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The Federal Institute for Geosciences and Natural Resources (BGR) has drilled the approx. 3,900m deep geothermal well Groß Buchholz Gt1 as part of the GeneSys geothermal project. The Bunter sandstone (Lower Triassic) was initially recovered as the target formation for the borehole. Due to the difficult geological conditions found, it was decided to abandon the Bunter sandstones. It is now intended to heat the properties of the GEOZENTRUM Hannover (GZH) using a geothermal doublet. In addition to the direct benefit for the GZH from the utilization of this regenerative energy source and the associated relief for the environment, the demonstration of the use of multiple, low thickness sandstone layers is of particular importance in this project. It should be proven that the sandstones of the Wealden formation (Lower Cretaceous / Berriasian) at a depth of approximately 1200m are suitable for geothermal usage. Moreover, a reference site for locations with similar geological conditions will be created.As a requirement for further technical work in the borehole, a bottom cementation will be carried out at the final depth of the wellbore. This seals the perforation at a depth of approx. 3,700m and the access to the Bunter sandstones. A sidetrack is then drilled out of the existing hole into the Wealden sandstones. For this, a window is milled at a depth of approx. 750m and the sidetrack is drilled down to a depth of approx. 1300m. The determination of the landing point of the sidetrack is the subject of current investigations, as the Wealden sandstones are spatially heterogeneous. The reinterpretation of existing seismic profiles is of great importance for this. After successful completion of the sidetrack and evaluating the production tests and after a successful production test a second well is drilled from the same drilling site to a target depth of approx. 1300 m. The calculated distance of the two holes at the target depth is approx. 500 m.In the previous project, the sandstone layers of the Lower Cretaceous were examined for their suitability for geothermal use. A maximum expected transmissibility can be estimated from the sum of six suitable sandstone units (total 46m), as well as the average permeability of these units of approx. 75mD. This results in a maximum transmissibility to be expected of approx. 3.5Dm. The measured temperature at a depth of 1200m is exceptionally high with 69°C.In order to further increase the productivity of the drilling, it is planned to open up or stimulate the target horizons with different drilling techniques: for example, radial jet drilling and / or acid stimulation.

This brochure is designed for scientists and engineers of upcoming drilling projects and explains the key steps and important challenges in planning and executing continental scientific drilling.

Downhole sensors of different types and in various environments provide substantial benefit to signal quality. They also add the depth dimension to measurements performed at the Earths’ surface. Sensor types that particularly benefit from downhole installation due to the absence of near-surface noise include piezometers, seismometers, strainmeters, thermometers, and tiltmeters. Likewise, geochemical and environmental measurements in a borehole help eliminate near-surface weathering and cultural effects. Installations from a few hundred meter deep to a few kilometer deep dramatically reduce surface noise levels—the latter noticeably also reduces the hypocentral distance for shallow microearthquakes. The laying out of a borehole network is always a compromise of local boundary conditions and the involved drilling costs. The installation depth and procedure for a long-term downhole observatory can range from time limited installations, with a retrieval option, to permanently cemented sensors. Permanently cemented sensors have proven to be long-term stable with non-deteriorating coupling and borehole integrity. However, each type needs to be carefully selected and planned according to the research aims. A convenient case study is provided by a new installation of downhole seismometers along the shoreline of the eastern Marmara Sea in Turkey. These stations are being integrated into the regional net for monitoring the North Anatolian Fault Zone. Here we discuss its design, installation, and first results. We conclude that, despite the logistical challenges and installation costs, the superior quality of downhole data puts this technique at the forefront of applied and fundamental research.

The joint project CO2MAN (CO2 Reservoir Management) was a scientific programme accompanying geological CO2 storage at the Ketzin pilot site in the German Federal State of Brandenburg. The project which was funded by the German Federal Ministry of Education and Research (BMBF) from 1 September 2010 to 31 December 2013 enclosed six scientific institutions and seven industry partners. The Ketzin pilot site is the longest-operating on-shore CO2 storage site in Europe. In advance of the CO2MAN project, CO2 injection had already started in June 2008 and storage operation had been accompanied by one of the world’s most extensive scientific research and development programmes. The CO2MAN project took advantage of this unique potential of the site in order to answer further technical and scientific questions on CO2 storage and to inform about this highly debated technology. The CO2MAN project demonstrates safe geological CO2 storage at the Ketzin site on a pilot scale.

At Ketzin, located west of Berlin, the GFZ German Centre for Geosciences is operating Europe's largest CO2 research storage site. This pilot site has been developed since 2004 and is comprised of one combined injection/observation well and four monitoring wells. From June 2008 to August 2013, a total of 67 kilotons of CO2 were safely injected into the sandstone units of the Upper Triassic Stuttgart Formation in a depth between 630 to 650 m. The paper discusses the well designs and lessons learned in drilling engineering and operations. The abandonment phase started in Ketzin with the first plug cementation of the observation well Ktzi 202 shortly after shut-in of CO2 injection. The experience with the first CO2 well killing operation will be reviewed.

At Ketzin, located west of Berlin, the GFZ German Research Centre for Geosciences operates Europe's longest- running on-shore CO 2 storage site. The Ketzin pilot site has been developed since 2004 and comprises three wells to depths of 750 m to 800 m and one shallow observation well, an injection facility and permanently installed monitoring devices. Since June 2008, CO 2 is injected into 630 m to 650 m deep sandstone units (Upper Triassic Stuttgart Formation) in an anticlinal structure of the Northeast German Basin. Until mid of May 2012, about 61,400 t of CO 2 have been stored safely. One of the most comprehensive monitoring concepts worldwide is applied and capable of detecting the behaviour of the CO 2 in the subsurface. The Ketzin project demonstrates safe CO 2 storage in a saline aquifer on a research scale and effective monitoring. This paper summarizes the key results obtained after four years of CO 2 injection.

This paper reports the well design, drilling and completion operation as well as the coring technique applied in the CO2SINK project. Three boreholes, one injection well and two observation wells have been drilled to a total depth of about 800 m. 200 m of recovered 6” core material has been real-time analysed in a research field lab. The wells have been completed as “smart” wells, containing a variety of permanently installed down-hole sensors for the continuous monitoring of the CO2 in the reservoir. All wells were cased with stainless final casings equipped with pre-perforated sand filters in the reservoir zone and wired on the outside with fiber-optical and multi-conductor copper cables. The reservoir casing section is externally coated with a fiber-glass-resin wrap for electrical insulation.