The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

Amann, Florian; Gischig, Valentin; Evans, Keith F.; Doetsch, Joseph; Jalali, Reza; Valley, Benoît; Krietsch, Hannes; Dutler, Nathan; Villiger, Linus; Brixel, Bernard; Klepikova, Maria; Kittilä, Anniina; Madonna, Claudio; Wiemer, Stefan; Saar, Martin O.; Loew, Simon; Driesner, Thomas; Maurer, Hansruedi; Giardini, Domenico

doi:10.5194/se-9-115-2018

Cited by 143 publications

(108 citation statements)

References 198 publications

(265 reference statements)

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“…These boreholes have been used for many purposes such as, for example, geophysical investigation, strain and pore pressure monitoring, stress measurements, petrophysical property characterization, and as injection boreholes for hydraulic stimulation of the shear zones (e.g., Jalali, Gischig, et al, 2018;Krietsch et al, 2017;Wenning et al, 2018). A detailed review of the ISC experiment is given in Amann et al (2018).…”

Section: Experimental Backgroundmentioning

confidence: 99%

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

et al. 2019

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In fractured rocks, the amplitudes of propagating seismic waves decay due to various mechanisms, such as geometrical spreading, solid friction, displacement of pore fluid relative to the solid frame, and transmission losses due to energy conversion to reflected and transmitted waves at the fracture interfaces. In this work, we characterize the mechanical properties of individual fractures from P wave velocity changes and transmission losses inferred from static full‐waveform sonic log data. The methodology is validated using synthetic full‐waveform sonic logs and applied to data acquired in a borehole penetrating multiple fractures embedded in a granodioritic rock. To extract the transmission losses from attenuation estimates, we remove the contributions associated with other loss mechanisms. The geometrical spreading correction is inferred from a joint analysis of numerical simulations that emulate the borehole environment and the redundancy of attenuation contributions other than geometrical spreading in multiple acquisitions with different source‐receiver spacing configurations. The intrinsic background attenuation is estimated from measurements acquired in the intact zones. In the fractured zones, the variations with respect to the background attenuation are attributed to transmission losses. Once we have estimated the transmission losses associated with a given fracture, we compute the transmission coefficient, which, on the basis of the linear slip theory, can then be related to the mechanical normal compliance of the fracture. Our results indicate that the estimated mechanical normal compliance ranges from 1 × 10−13 to 1 × 10−12 m/Pa, which, for the size of the considered fractures, is consistent with the experimental evidence available.

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Section: Experimental Backgroundmentioning

confidence: 99%

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

et al. 2019

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“…We take advantage of a new borehole array installed at the Grimsel Test Site, in the Swiss Alps, to characterize the hydraulic properties (Part 1) and track fluid flow dynamics (Part 2) in shear zones dissecting crystalline rock at 0.5‐km depth. Two different sets of shear zones are studied, both of which experienced multiple deformation stages and developed into multiple fault core systems with pervasive off‐fault damage; both sets were the target of an In‐situ Stimulation and Circulation (ISC) borehole‐based experiment (Amann et al, ), providing a unique opportunity to hydraulically test crustal faults, which is the focus of our study. We pay close attention to scale effects, well known in crystalline rock (Clauser, ; Martinez‐Landa & Carrera, ), and integrate results in two steps.…”

Section: Introductionmentioning

confidence: 99%

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

et al. 2020

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New in situ measurements to constrain the range, distribution, and spatial (meter-scale) variations of permeability in shallow crustal fault zones are reported based on systematic downhole tests at 0.5-km depth in crystalline rock. Single and cross-hole hydraulic packer tests were performed at a new dedicated test facility hosted in the Grimsel Test Site, in the Swiss Alps, following the technical instrumentation and isolation of discrete fault zones accessed by an array of boreholes. Single-hole test results are presented in this paper, while cross-hole experiments are reported in the companion paper. Our results reveal a sharp spatial falloff in permeability, from 10 -13 to 10 -21 m 2 , with off-fault distances of 1-5 m and characterized by a power-law relation with fracture density. Fractures linking subparallel faults were detected as high-permeability discrete spots several meters away from off-fault damage. Due to the narrow (centimeter-wide) thickness of fault cores, the hydraulic tests presented in this study do not characterize the permeability of fault core materials. The transmissivity of single fractures spans six orders of magnitude (10 -12 to 10 -6 m 2 /s) and is systematically higher in damage zones. In situ stresses appear to have a minor effect on natural, present-day fracture transmissivity at the borehole scale. We suggest that the geometrical and topological properties of fracture systems instead tend to control the permeability of the shallow crustal faults studied.Characterizing the structural properties of exhumed fault is key to understand their geometrical complexity at depth, and field studies have described how brittle damage around faults develops both along-strike and off-fault (i.e., off-plane) as fractures in damage zones organize themselves in response to off-fault stresses. Consequently, their spatial arrangement is not random (Kim et al., 2004;Peacock et al., 2017). Previous

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“…All cross‐hole tests were performed in 2017 at the Grimsel Test Site, prior to a series of high‐pressure fault stimulation experiments (Amann et al, ). A detailed description of the rock mass and internal structure of the faults intersected is provided in the first part of this study (Brixel et al, ).…”

Section: Datamentioning

confidence: 99%

Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

et al. 2020

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Cross-hole fluid injection tests were performed on shallow (0.5 km depth) crustal fault zones to characterize their internal flow structures on scales of 3-30 m. The data were acquired at the Grimsel Rock Laboratory, Switzerland, after the zonal isolation of damage zones in boreholes equipped with multipacker systems. We show that 80% of the pressure responses detected evolved as a power function of time due to fracture-controlled flow and were best described by a fractional model with a flow dimension (n) in the range of [1.3,1.5]. The scaling of characteristic times (t c ) and intermediate transitions in the flow dimension is shown to be inconsistent with the trend expected for normal diffusion, indicating a diffusion slowdown process caused by the spatial integration of structurally different damage zones. An analysis of the entire data set points to a subdiffusive regime where the mean squared displacement of pressure fronts, ⟨r 2 (t)⟩, scales as ⟨r 2 (t)⟩ ∝ t 0.59 , similar to an anomalous diffusion process on synthetic fractal systems. We characterize this diffusion slowdown by calculating a network connectivity exponent ( ) of 1.4, from which we estimate a fractal dimension of d f = 2.23 for the tested fault zones. Such a value close to 2.0 supports the view of fault damage zones as hierarchical systems promoting flow channeling. Our results provide further support to theoretical models of fractional flow and their application to fractured media in nature.

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The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

Cited by 143 publications

References 198 publications

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

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