4D Analysis of the current stress field in the stable Fennoscandian Shield

Stress field analysis using borehole data is well known approach. Our current understanding of breakout growth and development under constant and varying stress fields is mainly based on laboratory experiments and then described by numerical models. Verification of these models by long-term in-situ investigations in a borehole has so far not been performed because repeated access to an open borehole after drilling completion is not possible as wells are either cased or abandoned. The Outokumpu borehole, situated in the stable Svecofennian Shield, represents an exceptional opportunity for a 4D analysis of the stress field investigations. In 2006 and 2011 downhole measurements showed in some sections of the borehole a significant growth of existing induced features and also the emergence of new ones (borehole breakouts). This observed continuing generation and growth of stress‐induced borehole enlargements in the extremely tectonically stable and largely aseismic Northeastern part of the East European Plate contradicts breakout process models because there are no known temporal local stress variations. The ongoing deformation points either to additional sources of time-variable stress. This new GFZ-funded project aims to perform a third downhole logging campaign in June 2019 in order to analyze the 4D stress field investigations (3D + time) in the 2500 m deep Outokumpu borehole (Finland) and rock physics measurements on selected drill cores will provide a comprehensive and detailed analysis of both, the current mechanical stress state and, above all, its temporally variable effect on tectonic structures such as the development of fractures and fluid flow zones.