Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods

Krietsch, Hannes; Gischig, Valentin; Evans, Keith F.; Doetsch, Joseph; Dutler, Nathan; Valley, Benoît; Amann, Florian

doi:10.1007/s00603-018-1597-8

Cited by 67 publications

(58 citation statements)

References 35 publications

(54 reference statements)

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“…Stress heterogeneity near faults can be significant. Both stress rotation and drop toward the S3.1 fault, for example, have been characterized by Krietsch et al (). However, in shallow crustal settings where fractures are not critically stressed, such effects are likely secondary compared to the transmissivity variations with structural controls described in this study.…”

Section: Discussionmentioning

confidence: 92%

“…We first systematically determined the transmissivity of intact rocks, single fractures, and fracture clusters to explore relationships with fracture density and off‐fault distance. Shear and normal stresses acting on mapped fractures were then estimated based on the stress tensors determined by Krietsch et al (). Our aim in this second step was to examine if present‐day rock stresses exert any control on fracture transmissivity.…”

Section: Methodsmentioning

confidence: 99%

“…Hydraulic fracturing and overcoring tests from Krietsch et al () revealed that the stress field is highly heterogeneous and can be described by two stress tensors (Table ): (i) a far‐field strike‐slip stress state (measured using a dedicated borehole, drilled approximately >40 m from the fault zones studied) and (ii) a perturbed, near‐field stress state (measured around 5 m from the S3 fault zones) affected by one of the S3 faults (S3.1). The magnitudes of the principal stress components σ 1 , σ 2 , and σ 3 of the far‐field stress tensor were estimated to be 13.1, 9.2, and 8.7 MPa, respectively, whereas σ 2 and σ 3 of the near‐field stress tensor have reduced values of 8.2 and 6.5 MPa, respectively, in the vicinity of the S3 fault zones (potentially due to the increased fracture density).…”

Section: Methodsmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 92%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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

et al. 2020

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“…Details of the stress characterization campaign are given by Krietsch et al (2019b). Impression packers and microseismic monitoring were used to map hydraulic fracture orientation, which revealed consistent E-W, sub-vertical fracture extension.…”

Section: Stress Characterizationmentioning

confidence: 99%

Hydraulic fracture propagation in a heterogeneous stress field in a crystalline rock mass

Dutler¹,

Valley²,

Gischig³

et al. 2019

Preprint

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Abstract. As part of the In-situ Stimulation and Circulation (ISC) experiment, hydraulic fracturing (HF) tests were conducted in a moderately fractured crystalline rock mass at the Grimsel Test Site (GTS), Switzerland. The aim of these injection tests was to improve our understanding of processes associated with high-pressure fluid injection. A total of six HF experiments were performed in two inclined boreholes, where the surrounding rock mass was accessed with twelve observation boreholes, which allow high-resolution monitoring of fracture fluid pressure, strain and micro-seismicity in an exceptionally well-characterized rock mass. A similar injection protocol was used for all six experiments to investigate the complexity of the fracture propagation processes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle ductile shear zone), their similarity of injection pressures and their response to deformation and pressure propagation. The injection tests performed in the south connect upon propagation to the brittle ductile shear zone. Thus, the shear zone acts as a dominant drain and a constant pressure boundary. The experiments executed north of the shear zone, show smaller injection pressures and larger backflow during bleed-off phases. From a seismic perspective, the injection tests show high variability in seismic response independent of the location of injection. For two injection experiments, we observe re-orientation of the seismic cloud as the fracture propagated away from the wellbore. In both cases, the main propagation direction is normal to the minimum principal stress direction. The re-orientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking different than the propagating hydraulic fracture. The seismic activity was limited to about 10 m radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Transmissivities of injection intervals increase about 2–4 orders of magnitudes.

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“…The injection rates applied were in the range of 8.3 × 10 −8 to 1.2 × 10 −5 m 3 /s and kept constant during the testing period. A control valve was installed to maintain injection pressures below 600 kPa, that is, well below the minimum jacking pressure (Krietsch et al, ). We limited the active test duration from 15 min to 2.5 days to minimize boundary effects from nearby tunnels and research galleries, and special attention was paid to ensure that sufficient time elapsed between tests to let pressures recover.…”

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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Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods

Cited by 67 publications

References 35 publications

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

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

Hydraulic fracture propagation in a heterogeneous stress field in a crystalline rock mass

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

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