Transport and retention from single to multiple fractures in crystalline rock at Äspö (Sweden): 2. Fracture network simulations and generic retention model

The presence of fractures and discontinuities in the intact rock affects the hydraulic, thermal, chemical and mechanical behavior of the underground structures. Various techniques have been developed to provide information on the spatial distribution of these complex features. LIDAR, for instance, could provide a 2D fracture network model of the outcrop, Geophysical borehole logs such as OPTV and ATV can be used to investigate 1D geometrical data (i.e. dip and dip direction, aperture) of the intersected fractures, and seismic survey can mainly offer a large structure distribution of the deep structures. The ability to combine all the existing data collected from various resources and different scales to construct a 3D discrete fracture network (DFN) model of the rock mass allows to adequately represent the physical behavior of the interested subsurface structure.In this study, an effort on the construction of such a 3D DFN model is carried out via combination of various structural and hydrogeological data collected in fractured crystalline rock. During the pre-characterization phase of the In-situ Stimulation and Circulation (ISC) experiment [Amann et al., 2018] at the Grimsel Test Site (GTS) in central Switzerland, a comprehensive characterization campaign was carried out to better understand the hydromechanical characteristics of the existing structures. The collected multiscale and multidisciplinary data such as OPTV, ATV, hydraulic packer testing and solute tracer tests [Jalali et al., 2018; Krietsch et al., 2018] are combined, analyzed and interpreted to form a combined stochastic and deterministic DFN model using the FracMan software [Golder Associates, 2017]. For further validation of the model, the results from in-situ hydraulic tests are used to compare the simulated and measured hydraulic responses, allowing to evaluate whether the simulated model could reasonably represent the characteristics of the fracture network in the ISC experiment.&#160;References<ul><li>Amann, F., Gischig, V., Evans, K., Doetsch, J., Jalali, M., Valley, B., Krietsch, H., Dutler, N., Villiger, L., Brixel, B., Klepikova, M., Kittil&#228;, A., Madonna, C., Wiemer, S., Saar, M.O., Loew, S., Driesner, T., Maurer, H., Giardini, D., 2018. The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment. Solid Earth 9, 115&#8211;137.</li> <li>Golder Associates, 2017. FracMan User Documentation.&#160; Golder Associates Inc, Redmond WA.</li> <li>Krietsch, H., Doetsch, J., Dutler, N., Jalali, M., Gischig, V., Loew, S., Amann, F., 2018. Comprehensive geological dataset describing a crystalline rock mass for hydraulic stimulation experiments. Scientific Data 5, 180269.</li> <li>Jalali, M., Klepikova, M., Doetsch, J., Krietsch, H., Brixel, B., Dutler, N., Gischig, V., Amann, F., 2018. A Multi-Scale Approach to Identify and Characterize the Preferential Flow Paths of a Fractured Crystalline Rock. Presented at the 2nd International Discrete Fracture Network Engineering Conference, American Rock Mechanics Association.</li> </ul>

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The Virtual Element Method for large scale Discrete Fracture Network simulations: fracture-independent mesh generation

PAMM ◽

10.1002/pamm.201510006 ◽

2015 ◽

Vol 15 (1) ◽

pp. 19-22 ◽

Cited By ~ 2

Author(s):

Stefano Berrone ◽

Matìas Fernando Benedetto ◽

Andrea Borio ◽

Sandra Pieraccini ◽

Stefano Scialò

Keyword(s):

Mesh Generation ◽

Large Scale ◽

Fracture Network ◽

Discrete Fracture Network ◽

Virtual Element Method ◽

Discrete Fracture ◽

Network Simulations ◽

Virtual Element ◽

Element Method

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Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock

Solid Earth ◽

10.5194/se-11-627-2020 ◽

2020 ◽

Vol 11 (2) ◽

pp. 627-655 ◽

Cited By ~ 5

Author(s):

Linus Villiger ◽

Valentin Samuel Gischig ◽

Joseph Doetsch ◽

Hannes Krietsch ◽

Nathan Oliver Dutler ◽

...

Keyword(s):

Seismic Response ◽

Fluid Pressure ◽

Shear Zones ◽

Test Site ◽

Fracture Network ◽

Crystalline Rock ◽

Hydraulic Fractures ◽

Geothermal Systems ◽

Hydraulic Stimulation ◽

Shear Dilation

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

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