Geothermal Research Well in a Deep Sedimentary Reservoir

Abstract

Lower Permian siliciclastic sediments and volcanics are widespread strata throughout the Central Europe forming deeply buried aquifers in the North German Basin with formation temperatures of up to 150°C. These on average 4000 m deep structures are investigated to develop stimulation methods to increase the permeability by enhancing or creating secondary porosity and flow paths. The final goal is to test the generation of geothermal electricity from such low-enthalpy reservoirs using a doublet of borehole, one to produce deep natural water and the other to re-inject the utilized water. For these purposes, an in-situ downhole laboratory was established in Groß Schönebeck, north of Berlin, Germany. At present, two 4.3 km deep boreholes have been drilled. The first well GrSk 3/90, originally completed in 1990 as a gas exploration well and abandoned due to non-productivity, was re-opened in 2000 and was hydraulically stimulated in several treatments between 2002 and 2005. In 2006, the second well GrSk 4/05, planned for extraction of thermal waters, was drilled in order to realize a doublet system with two hydraulically connected boreholes. It is planned to stimulate in the second well both, the Lower Permian sandstones and the underlying volcanic rock. The resulting engineered reservoir should have an increased productivity being operated with minimized auxiliary energy to drive the thermal water loop and should have a minimized risk of a temperature short circuit of the system during a planned 30-year utilization period. The forthcoming phase is designed to demonstrate sustainable hot water production from the reservoir between the two wells through a long-term circulation experiment. This article describes the challenges and experiences of drilling the geothermal research well into a deep sedimentary geothermal reservoir. The lessons learnt covers drilling large diameter in sheet silicate bearing rocks, directional drilling through and beneath salty formations, and various mud concepts with the goal of minimized formation damage.