In Situ Stress Measurements in Siliceous Mudstones at Horonobe Underground Research Laboratory, Japan

Abstract

The stress measurement methods implemented during the surface-based investigations and during construction of the underground facilities in the Horonobe mudstones, as well as information on the initial stress state around the Horonobe URL, are described in this paper. During the surface-based investigations, determination of deep in situ stress was conducted using HF, BB information in deep boreholes and core-based methods such as AE and DSCA. During construction of the underground facilities, subsurface investigations utilizing CCBO, HTPF and the monitoring of spalling around the shafts were conducted in order to verify results from initial stress measurements in the surface-based investigations. HF results indicate that magnitude of the horizontal maximum and minimum principal stresses increases linearly with depth. The maximum principal stress estimated from the HF and borehole breakout data is almost E-W. This is similar to the tectonic movement direction in the vicinity of the Horonobe URL. Due to tectonic movement, horizontal maximum stress is almost 1.5 times larger than the horizontal minimum stress. The minimum horizontal principle stress is almost equivalent to overburden pressure. Stress condition determined from HTPF in the investigations during construction of the underground facilities is almost equal to the results during surface-based investigations.

Predictions of the highest potential transmissivity of fractures in fault zones from rock rheology: Preliminary results

“…In this study, when a flow anomaly exists at a distance of less than 10 m from the fault core in the boreholes, it is identified as a flow anomaly within the fault zone. The fault cores of the fault zones are commonly oriented at a low angle to the maximum principal stress direction (i.e., the E-W direction) (e.g., Figure 4c), and the Journal of Geophysical Research: Solid Earth 10.1002/2014JB011756 damage-zone fractures that are less than 1 m from the flow anomalies are typically oriented at a high angle to the minimum or intermediate principal stress direction (i.e., the N-S direction) or at a low angle to the maximum principal stress direction (e.g., Figure 4c) [Funaki et al, 2009;Sanada et al, 2010]. Such characteristics are also observed in pregrouted fault zones in underground outcrops (e.g., Figure 5), implying the generation of shear-induced dilation in the fault zones ( Figure 1).…”

“…The assigned stresses are shown in Table 3. The mean and minimum principal stresses for the Horonobe site were obtained by the hydraulic fracturing method in Sanada et al [2009Sanada et al [ , 2010 ( Figure 7a). The mean and minimum principal stresses for the Palfris Formation at the Wellenberg site were calculated from the primary data of Nagra [1997] (Figure 7b), which were obtained by the hydraulic fracturing method in three boreholes (SB1, SB3, and SB4a/v), assuming σ H = 3P s À P r and σ h = P s , where P s and P r are the shut-in pressure and the fracture reopening pressure, respectively.…”

“…logT ¼ À1:60logσ' 3 À 6:09 standard error ¼ 1:51 in logT ð Þ (3) Figure 7. Minimum principal stresses measured by the hydraulic fracturing method at (a) Horonobe (HDB-1 to HDB-6, HDB-9, and HDB-11) and (b) Wellenberg (SB1, SB3, and SB4a/v), after Sanada et al [2009Sanada et al [ , 2010 and Nagra [1997], respectively. Representative minimum principal stresses against depths were determined by regression analyses for each site.…”

“…Furthermore, the fault zones at Horonobe and Wellenberg are oriented at a low angle to the maximum principal stress, and the damage-zone fractures near the flow anomalies are generally oriented at a high angle to the minimum or intermediate principal stress or at a low angle to the maximum principal stress (e.g., Figures 1c, 4c, and 5) [Funaki et al, 2009;Nagra, 1997;Sanada et al, 2010]. The magnitudes of the minimum and intermediate principal stresses are similar at each site [Nagra, 1997;Sanada et al, 2010]. Thus, the fault zones at Horonobe and Wellenberg can be correlated with the A-type orientation.…”

Assessment of Hydraulic Connectivity of Fractures in Mudstones by Single‐Borehole Investigations

“…The tensile strengths of the intact rocks of the three formations are listed in Table (Ishii, ), based on the results of indirect laboratory tension (Brazilian) tests (Ishii et al, ; Nagra, ). Mean stress for a given depth is formulated as shown in Table (Ishii, ), using the hydraulic fracturing method of Nagra () and Sanada et al ().…”