The original approach of a single borehole to recover hot water for direct use (heat) from tight sedimentary rock formations at great depth has been tested in the abandoned gas well Horstberg Z1 in the Northern German Basin about 80 km NE of Hannover. The concept assumes that though the overall permeability of these formations is low, open flow paths (faults, fracture zones, or intersections of them) exist even at great depths and that these paths can be accessed from a borehole by creating extremely large fractures opened by hydraulic fracturing. The hot water produced will then be reinjected after use via the annulus in the same borehole into a permeable rock formation at more shallow depths. Massive water-frac tests were performed in a sandstone layer of the Middle Buntsandstein-formation (Detfurth) at a depth of 3800 m by injecting more than 20,000 m3 of fresh water at flow rates up to 50 l/s and at a wellhead pressure of about 330 bar. Fracture propagation was monitored by a comprehensive geophysical programme including microseismicity, electric self potential and tiltmeter measurements.
The results of post-frac hydraulic experiments and several production tests showed that in the case of the test well Horstberg Z1 the original concept proved only partially successful: extrapolations showed that the required flow rate of 25 m3/h can not be maintained due to the insufficient yield of the formation accessed by the fracture. Therefore alternative concepts taking advantage of the hydraulically produced tensile frac with high storage capacity and substantial transmissivity were developed and then successfully tested. First a cyclic “huff puff” production and injection pattern using the frac plane as a cyclic heat exchanger. Second a deep circulation concept with communication between two sandstones layers connected by the frac and separated by a packer within the well. By production from one sandstone layer and reinjection into another sandstone layer, a continuous circulation similar to a borehole doublet is created. Numerical modelling shows that these concepts could provide the required thermal power of approximately 2 MW for more than 20 years.
The GeneSys single-well-concept (Generated Geothermal Energy Systems) is considered to be economically relevant for consumers of medium size (a few MW thermal power) and will be demonstrated in a 3800 m deep borehole planned for 2009 on the campus of the GEOZENTRUM Hannover for space heating of its offices and laboratories.
R&D at the well site Horstberg is funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety.
Promotion / Document number: BMU Grant Number: 0327116 (until 2005), 0329996 (2006-2008)