Reactivation potential of fractures and faults

Faults and fractures represent major controlling factors for subsurface fluid flow in sedimentary basins (Ruhr Basin and Upper Rhine Graben) in Germany and similar settings world-wide, and are therefore important for a range of geo-energy applications. Further, faults can present socio-technical challenges due to reactivation-induced seismicity, compromising the feasibility of geo-energy projects. This study aims to combine structural geological analysis with geomechanical approaches to assess the structural control of
fractures and faults on fluid flow in two sedimentary basin settings (Ruhr Basin and Upper Rhine Graben, Germany). Moreover, this study aims to evaluate the potential risk of fault reactivation in the context of post-mining related flooding of abandoned hard coal mines (Ruhr Basin).
For the tight Upper Carboniferous sandstones of the Ruhr Basin (Germany), a better understanding of the fracture networks is crucial to understand subsurface fluid flow pattern, which is beneficial for the successful development of geo-energy utilization.
Two outcrops of Upper Carboniferous sandstones (Namurian C and Westphalian A), located along the southern part of the Variscan Ruhr Basin, are studied utilizing digital outcrop models for fracture detection and attribute quantification. ... mehrThe Namurian C outcrop near Hattingen is situated approximately 13 km west of the Westphalian A outcrop near Witten. In addition, the Bork 10 well provides a core description-based fracture data set, that allows the comparability of outcrop and subsurface to be assessed. The well penetrates the Upper Carboniferous (Westphalian A) approximately 30 to 40 km northeast of the outcrops, providing insights into the fracture inventory of the subsurface.
Comparable fracture orientation statistics are observed in both outcrops, with three dominant fracture trends detected: NNE-SSW to NE-SW (set 1), WNW-ESE (set 2), and NNW-SSE to N-S (set 3). For the additional analysis of the orientation statistics of the fracture data set of Bork 10, there is an acceptable agreement for fracture orientations between outcrop and subsurface. Two dominant N-S and W-E trends occur in the
Bork 10 along with subordinate NNE-SSW and NW-SE striking fractures.
The Terzaghi-corrected averaged fracture intensity (P10) exhibits variations between both outcrops for comparable fracture trends, ranging from 0.4 to 3.1 m-1. These fracture intensities values are 1.1 to 4.5 times higher than the P10 values of comparable fracture orientations in the Bork 10 well. Outcrop analysis indicates that the fracture height is limited by the thickness of the individual sandstone beds since the majority of the detected fractures is bed-restricted. Both outcrops are dominated by high-angle fractures with respect to bedding, while the Bork 10 data set is characterized by a mixture of inter-
mediate to high-angle (w.r.t. bedding) fractures, indicating differences in the fracture inventory despite comparable orientation statistics. This discrepancy might be attributed to the different burial and uplift histories, as outcrops experienced uplift-related stress relief.

Identified fracture trends of the Bork 10 are geomechanically analyzed following the slip and dilation tendency concept to gain better understanding of stress-controlled subsurface fluid flow patterns in tight Upper Carboniferous sandstones. N-S as well as NW-SE trending near-vertical fractures are favorably oriented within the present-day stress field to be hydraulically conductive. Highest dilation tendencies are associated with NNW-SSE striking fractures.
Post-mining related flooding of abandoned hard coal mines in the water province “Haus Aden” (Ruhr Basin, Germany) might affect fault stability as stress conditions change. Based on a constructed 3D structural model of the NE part of the water province “Haus Aden” NW-SE to NNW-SSE trending normal faults are identified as the most susceptible fault trend for reactivation. The probabilistic fault reactivation assessment, focused on NW-SE to NNW-SSE trending normal faults, is performed for a depth level of 1000 m below ground representing former mining levels. No reactivation potential is shown for considering a mine water rebound up to a depth level of 640 m below ground. Under hydrostatic conditions, a low reactivation potential is determined, requiring high differential stresses to
exceed the chosen frictional fault strength (µ = 0.5, µ = 0.67). The overall low potential for fault reactivation aligns with the absence of observed natural seismicity in the eastern Ruhr Basin. This provides a promising setting for future geothermal applications, given that fault instability can pose a significant risk. Further, a workflow is tested to account for the spatial variability of the frictional fault strength. To better capture the phenomena of along-strike and dip changes of the frictional strength, a model is established utilizing a non-perfect clay mixing model based on fault displacement and host rock clay content. Using the derived frictional fault strength model as input, a fault stability analysis is performed for a selected NNW-SSE trending normal fault. The fault is characterized by mostly stable, horizontally elongated patches and subordinate unstable zones. An additional low fault cohesion results in overall stable conditions. Generally, the tested workflow offers a suitable approach to improve the representation of the frictional fault strength in the context of fault stability analysis.
For the Upper Rhine Graben rift basin (Germany), a better understanding of fault-controlled fluid flow is crucial for de-risking the subsurface utilization in the context of fault-related geothermal applications. The stress control on hydrocarbon migration along a fault is exemplary demonstrated by the case of the fault-controlled hydrocarbon migration that has charged the former Leopoldshafen oil field (central Upper Rhine Graben).
The Leopoldshafen fault is structurally composed of two NNE-SSW trending segment, linked by a NNW-SSE to N-S striking left-stepping fault bend segment. The fault formed during the Oligocene rifting phase, generally providing a potential pathway for vertical migration of hydrocarbons that were expulsed during the Miocene. During the strike-slip stress field that has been active since the Miocene, highest slip and dilation tendencies are observed along the fault bend segment, indicating the preferential alignment for fluid flow. The fault was likely reactivated by (oblique) sinistral strike slip kinematics during the Miocene, inducing dilational strain concentrated along the bend segment, as indicated by geomechanical modeling. Therefore, the Leopoldshafen fault bend might have acted as releasing bend, providing vertical pathways for hydrocarbon migration. This understanding of stress control on structural fault configurations might benefit the de-risking of exploration for geothermal and lithium brine mining in the Upper Rhine Graben.