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(a) Image of borehole wall and (b-d) the drill core containing the injected fault in borehole 350-FZ-02.
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A constant-head step injection test using a conventional straddle-packer system was performed for a normal fault in siliceous mudstone. The test applied a new method whereby axial displacements of isolated test sections in a borehole during injection are monitored by measuring the pressures of sliding packers and the pore pressure in the test secti...
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... = 1.8 ± 0.7 MPa ( ± 1σ), unconfined compressive strength = 22.4 ± 5.4 MPa ( ± 1σ), Young's modulus = 1-5 GPa, Poisson's ratio = ~0.2, porosity = ~40%, and intrinsic permeability = 10 −19 m 2 ( Ishii et al., 2011;Miyazawa et al., 2011;Niunoya and Matsui, 2007). The fault at 99.5 mabh (479.3 mbgl) exhibits a dip direction/dip angle of 181°/71° (Fig. 2a), a thin layer of fault breccia (millimeters or less) (Fig. 2b, c), and striations (rake ≈ 90°) on the fault surface (Fig. 2d), although slickensteps are not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault ...
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... 22.4 ± 5.4 MPa ( ± 1σ), Young's modulus = 1-5 GPa, Poisson's ratio = ~0.2, porosity = ~40%, and intrinsic permeability = 10 −19 m 2 ( Ishii et al., 2011;Miyazawa et al., 2011;Niunoya and Matsui, 2007). The fault at 99.5 mabh (479.3 mbgl) exhibits a dip direction/dip angle of 181°/71° (Fig. 2a), a thin layer of fault breccia (millimeters or less) (Fig. 2b, c), and striations (rake ≈ 90°) on the fault surface (Fig. 2d), although slickensteps are not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault surface (Fig. 2d). The orientations of the splay crack and ...
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... = ~0.2, porosity = ~40%, and intrinsic permeability = 10 −19 m 2 ( Ishii et al., 2011;Miyazawa et al., 2011;Niunoya and Matsui, 2007). The fault at 99.5 mabh (479.3 mbgl) exhibits a dip direction/dip angle of 181°/71° (Fig. 2a), a thin layer of fault breccia (millimeters or less) (Fig. 2b, c), and striations (rake ≈ 90°) on the fault surface (Fig. 2d), although slickensteps are not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault surface (Fig. 2d). The orientations of the splay crack and striations on the fault surface indicate that the fault is a normal ...
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... at 99.5 mabh (479.3 mbgl) exhibits a dip direction/dip angle of 181°/71° (Fig. 2a), a thin layer of fault breccia (millimeters or less) (Fig. 2b, c), and striations (rake ≈ 90°) on the fault surface (Fig. 2d), although slickensteps are not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault surface (Fig. 2d). The orientations of the splay crack and striations on the fault surface indicate that the fault is a normal fault without a strike-slip component (Fig. 2d). Although the shear displacement of this fault is unknown, Ishii et al. (2010) reported the shear ...
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... 2a), a thin layer of fault breccia (millimeters or less) (Fig. 2b, c), and striations (rake ≈ 90°) on the fault surface (Fig. 2d), although slickensteps are not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault surface (Fig. 2d). The orientations of the splay crack and striations on the fault surface indicate that the fault is a normal fault without a strike-slip component (Fig. 2d). Although the shear displacement of this fault is unknown, Ishii et al. (2010) reported the shear displacements of similar faults exposed on a surface outcrop to be up to 0.6 m ...
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... not clearly identified. A splay crack (hybrid fracture; Ishii, 2016) is also observed as a secondary fracture (Fig. 2a, c), which propagates at an angle of 30°-40° from the main fault surface (Fig. 2d). The orientations of the splay crack and striations on the fault surface indicate that the fault is a normal fault without a strike-slip component (Fig. 2d). Although the shear displacement of this fault is unknown, Ishii et al. (2010) reported the shear displacements of similar faults exposed on a surface outcrop to be up to 0.6 m (exceptionally ~10 m for a largest ...
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... by reverse/strike-slip faulting ( Fig. 3; Sanada et al., 2010Sanada et al., , 2012. However, a normal faulting stress regime (E-W-directed horizontal maximum principal stress) was also observed within a range of decameters from the test section (HDB-6_416.0 m in Fig. 3), which is consistent with the sense of displacement of the fault (Fig. 2d). The pore pressure around the test section before excavation of the underground facility and subsequent pumping was ~4.9 MPa ( Yoshino et al., 2015), but dehydration due to the pumping has reduced this to the current value of 3.9-4.5 ...
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... constant while the pressure in the other packer decreases (as shown by the gray zones in Fig. 4h, i). Furthermore, the estimated shortening of the test section reflects shear movement along the fault in the sense of normal faulting (section 4.2), which is consistent with the previous displacement interpreted from observations of drill cores (Fig. ...
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Citations
... Recently, the author developed a straightforward method using a conventional straddle-sliding-packer system for hydraulic tests as a borehole extensometer (Ishii 2020). In this method, during injection, an axial displacement of a packered-off test section can be estimated from a change in the packer pressures through a calibration based on simple laboratory experiments. ...
