The GeneSys-Project: Extraction of Geothermal Heat from Tight Sediments

Abstract

A single hole concept to recover hot water for direct use from tight sedimentary rock formations at great depth has been tested in an abandoned gas well in the Northern Ger-man Basin about 80 km NE of Hanover. The concept as-sumes that though the overall permeability of these forma-tions is low open flow paths (faults, fracture zones, or inter-sections of them) exist even at great depths and that these paths can be accessed from a borehole by creating ex-tremely large hydraulic fractures. The hot water produced from these features will be reinjected after use via the annu-lus of the same borehole into a permeable rock formation at more shallow depths. The GeneSys concept (Generated Geothermal Systems) can be economic for consumers of medium size (a few MW) and will in case of success be applied in a 3500 m deep borehole planned for 2005 on the campus of the GEOZENTRUM Hannover for space heating of its offices and laboratories. The concept should be appli-cable at almost any site in the Northern German Basin and is attractive especially for sites where abandoned gas or oil wells are available.

In order to test the concept massive water-frac tests were performed in a sandstone layer of the Buntsandstone-formation at a depth of 3800 m by injecting more than 20,000 m? of water at flow rates up to 50 l /s and at a well-head pressure of about 330 bar. Post-frac venting tests showed that the created fracture has a high storage capacity (about 100 m?/bar) and covers an area of several 100.000 m?, indicating that the fracture not only propagated in the sandstone layer, but also fractured the adjacent clay-stone horizons. They also showed that the fracture or at least part of the fracture stayed open during pressure release thus allowing venting flow rates of about 30 m?/h at fluid pres-sures well below the frac-extension pressure. Long term extrapolations of the venting flow rate however showed that the desired flow rate of 25 m?/h can not be maintained over a prolonged time period since the production and reinjec-tion horizon (at 1200 m depth) do not communicate and the overall yield of the formation accessed by the fracture is too low.

The results of cyclic tests ("huff-puff"), consisting of a cold water injection period, a warm-up period and a venting period on contrary were very promising. The fluid volumes and production temperatures achieved during these tests show that this can be an alternative concept for heat extrac-tion from tight sedimentary rock.