TY - JOUR

T1 - Constraining the stress field and its variability at the BedrettoLab: Elaborated hydraulic fracture trace analysis

AU - Bröker, Kai

AU - Ma, Xiaodong

AU - Zhang, Shihuai

AU - Doonechaly, Nima Gholizadeh

AU - Hertrich, Marian

AU - Klee, Gerd

AU - Greenwood, Andrew

AU - Caspari, Eva

AU - Giardini, Domenico

N1 - Publisher Copyright: © 2024 The Authors

PY - 2024/5/10

Y1 - 2024/5/10

N2 - The measurement of the stress field and its natural heterogeneity is a common challenge in the field of geophysics and underground engineering. This study presents findings from an extensive mini-frac campaign conducted at the Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Six dedicated stress measurement boreholes, spanning 500 m along the Bedretto Tunnel, were utilized to prepare for meso-scale hydraulic stimulation experiments in a granitic rock volume. A detailed analysis of the induced hydraulicfracture trace morphology was carried out using acoustic televiewer logging data, with an interpretation based on the local geological context. The results show that the far field orientation of the maximum horizontal stress (𝑆Hmax) is on average N112°E and the overburden stress can be assumed to be a principal stress direction. However, the en-echelon shape of the observed hydraulic fracture traces suggests that there is a slight inclination of the greatest principal stress away from the vertical stress direction. The mini-frac testintervals along boreholes inside the enlarged niche of the BedrettoLab indicate a counterclockwise rotated horizontal stress by up to 37° and potentially stronger inclination of the greatest principal stress from the vertical stress direction. The most likely causes of this stress field rotation, along with a considerably larger scatter in stress magnitudes, appear to be the larger laboratory niche dimensions compared to the tunnel and the presence of natural fractures that are preferentially oriented for slip in the far field stress state. Althoughthe uncertainties and variability of the stress field around the BedrettoLab are not yet fully understood, our results are useful for strategic planning and design of meso-scale stimulation experiments in the rock volume. Furthermore, our detailed analysis procedure contributes to advancing the understanding of mini-frac tests in similar geological settings.

AB - The measurement of the stress field and its natural heterogeneity is a common challenge in the field of geophysics and underground engineering. This study presents findings from an extensive mini-frac campaign conducted at the Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Six dedicated stress measurement boreholes, spanning 500 m along the Bedretto Tunnel, were utilized to prepare for meso-scale hydraulic stimulation experiments in a granitic rock volume. A detailed analysis of the induced hydraulicfracture trace morphology was carried out using acoustic televiewer logging data, with an interpretation based on the local geological context. The results show that the far field orientation of the maximum horizontal stress (𝑆Hmax) is on average N112°E and the overburden stress can be assumed to be a principal stress direction. However, the en-echelon shape of the observed hydraulic fracture traces suggests that there is a slight inclination of the greatest principal stress away from the vertical stress direction. The mini-frac testintervals along boreholes inside the enlarged niche of the BedrettoLab indicate a counterclockwise rotated horizontal stress by up to 37° and potentially stronger inclination of the greatest principal stress from the vertical stress direction. The most likely causes of this stress field rotation, along with a considerably larger scatter in stress magnitudes, appear to be the larger laboratory niche dimensions compared to the tunnel and the presence of natural fractures that are preferentially oriented for slip in the far field stress state. Althoughthe uncertainties and variability of the stress field around the BedrettoLab are not yet fully understood, our results are useful for strategic planning and design of meso-scale stimulation experiments in the rock volume. Furthermore, our detailed analysis procedure contributes to advancing the understanding of mini-frac tests in similar geological settings.

KW - In situ stress field

KW - Hydraulic fracture traces

KW - Natural fractures

KW - Underground research

KW - laboratory

KW - Fractured crystalline rock

KW - Bedretto underground

KW - laboratory for

KW - geoenergies and

KW - geosciences

KW - Underground research laboratory

KW - Bedretto underground laboratory for geoenergies and geosciences

UR - http://www.scopus.com/inward/record.url?scp=85192675679&partnerID=8YFLogxK

U2 - 10.1016/j.ijrmms.2024.105739

DO - 10.1016/j.ijrmms.2024.105739

M3 - Article

VL - 178.2024

JO - International Journal of Rock Mechanics and Mining Sciences

JF - International Journal of Rock Mechanics and Mining Sciences

SN - 1365-1609

IS - June

M1 - 105739

ER -