In an abandoned 4000 m deep gas well the GEOZENTRUM HANNOVER is testing a new concept for accessing heat from tight sedimentary rocks (triassic sandstone and claystone layers) in the Northern German Basin. Key technology of the ìGeneSys-Conceptî are massive waterfrac-tests, which after successful application in granite complexes (Hot-Dry-Rock-Research) are now applied to create extremely large fracture surfaces in tight sedimentary formations. Our hope is to enhance the productivity from these formations to an extent that commercially relevant production flow rates of geothermal fluid (> 25 m3/h) are to be achieved.

The concept foresees to re-inject the geothermal fluid via the annulus of the same borehole into a more shallow permeable layer. In case of success this single-hole concept will be applied on the campus of the GEOZENTRUM HANNOVER for space heating of its offices and laboratories (2 MWt). Since the geological conditions in the Northern German Basin are similar over wide areas this concept can be applied at almost any location in Northern Germany to supply heat for consumers of medium size.

During two massive waterfrac-tests in October/November 2003 at a depth of 3800 m approximately 20.000 m? of freshwater were injected at flow rates of up to 50 l/s and at wellhead pressures of up to 33 MPa. An analysis of the test results shows that the created fracture has a high storage capacity (> 50 m?/bar) and covers an area of several 100.000 m?, indicating that the fracture not only propagated in the sandstones, but also fractured the adjacent claystone layers.

Seismic monitoring during the stimulation did not reveal pronounced micro-seismicity as was observed during similar tests in granite. During venting tests 7000 m? of brine were recovered from the fracture and re-injected in a permeable crateceous layer at about 1000 m depth. The venting flow rate (for constant wellhead pressure) decreased proportional to 1/√t, which is typical for formation linear flow into a fracture of infinite hydraulic conductivity.

The long time extrapolation of the data showed that the flow rate would not be sufficient for geothermal energy production in the long run. Cyclic tests (Huff-Puff) with each cycle consisting of a cold water injection period, a shut-in period, and a venting period for producing the heated water however showed promising results and may be an alternative concept.

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