... In this method, during injection, an axial displacement of a packered-off test section can be estimated from a change in the packer pressures through a calibration based on simple laboratory experiments. This method cannot measure a slight displacement of 10 −5 m or less but can measure an axial displacement of 10 −4 m or more without any special device (Ishii 2020). Thus, this method might help investigate the shear capability of minor faults. ...
... Thus, this method might help investigate the shear capability of minor faults. However, this method is applicable for investigating the effect of fault shear displacement on hydraulic transmissivity (i.e., hydraulic dilation angle) (Ishii 2020) or hydraulic connectivity (Ohno and Ishii 2022), but the applicability for assessing the shear capability of faults with fault rocks/filling materials has yet to be verified. ...
Low-permeability rock is suitable as the host rock of an underground repository for radioactive waste disposal; however, minor faults might develop there. Investigating the shear capability (= shear compliance) of those faults is crucial because they could be elastically sheared by the thermal effect of the waste to damage the waste’s engineered barriers. This study performed constant-head step-injection tests along with a recently developed packer-pressure-based extensometer method for assessing the applicability of this method to investigate the shear capability of minor faults. Herein, two neighboring minor faults (A and B) in siliceous mudstone were evaluated. The results showed that fault A, with centimeter-thick fault breccia, exhibited high shear capability, whereas fault B, with millimeters or less-thick fault breccia, displayed low shear capability despite containing an incohesive fault rock. An elastic shear displacement occurred for fault A during injection and reached 15–66 mm when the test-section pressure increased from 4.1 to 4.3 MPa. Here, the shear capability was 10¹ mm/MPa or more. Conversely, fault B had cohesion, and no shear displacement was detected even when the test-section pressure increased from 4.0 to 6.0 MPa. In this case, the shear capability was 10⁻¹ mm/MPa or less. The estimated shear capabilities were consistent with the results from previous shear experiments, and therefore, the applied method helps investigate the shear capabilities of minor faults.
... (1) JRC 0 = 4.4192R 0.6482 z , and insensitive to the problem of the sampling interval of data points. In addition, several works have applied Eq. (1) to estimate the JRC 0 of natural and EDZ fractures at the Horonobe URL (Aoyagi et al. 2022;Ishii 2020Ishii , 2021Ishii , 2022. Figure 10 shows photographs of representative rock core surfaces and trace profiles. Measurements of R z using 12 fracture surfaces led to JRC 0 values ranging from 3.7 to 7.3, with an average of 5.1. ...
The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility’s hydro-mechanical characteristics to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures’ transmissivity were assessed over a period of 4 years. The transmissivity continuously decreased in this period to about 41% of its value measured prior to the swelling. Using the Barton–Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton–Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.
... Packer is widely used in oilfield water injection, oil production, fracturing, well completion, oil testing, log testing and other downhole operations (Sun et al. 2017;Liu et al. 2018;Lo et al. 2014;Zhou et al. 2018;Tong et al. 2018;Dong et al. 2021;Kao et al. 2020;Guo et al. 2020;Deng et al. 2020;Cheng et al. 2021;Franquet et al. 2019;Ishii 2020). ...
The staged and layered fracturing technology plays an important role in unconventional tight reservoirs. And the gas well fracturing and completion integration is the core component to realize the fracturing and completion integration process, which can realize the integration of acid fracturing and later drainage production so as to reduce the secondary pollution to the reservoir. The packer rubber barrel’s performance directly affects the long-term effective sealing reliability itself in high temperature and high pressure environment. In this paper, the constitutive model of rubber tested from high temperature and high pressure curing kettle to simulate the high-temperature and highly corrosive environment of the formation. On this basis, the structure of the packer’s shoulder and the protective ring of the rubber barrels are optimized through Abaqus to reduce its stress failure under high pressure, and its corrosion resistance is improved by improving the rubber material. The sealing performance of the packer rubber cylinder under the field underground requirements is tested through laboratory evaluation test and field test. The results show that the protective ring and rubber tube shoulder at 30° angle are a reasonable result of optimization, and the optimized packer can meet the requirements of 154 °C temperature resistance, 79 MPa pressure bearing and long-term effective sealing. The successful development of packer rubber and the integrated analysis process can lay a solid foundation for the realization of integrated fracturing and completion process for exploration and development of deep volcanic or carbonate reservoirs.
... Recently, the author developed a straightforward method using a conventional straddle-sliding-packer system for hydraulic tests as a borehole extensometer (Ishii 2020). In this method, during injection, an axial displacement of a packered-off test section can be estimated from a change in the packer pressures through a calibration based on simple laboratory experiments. ...
... In this method, during injection, an axial displacement of a packered-off test section can be estimated from a change in the packer pressures through a calibration based on simple laboratory experiments. This method cannot measure a slight displacement of 10 − 5 m or less but can measure an axial displacement of 10 − 4 m or more without any special device (Ishii 2020). Thus, this method might help investigate the shear compliance of minor faults. ...
