Fracture network characterisation of the naturally fractured Upper Carboniferous sandstones combining outcrop and wellbore data, Ruhr Basin, Germany.

Fractures in subsurface lithologies globally host economically significant energy resources and can be preferential fluid migration pathways. With the naturally fractured Upper Carboniferous sandstones of the Ruhr Basin located within Germany’s most densely populated area, the utilisation for powering one of Europe’s largest district heating infrastructure can be an option. Since the Upper Carboniferous sandstone matrix exhibits poor reservoir quality (average porosity: 8%, permeability: up to 3 mD), fractures are believed to primarily control fluid flow. By combining data from two outcrops (exposing the Namurian C and Westphalian A) with a wellbore data set (penetrating the Westphalian A), this study aims to develop a refined understanding of the fracture network. The present-day stress control on hydraulically conductive fracture orientations is studied by performing critically stressed fracture analysis for three different SHmax orientation scenarios. Kaisberg (Namurian C) and Finefrau (Westphalian A) Sandstone outcrops show three dominant fracture trends: NNW– SSE to N–S, NNE–SSW to NE–SW and WNW–ESE. The characterised fracture networks of both outcrops follow a top-bounded height distribution pattern, with fractures being mostly bed restricted. Therefore, bed thickness is identified as a controlling factor limiting fracture height. Based on 600 m of oriented core material from the Upper Carboniferous strata (Bork 10 wellbore), the analysis of the in-situ fracture network revealed two dominant fracture orientations: N–S and W–E. Fracture intensity indicates a varying degree of fracturing of the drilled sandstone layers for both trends (N–S: 0 to 9.1 m-1, W–E: 0 to 8.8 m-1). Comparison of dipmeter log fracture interpretations with reliable core description data shows a poor match (11%) for depth location (± 6 m) and dip azimuth (± 25°). Slip tendency analysis highlighted near-vertical N–S and NW–SE striking fractures to be generally favourably oriented for reactivation within the active stress field. In total, 10% of the detected fracture population (Bork 10 wellbore) are critically stressed under present-day stress conditions and may contribute to large scale fluid flow in the subsurface. [ABSTRACT FROM AUTHOR]