In-situ experiments
Since 2003, the well bore Horstberg Z1 serves as an in-situ laboratory for the development and installation of new concepts for deep geothermal heat extraction. Mainly, the cyclic (Huff-Puff) and the two-strata deep circulation concepts have been implemented successfully. Hence rocks in 3.700 m depth elude a direct examination, the interpretation of the reservoir’s transient pressure response (hydraulic tests) a proven technique for the assessment of the hydraulic properties of a reservoir. Aim of the examinations is to verify the feasibility of the concepts for direct use of geothermal heat from sedimentary rocks mentioned above. The concepts will then be implemented at the location Hannover. Furthermore, the well Horstberg Z1 will act as an experimental laboratory.
In autumn 2003, the sandstone formations Detfurth and Solling of the mid Triassic were connected to the well by perforation of the well casing. Then, a production pacer was placed in the well, in a way to prevent hydraulic communication of the two formations via the well bore. After that, the formations were connected by a large-scale hydraulic fracture, beginning in the Detfurth formation, spanning a height of approximately 120 m. The fracture-plane is supposed to cover more than 100,000 m². This artificial fracture is an essential prerequisite for the implementation of the two concepts for heat extraction from tight (permeability < 40 mD) sedimentary rocks. The fracture is hydraulically highly permeable and acts as the surface of a heat exchanger.
Amongst the classical well test procedures are injection-, production- (long- and step rate tests) and circulation tests with varying flow rates and associated reservoir pressure changes.
In numerous hydraulic tests, amongst others, the hydraulic communication between the Detfurth and Solling sandstone could be investigated. This showed, that also in sedimentary rocks, a self propping effect takes place which keeps the fracture open, like it is known form the crystalline reservoirs of the HDR (figure). This knowledge was crucial for the ling-term implementation of the new concepts in Hannover.
Results of a „low-rate“ injection test in September 2007. The interpretation of the shut-in phase after a step-wise injection into the Detfurth/Frac with flowrates < 2 l/s in the log-log-plot. The change of pressure (dP) and it’s deviation (dP’) show a fracture linear flow behaviour with an inclination of approximately ½ of dP and dP’Results of a „low-rate“ injection test in September 2007