Slip tendency analysis of major faults in Germany

Natural seismicity and tectonic activity are important processes for the site-selection and for the long-term safety assessment of a nuclear waste repository, as they can influence the integrity of underground structures significantly. Therefore, it is crucial to gain insight into the reactivation potential of faults. The two key factors that control the reactivation potential are (a) the geometry and properties of the fault such as strike direction and friction angle, and (b) the orientations and magnitudes of the recent stress field and future changes to it due to exogenous processes such as glacial loading as well as anthropogenic activities in the subsurface. One measure of the reactivation potential of faults is the ratio of resolved shear stress to normal stresses at the fault surface, which is called slip tendency. However, the available information on fault properties and the stress field in Germany is sparse. Geomechanical numerical modelling can provide a prediction of the required 3D stress tensor in places without stress data. Here, we present slip tendency calculations on major faults based on a 3D geomechanical numerical model of Germany and adjacent regions of the SpannEnD project (Ahlers et al., 2021). Criteria for the selection of faults relevant to the scope of the SpannEnD project were identified and 55 faults within the model area were selected. For the selected faults, simplified geometries were created. For a subset of the selected faults, vertical profiles and seismic sections could be used to generate semi-realistic 3D fault geometries. Slip tendency calculations using the stress tensor from the SpannEnD model were performed for both 3D fault sets. The slip tendencies were calculated without factoring in pore pressure and cohesion, and were normalized to a coefficient of friction of 0.6. The resulting values range mainly between 0 and 1, with 6 % of values larger than 0.4. In general, the observed slip tendency is slightly higher for faults striking in the NW and NNE directions than for faults of other strikes. Normal faults show higher slip tendencies than reverse and strike slip faults for the majority of faults. Seismic events are generally in good agreement with the regions of elevated slip tendencies; however, not all seismicity can be explained through the slip tendency analysis.

Field Scale Geo-Mechanical Analysis To Identify Fracture Sweet Spots Within Deccan Trap, Western Onshore, India

Hydrocarbon exploration continues to venture into new avenues. This paper elaborates the 3D geomechanical study carried out to identify sweet spots in Deccan Trap Basalts in depth ranging from 500m-1100m in Cambay basin field of India. The main challenge is wide variation in the rock mechanical properties and stress profiles along various azimuths resulting from different tectonic incidents over the geological ages. Several drilling complications and held ups during electro logging in highly deviated wells are also reported. The normal fault tectonic framework has the imprint of two sets of faults viz., NNW-SSE and ENE-WSW. Deccan Trap acts as reservoirs due to the presence of connected open fracture network and to assess the potential reserves a comprehensive 3D Critically stressed fracture analysis has been performed using 3D numerical simulation-based rock properties, in-situ stress and seismic data. Open hole geophysical logs like sonic dipole and borehole images have been used to estimate rock mechanical properties and stress profiles in 18 key wells. Available core data of Basalt in the area have been used for dynamic to static rock properties estimation along with available published literature data. Critically stressed fracture analysis using 1D MEM outputs and dips dataset has been performed at well scale to history match production logging and testing results of 23 wells located in different fault blocks. 3D stress model has been built using plasticity model while taking into account faults and fracture sets. Utilizing 3D Geomechanical properties and Discrete fracture network model, critically stressed fracture sets have been identified across the field with slip tolerance and effective drawdown pressures. The study suggests that structurally high locations are good producers if seals are present above Trap. Sub-horizontal fractures have a higher closing tendency with decline in pressure in layers with SHmax>SHmin>Sv inside stiff Trap layer. There is variation of slip tolerance in the range of 0.2-1.4 in fracture sets which indicates slip tendency to be varying both vertically and laterally. Faults with ENE-WSW strike seem to be fluid migratory conduits and their intersection with NNW-SSE discontinuities are the areas where fracture sets have a higher slip tendency. Most of the producing layers are within 25m-55m of Trap with water being encountered at deeper depth intervals. These are mostly weathered fractured layers within the trap. The stress map suggests rotation of the maximum horizontal stress azimuth from NW to E which also affects fracture intensity in the field. Few fracture sets have tendency to be slip prone even with depletion up to 300psi-800psi while others will require stimulation or acid clean up job. Eight exploration wells drilled based on the study have shown good flow rate on initial well testing in the area providing validation to the study.

