formation linear flow regime was observed from which a remaining “infinite†conductive fracture area in the order of 10.000 m2 was derived. Over several years no deterioration of the fracture performance was observed. The comparison of hydraulic tests before and after the stimulation indicates a more than tenfold increase in productivity. However, the productivity is still insufficient for commercial energy production in a geothermal doublet system. Only if the created fracture acts as heat exchanger itself the geothermal utilisation might be possible. The great differences in the success of the hydraulic stimulation operations in the two sandstone layers are noteworthy. Reasons for the poor fracture performance in the Volpriehausen might be the type of perforation, the low stimulation volume and the different state of stress. Due to the applied special perforation technique (propellant stimulation) some deep channels might have been created which are not favourable aligned to the stress field and thereby lead to friction losses while producing. Further, a large injection volume seems to be necessary to induce a substantial and remaining fracture and the stress conditions are less favourable in the Volpriehausen for shearing. Due to the stimulation operation in the Detfurth sandstone a hydraulic link to the upper and significantly higher permeable Solling sandstone was formed. This opened up the possibility for thermal water circulation between two sandstones via an artificial fracture. A first circulation experiment was conducted already in 2004. Now further circulation experiments are planned in order to study the fluid flow over the fracture and to determine its potential for heat extraction. The hydromechanical properties of the fracture are of special importance in those circulation experiments. |