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- PublicationAccès libreFractal characteristics of fractures in crystalline basement rocks: Insights from depth-dependent correlation analyses to 5 km depth(2022-5-27)
;Afshari Moein, Mohammad Javad ;Evans, Keith F. ; ;Bär, KristianGenter, AlbertThe scaling laws describing the spatial arrangement of fractures along six deep boreholes penetrating the crystalline rocks in the Rhine Graben were derived using a correlation analysis. Five of the wells, two to 5 km depth, were located at the Soultz geothermal site and one well to 5 km depth was located at Basel, some 150 km from Soultz. Five datasets were derived from borehole imaging logs, whilst one stemmed from the analysis of 810 m of continuous core at Soultz. The two differed inasmuch as the core dataset included essentially all fractures, whereas the image log dataset had few fractures narrower than 1–3 mm. The results of the analysis for all image datasets showed that the spatial arrangement of fractures followed fractal behavior at all scales from meters to several hundred meters, the largest scale amenable to assessment, and that the fractal dimensions were confined to the narrow range 0.85–0.9. However, the core dataset showed significant deviation from fractal behavior, the best-fit fractal dimension of 0.8 being somewhat lower than values obtained from imaging logs in neighboring wells. Eliminating fractures with apertures less than 1 mm from the core dataset to improve comparability led to even lower fractal dimension estimates, indicating the discrepancy was not due to imaging log resolution. Analysis of successive depth sections of the core log suggested the discrepancy was due to the presence of a localized zone between 1750 and 2070 m where the fractal organization is disturbed or takes a lower dimension than elsewhere. Aside from this zone, no systematic variation of fractal dimension with depth was observed in any dataset, implying that a single exponent together with intensity adequately describes the arrangement of fractures along the entire length of the boreholes. The results are relevant to the parameterization of DFN models of deep rock masses.
- PublicationAccès libreOn the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity(2018-7-1)
;Gischig, Valentin ;Doetsch, J. ;Maurer, Hansruedi ;Krietsch, Hannes ;Amann, Florian ;Evans, Keith F. ;Nejati, M. ;Jalali, Mohammadreza ; ;Obermann, A. ;Wiemer, StefanGiardini, Domenico
- PublicationMétadonnées seulementStress Characterisation and stress modeling at the Grimsel Test Site for the In-situ Stimulation and Circulation Project
- PublicationAccès libreHydromechanical processes and their influence on the stimulation effected volume: observations from a decameter-scale hydraulic stimulation project(2020-9-4)
;Krietsch, Hannes ;Gischig, Valentin ;Doetsch, J. ;Evans, Keith F. ;Villiger, Linus ;Jalali, Mohammadreza ; ;Loew, SimonAmann, FlorianSix hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to 3 orders in magnitude. With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical response. The elevated fracture fluid pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15–20 m from the injection point is characterized by variable extensional and compressive strain produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud and can be subdivided into a primary stimulated and secondary effected zone.
- PublicationMétadonnées seulementStress state at Soultz to 5 km depth from wellbore failure and hydraulic observations(2006)
;Evans, Keith F.
- PublicationAccès libreHydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems(2020-1-2)
;Gischig, Valentin ;Giardini, Domenico ;Amann, Florian ;Hertrich, Marian ;Krietsch, Hannes ;Loew, Simon ;Maurer, Hansruedi ;Villiger, Linus ;Wiemer, Stefan ;Bethmann, Falco ;Brixel, Bernard ;Doetsch, J. ;Gholizadeh, Nima ;Driesner, Thomas ; ;Evans, Keith F. ;Jalali, Mohammadreza ;Jordan, D. ;Kittilä, A. ;Ma, Xiadong ;Meier, Peter ;Nejati, M. ;Obermann, A. ;Plenkers, K. ;Saar, Martin O. ;Shakas, A.The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS
- PublicationMétadonnées seulementPreliminary stress characterization for an in-situ stimulation experiment at the Grimsel Underground Laboratory(2016-4-1)
;Krietsch, Hannes ;Doetsch, J. ;Gischig, Valentin ;Amann, Florian ;Jalali, Mohammadreza ;Madonna, Claudio ;Evans, Keith F. ; ;Giardini, Domenico ;Wiemer, Stefan ;Maurer, HansruediLoew, SimonA decameter-scale in-situ stimulation experiment is currently being executed at the Grimsel Test Site in Switzerland, spanning from hydraulic fracturing to controlled fault-slip experiments. For the feasibility of this project the in-situ stress tensor is of foremost importance. Therefore a unique stress characterization campaign combining stress relief methods (overcoring of USBM and CSIRO-HI probes) with hydraulic fracturing (HF) and hydraulic testing on pre-existing fractures (HTPF) in three boreholes was conducted in a first phase of this project. During all hydraulic stress measurements, micro-seismicity was monitored and localized in real time utilizing a dense network of piezo-electric sensors. In this contribution, we present preliminary results of the stress characterization and compare the derived stress tensor with previous estimates of the stress state. The stress characterization campaign was conducted in three boreholes, one sub-vertical and two sub-horizontal boreholes, assuming that the sub-vertical and one sub-horizontal are parallel to a principal stress component. A major task in this contribution is the integration of the different stress characterization methods. Our results of the different methods (overcoring and HF) are largely consistent, but disagree with some of the previous stress orientation estimates. From the new campaign the overcoring measurements indicate a sub-horizontal sigma1 of 17.3 MPa with a strike of 145°, a sigma2 of 9.7 MPa with 241°/69° and a sigma3 of 8.3 MPa with 055°/21° using an isotropic approach for inversion calculation. Whereas the USBM-Probe measures a projection of the principal stresses in a plane normal to borehole axis, the CSIRO-HI Probe provides the real 3D stress tensor. The HF and HTPF measurements indicate a far-field minimum horizontal stress between 8.7 and 9.1 MPa, consistent with the overcoring. Principal stresses, estimated by location of micro-seismic events during HF and HTPF, suggest that the maximum horizontal stress strikes EW, the minimum horizontal stress strikes NS and sigma2 stress direction is sub-vertical dipping towards south. One sub-horizontal borehole dedicated to stress characterization penetrates one of the fault zones targeted for a future fault-slip experiment. Results reveal a significant drop in the minimum stress component towards the fault zone. This stress information will be critical for the planning of the stimulation phase of the project.
- PublicationMétadonnées seulement
- PublicationMétadonnées seulementStress magnitudes estimate from borehole failure at the Basel EGS reservoir(2015-11-26)
;Evans, Keith F.
- PublicationMétadonnées seulement