ALBERT

Description: Between 1986 and 1995, 429 hydrofracturing tests have been carried out in six now-abandoned coal mines and two coal bed methane boreholes at depths between 600 and 1950 m within the greater Ruhr region in western Germany. From these tests, stress magnitudes and orientations of the stress tensor are derived. The majority of hydrofracturing tests were carried out from mine galleries away from mine workings in a relatively undisturbed rock mass. These data along with detailed information have been disclosed recently. In combination with already published material, we provide the first comprehensive stress database of the greater Ruhr region. Our study summarises the results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project. The stress magnitudes suggest predominantly strike-slip stress regime, where the magnitude of the minimum horizontal stress, Shmin, is half of the magnitude of the maximum horizontal stress, SHmax, implying that the horizontal differential stress is high. We observe no particular change in the stress gradient at depth throughout the Carboniferous layers and no significant difference between tests carried out in coal mines and deep boreholes. The mean SHmax orientation varies between 133 ± 13∘ in the westernmost located Friedrich Heinrich coal mine and 168 ± 23∘ in the easternmost located Westphalia coal mine. The mean SHmax orientation, based on 87 data records from this and already published studies, of 161 ± 43∘ is in good agreement with the regional stress orientation observed in northwestern Europe. The presented public database provides in situ stress magnitude and stress orientation data records that are essential for the calibration of geomechanical numerical models on regional and/or reservoir scales for, among others, assessing stability issues of borehole trajectories, caverns, and georeservoirs in general. For an application example of this database, we estimate slip and dilation tendencies of major geological discontinuities, discovered during the 700-year-long coal mining activities in the region. The result, although burdened by high uncertainties, shows that the discontinuities striking in the N–S and NW–SE directions have a higher slip tendency compared to the ones striking ENE–WSW and NNW–SSE, whereas a high dilation tendency is observed for discontinuities striking NNW–SSE and a low dilation tendency for the ones striking ENE–WSW. The stress orientation database is available under https://doi.org/10.24406/fordatis/200 (Kruszewski et al., 2022a), the stress magnitude database is available under https://doi.org/10.24406/fordatis/201 (Kruszewski et al., 2022b), whereas the hydrofracturing test reports are available under https://doi.org/10.24406/fordatis/222 (Kruszewski et al., 2022c).

Description: For around 700 hundred years, the greater Ruhr region has been the heart of the coal mining industry in Germany. In 2018, the last coal mine ceased operation in the country, and the process of abandonment and repurposing of the vast amounts of unused subsurface infrastructure has begun. The operations of mine flooding as well as the recent geothermal exploration campaigns, aiming to utilize old mine infrastructure as well as permeable Massenkalk formations found below mining depths, pose serious questions regarding safety and potential seismic risks in the densely populated greater Ruhr region. To alleviate these concerns, we assess the probability of reactivation of major fault in the region. This study benefits from the astonishing amount of 429 stress magnitude and 38 stress orientation data records derived from hydrofracturing tests and borehole logs carried out in six coal mines and two coal bed methane boreholes between 1986 and 1995. Due to the change in subsurface data regulations, this data has been just recently made available to the public. Our study summarizes the results of this extensive in situ stress test campaign, assigns quality to each data record using the established quality ranking schemes of the World Stress Map project for both stress orientations and magnitudes, and, in combination with already published material, provides the first comprehensive stress database of the region. This unique data set, together with geometries of major faults revealed from coal mining activities and already published results from laboratory studies, serves as a base for the probabilistic assessment of fault reactivation potential, incorporating uncertainty of each Mohr-Coulomb parameter (i.e., stress tensor, pore pressure, frictional properties, and fault geometry). The probabilistic assessment of fault reactivation potential, as presented in this study, aims to de-risk future subsurface operations facilitating the green transition of the greater Ruhr region.