Numerical investigation of the influence of discontinuity orientations on fault permeability evolution and slip displacement

A pre-existing plane of weakness along the fault is comprised of a particular pattern of joints dipping at different orientations. The fault stress state, partially defined by the orientation of fault, determines the potential of slip failure and hence the evolution of fault permeability. Here the influence of fault orientation on permeability evolution was investigated by direct fluid injection inside fault with three different sets of fault orientations (45°, 60° and 110°), through the coupled hydromechanical (H-M) model TOUGHREACT-FLAC3D. The influence of joints pattern on slip tendency and magnitude of potential induced seismicity was also evaluated by comparing the resulted slip distance and timing. The simulation results revealed that decreasing the dip angle of the fault increases the corresponding slip tendency in the normal fault circumstance. Also, with changing joints dip angle associated with the fault, the tendency of the fault slip changes concurrently with the permeability evolution in a noticeable manner. Permeability enhancement after the onset of fault slip was observed with the three sets of fault angles, while the condition of 60° dipping angle resulted in highest enhancement. Joints pattern with a dip angle of 145° (very high dip) and 30° (very low dip) did not trigger a shear slip with seismic permeability enhancement. However, high dip and intermediate dip angles (135°, 50° and 70°) yielded high permeability in varying orders of magnitude. The large stress excitation and increasing permeability during shear deformation was noticeably high in intermediate joint dip angles but decreases as the angle increases. Article highlights The magnitude of injection-induced permeability enhancement is largely influenced by the fault and joint spatial orientations. With a slight change in the joint direction, there is an increasing possibility for fault to approach a different critical state of failure. Stress elevation at the point of failure is controlled by the orientations of fault/joint planes with respect to the direction of maximum principal stress.

Resolved stress analysis, failure mode, and fault-controlled fluid conduits

Failure behaviors can strongly influence deformation-related changes in volume, which are critical in the formation of fault and fracture porosity and conduit development in low-permeability rocks. This paper explores the failure modes and deformation behavior of faults within the mechanically layered Eagle Ford Formation, an ultra-low permeability self-sourced oil and gas reservoir and aquitard exposed in natural outcrop in southwest Texas, USA. Particular emphasis is placed on analysis of the relationship between slip versus opening along fault segments and the associated variation in dilation tendency versus slip tendency. Results show that the failure mode and deformation behavior (dilation versus slip) relate in predictable ways to the mechanical stratigraphy, stress field, and specifically the dilation tendency and slip tendency. We conclude that dilation tendency versus slip tendency patterns on faults and other fractures can be analyzed using detailed orientation or structural geometry data and stress information and employed predictively to interpret deformation modes and infer volume change and fluid conduit versus barrier behavior of structures.

Multiscale 3D stress field modelling for the URL 'Reiche Zeche' using a discontinuum model approach

<p>Underneath the small town of Freiberg, Saxony, stretches the ore mine complex 'Reiche Zeche'. The underground laboratory (URL) inside the mine was inaugurated in 1919 and is an internationally acknowledged institution for experimental work of variable scales and subjects. Our work is part of the Stimtec project, which aims on improving planning and conducting hydraulic stimulation in anisotropic, crystalline rocks. The project comprises numerical modelling and field work inside the URL. Prior to the numerical analysis, we implemented a tool to perform a slip tendency analysis of faults that were mapped along the tunnel walls at the project site. It allows to assess the slip tendency of arbitrarily oriented faults and stress fields. The tool is used for preselection of stimulation intervals, enabling identification of faults which are likely to be reactivated by hydraulic stimulation. <br>We perform the stress field modelling using a multiscale numerical model approach. Therefore, we set up three different sized models deriving from a large scale 3D geomodel. The geomodel contains the topography, drifts and 47 fault structures taken from mine maps. The project site and measurement points are positioned in the center of the model. From the large scale geomodel, we developed a simplified numerical model geometry with 12 major faults, disregarding the galleries. We use the distinct element code 3DEC for discontinuous numerical modelling of the stress field. This allows to take into account discrete displacements along the faults. Far field stress is taken from previous investigations and literature as boundary and initial conditions. The resulting stress  field provides the stress tensors for calculating the corresponding forces for each gridpoint at the model boundaries of the small scale model. The small scale numerical model is smaller by a factor of 10, including two major fault segments, the galleries and mapped local faults. Hydraulic fracturing stress measurements taken during the field tests indicate that the stress field is strongly distorted in the vicinity of the tunnels and excavations along the ore veins. Hence, we developed a third model approach, a 2.5D slice model, to investigate the influence of the assumed excavation damage zones.<br>With this work, we provide an approach to predict the stress field inside the complex, anisotropic rock volume. Within the framework of the Stimtec project, we developed a workflow for planning hydraulic stimulation tests and 3D geological models for a diverse set of further appliations in the URL 'Reiche Zeche'.</p>

