On petrophysical and geomechanical properties of Bedretto Granite

David, Christian; Nejati, Morteza; Geremia, Davide

doi:10.3929/ethz-b-000428267

Cited by 4 publications

(10 citation statements)

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“…In one case because of a potential fluid bypass around the packer and in the other case because of the development of the hydraulic fracture entirely below the packer, which both lead to reopening pressures close to the The difference between P b and P r is commonly used to estimate the hydraulic tensile strength (T) of the rock (Bredehoeft et al 1976;Haimson and Cornet 2003). The inferred hydraulic tensile strength lies between 5.4 and 11.6 MPa (Table 2), which is consistent with estimates from laboratory Brazilian tests (David et al 2020). In the aforementioned intervals, where a reliable estimate of the reopening pressure was not possible, we assumed the average tensile strength in the borehole.…”

Section: Reopening Pressuresupporting

confidence: 72%

“…For the range of the SB boreholes, the overburden increases from about 940 m at TM1750 to about 1110 m at TM2250. We estimate the average magnitude of the vertical stress ( S v ) exerted by the overburden to be about 26.5 MPa by assuming a thickness of the overlying granite of 1000-1050 m with a bulk density of 2.62 g cm −3 from laboratory measurements (David et al 2020). A previous study by Meier (2017) showed that the topographic effect diminishes with increasing overburden, and one principal stress direction can be assumed vertical behind approximately TM1500.…”

Section: Site Descriptionmentioning

confidence: 99%

“…This indicates that some natural fractures intersecting the borehole are permeable (see Sect. 4.5), as the Rotondo granite only has a permeability in the order of 10 −18 m 2 (David et al 2020). We assumed that the hydraulic fractures created during the minifrac tests likely connect to one of these permeable structures, which will be discussed in Sect.…”

Section: Pore Pressurementioning

confidence: 99%

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Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

Bröker

2022

Rock Mech Rock Eng

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The hydraulic fracturing technique (also termed mini-frac test) is commonly used to estimate the in situ stress field. We recently conducted a mini-frac stress measurement campaign in the newly-established Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Four vertical boreholes, dedicated for stress characterization of the granitic rock mass, hosted a total of 19 mini-frac test intervals. Systematic pressure transient analysis was performed to carefully estimate the magnitude of the least principal stress ($$S_\mathrm {3}$$ S 3 ). We compared five different methods (inflection point, bilinear pressure decay rate, tangent, fracture compliance, and jacking pressure) to identify an adequate approach best suited for our test scale and the host rock mass. We found that the methods used to determine the fracture closure pressure underestimate the magnitude of $$S_\mathrm {3}$$ S 3 , presumably due to the rapid closure of the hydraulic fracture after shut-in. The most consistent results were found using the inflection point and bilinear pressure decay rate method, which both determine the (instantaneous) shut-in pressure as the proxy for the $$S_\mathrm {3}$$ S 3 magnitude. The determined shut-in pressure, or $$S_\mathrm {3}$$ S 3 magnitude, is $$14.6\pm 1.4$$ 14.6 ± 1.4 MPa from the inflection point method. This allowed us to further estimate the stress environment around the BedrettoLab, which is transitional between normal and strike-slip faulting. The measured local pore pressures from extended shut-in periods are between 2.0 and 5.6 MPa, significantly below hydrostatic. A combination of drainage, cooling, and the excavation damage zone of the tunnel may have significantly perturbed the in situ stress field in the vicinity of the BedrettoLab.

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Section: Reopening Pressuresupporting

confidence: 72%

Section: Site Descriptionmentioning

confidence: 99%

Section: Pore Pressurementioning

confidence: 99%

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Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

Bröker

2022

Rock Mech Rock Eng

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“…BULG is situated in a cavern with 100 m length, under an overburden of about 1000-1200 m, and is hosted in a granitic intrusion called Rotondo (Bedretto) granite. Bedretto Granite is a rather homogeneous rock with small to medium grain size, and a mild anisotropic behaviour (David et al 2020). Experimental data show that the Bedretto Granite holds to following mechanical properties: the secant Young's modulus of 75 GPa, the uniaxial compressive strength of 170 MPa and the indirect tensile strength of 8 MPa (David et al 2020).…”

Section: Rock Samplesmentioning

confidence: 99%

“…Bedretto Granite is a rather homogeneous rock with small to medium grain size, and a mild anisotropic behaviour (David et al 2020). Experimental data show that the Bedretto Granite holds to following mechanical properties: the secant Young's modulus of 75 GPa, the uniaxial compressive strength of 170 MPa and the indirect tensile strength of 8 MPa (David et al 2020). Two sets of experiments were conducted on Bedretto Granite in order to investigate the effects of ligament length (2l) and loading angle ( ) on the measured values of K IIIc using the ANBD test.…”

Section: Rock Samplesmentioning

confidence: 99%

True Mode $$\mathrm {III}$$ Fracturing of Rocks: An Axially Double-Edge Notched Brazilian Disk Test

et al. 2022

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A new test, referred to as axially double-edge notched Brazilian disk (ANBD), is proposed to measure true mode $$\mathrm {III}$$ III fracture toughness ($$K_{\mathrm {IIIc}}$$ K IIIc ) of rock materials. The term true denotes a shear-induced fracturing via self-planar crack extension as opposed to a twisted tension-based one commonly observed in many mode $$\mathrm {III}$$ III experiments of rocks. The ANBD test follows a straightforward procedure thanks to its simple core-based geometry and diametrical compression loading setup. Finite element analyses are employed to evaluate the stress intensity variations along the crack front and to calculate the point-wise stress intensity factors (SIFs) for different geometry and loading configurations. The results of ANBD tests conducted on granitic samples demonstrate the good performance of this test in yielding true mode $$\mathrm {III}$$ III fracturing. The influences of the test parameters of ligament length and loading angle on $$K_{\mathrm {IIIc}}$$ K IIIc are also investigated. A comparison study shows that $$K_{\mathrm {IIIc}}$$ K IIIc values are similar to $$K_{\mathrm {IIc}}$$ K IIc but almost 2.5 times greater than $$K_{\mathrm {Ic}}$$ K Ic . This demonstrates that the true mode $$\mathrm {III}$$ III test offers a similar shear-based fracturing mechanism to the true mode $$\mathrm {II}$$ II , which is significantly more energy-consuming than the tension-based mode $$\mathrm {I}$$ I failure type.

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Mechanical Compliance of Individual Fractures in a Heterogeneous Rock Mass From Production‐Type Full‐Waveform Sonic Data

et al. 2022

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The presence of fractures, which are typically hydraulically much more transmissive and mechanically much more compliant than their embedding background, is ubiquitous throughout the upper crust (e.g., Barton, 2006). Correspondingly, there is increasing interest not only in the detection, but also, and in particular, in the geometrical, mechanical, and hydraulic characterization of fractures for a wide range of applications throughout the Earth, environmental, and engineering sciences, such as, for example, hydrocarbon exploration (e.g.,

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On petrophysical and geomechanical properties of Bedretto Granite

Cited by 4 publications

References 0 publications

Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis

True Mode $$\mathrm {III}$$ Fracturing of Rocks: An Axially Double-Edge Notched Brazilian Disk Test

Mechanical Compliance of Individual Fractures in a Heterogeneous Rock Mass From Production‐Type Full‐Waveform Sonic Data

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