Variation of slip tendency of a seismogenic fault associated with fluid extraction and injection in the Hutubi underground gas storage, China

<p>We conduct geomechanical study for a seismogenic fault in Hutubi underground gas storage site, northwestern China. The Hutubi reservoir has undergone active production from 1990s to 2012, leading to a complete depletion, and then sequential gas injection and extraction from 2013 for the gas storage project. First, we constrain the orientation and magnitudes of the stress state at the reservoir depths (~3.6 km depth) at the time of a complete depletion in 2012, using image-logged wellbore breakouts in a borehole. Then we estimate the variation of the stress state with time as a result of pore pressure change based on a simple assumption of coupling between horizontal stresses and pore pressure. Our results show that the stress state was initially in a reverse faulting regime before production and switched to a strike-slip faulting regime during production. Gas injection from 2013 turned the stress regime again in favor of reverse faulting. We use the estimated variation of the reservoir stress state with time to calculate temporal changes of slip tendency of the major earthquake fault (Hutubi fault) in the reservoir. Slip tendency of the fault decreased continuously with production, and then increased with injection. The first earthquake swarm associated with gas injection occurred ~2 months after the commencement of injection, possibly due to slow pore pressure diffusion. Thereafter, earthquakes were induced whenever gas was injected, while few earthquakes were detected during gas extraction phases. Our preliminary assessment of slip tendency suggests that earthquake swarms are induced during increasing phases of pore pressure when slip tendency reaches a value between 0.4 and 0.5, which can provide information on friction coefficient of the fault.</p><p>Funding information: This work is supported by the National Natural Science Foundation of China (41574088,41704096) </p>

Resolved stress analysis, failure mode, and fault-controlled fluid conduits in low-permeability strata

Failure behaviour can strongly influence deformation-related changes in volume, which is critical in the formation of fault and fracture porosity and conduit development in low permeability rocks. This paper explores the failure modes and deformation behaviour of faults within the mechanically layered Eagle Ford Formation, an ultra-low permeability self-sourced oil and gas reservoir and aquitard exposed in natural outcrop in southwest Texas, USA. Particular emphasis is placed on analysis of the relationship between slip versus opening along fault segments, and the associated variation in dilation tendency versus slip tendency. Results show that the failure mode and deformation behaviour (dilation versus slip) relate in predictable ways to the mechanical stratigraphy, stress field, and specifically the dilation tendency and slip tendency. We conclude that dilation tendency versus slip tendency patterns on faults and other fractures can be analysed using detailed orientation or structural geometry data and stress information, and employed predictively to interpret deformation modes and infer volume change and fluid conduit versus barrier behaviour of structures.

Fault failure modes, deformation mechanisms, dilation tendency, slip tendency, and conduits v. seals

Faults have complicated shapes. Non-planarity of faults can be caused by variations in failure modes, which in turn are dictated by mechanical stratigraphy interacting with the ambient stress field, as well as by linkage of fault segments. Different portions of a fault or fault zone may experience volume gain, volume conservation and volume loss simultaneously depending on the position along a fault's surface, the stresses resolved on the fault and the associated deformation mechanisms. This variation in deformation style and associated volume change has a profound effect on the ability of a fault to transmit (or impede) fluid both along and across the fault. In this paper we explore interrelated concepts of failure mode and resolved stress analysis, and provide examples of fault geometry in normal faulting and reverse faulting stress regimes that illustrate the effects of fault geometry on failure behaviour and related importance to fluid transmission. In particular, we emphasize the utility of using relative dilation tendency v. slip tendency on fault patches as a predictor of deformation behaviour, and suggest this parameter space as a new tool for evaluating conduit v. seal behaviour of faults.