... Thus, this method might help investigate the shear compliance of minor faults. However, this method is applicable for investigating the effect of fault shear displacement on hydraulic transmissivity (i.e., hydraulic dilation angle) (Ishii 2020) or hydraulic connectivity (Ohno and Ishii 2022), but the applicability for assessing the shear compliance of faults with fault rocks/ lling materials has yet to be veri ed. ...
Low-permeability rock is suitable as the host rock of an underground repository for radioactive waste disposal; however, minor faults might develop there. Investigating the shear compliance of those faults is crucial because they could be elastically sheared by the thermal effect of the waste to damage the waste’s engineered barriers. This study performed constant-head step-injection tests along with a recently developed packer-pressure-based extensometer method for assessing the applicability of this method to investigate the shear compliance of minor faults. Herein, two neighboring minor faults (A and B) in siliceous mudstone were evaluated. The results showed that fault A, with centimeter-thick fault breccia, exhibited high shear compliance, whereas fault B, with millimeters or less-thick fault breccia, displayed low shear compliance despite containing an incohesive fault rock. An elastic shear displacement occurred for fault A during injection and reached 15–66 mm when the test-section pressure increased from 4.1 MPa to 4.3 MPa. Here, the shear compliance was 10 ¹ mm/MPa or more. Conversely, fault B had cohesion, and shear displacement was undetected even when the test-section pressure increased from 4.0 MPa to 6.0 MPa. In this case, the shear compliance was 10 − 1 mm/MPa or less. The estimated shear compliances were consistent with the results from previous shear experiments, and therefore, the applied method helps investigate the shear compliances of minor faults.
... The joint closure is unlikely to be size-dependent, unlike the shear stress-displacement behavior, hence these equations enable one to link E and e even at the field scale (Barton, 1982;Quadros, 1997, 2019). The above aperture equations are approximate (Barton, 2020), but the formulae have been widely applied to converting hydraulically measured e into E at field scale for various geoengineering problems (Barton, 1972(Barton, , 1982Bandis et al., 1985;Barton and Bakhtar, 1987;Bhasin et al., 2002;Ishii, 2020) including for assisting in the choice of grout particle sizes using the E > 4d 95 rule of thumb. This is apparently not used in Swedish pre-grouting of tunnels, where the theoretical hydraulic e-aperture is (surprisingly) used for estimating grout entry. ...
Abstract: The joint roughness coefficient (JRC), introduced in Barton (1973) represented a new method in rock mechanics and rock engineering to deal with problems related to joint roughness and shear strength estimation. It has the advantages of its simple form, easy estimation, and explicit consideration of scale effects, which make it the most widely accepted parameter for roughness quantification since it was proposed. As a result, JRC has attracted the attention of many scholars who have developed JRC-related methods in many areas, such as geological engineering, multidisciplinary geosciences, mining mineral processing, civil engineering, environmental engineering, and water resources. Because of such a developing trend, an overview of JRC is presented here to provide a clear perspective on the concepts, methods, applications, and trends related to its extensions. This review mainly introduces the origin and connotation of JRC, JRC-related roughness measurement, JRC estimation methods, JRC-based roughness characteristics investigation, JRC-based rock joint property description, JRC's influence on rock mass properties, and JRC based rock engineering applications. Moreover, the representativeness of the joint samples and the determination of sampling interval for rock joint roughness measurements are discussed. In the future, the existing JRC-related methods will likely be further improved and extended in
rock engineering
... Constant-head step injection testing is an effective in situ test method for confirming the stress dependence of fracture transmissivity on the field scale; it can be feasibly conducted using a conventional packer system without large-scale, specialized equipment such as a plate-loading system or mega-packer. [19][20][21] Constant-head step injection testing may also be helpful for quantifying or verifying the stress dependence of fracture transmissivity on the field scale. Nevertheless, although case studies have reported constant-head step injection testing in an EDZ, 22 reports of using injection tests to quantify or verify the stress dependence of EDZs' fracture transmissivity are scarce. ...
... The present study determined the transmissivity of the single fracture during each injection step by fitting the measured and simulated flow rates, while the simulation considered entire injection steps as the pressure history ( Fig. 7), similar to previous studies. 21 Each single fracture was modeled as a homogeneous, horizontal, and finite aquifer with the same thickness as the test section (i.e., 0.4 m for H4-1 and 0.7 m for H4-3) and surrounded by a constant-head boundary, where a radial laminar flow was assumed. This boundary condition is based on the assumption that the tested EDZ fractures link hydraulically to numerous other EDZ fractures (as observed on the excavation faces in Fig. 3), which may together behave as a significantly more permeable zone than the tested "single" EDZ fractures; thus the tested single fractures can be considered to be hydraulically surrounded by a constant-head boundary (Fig. 8a). ...
In an underground repository for high-level radioactive waste disposal, fracture transmissivity in an excavation damaged zone (EDZ) along tunnels or deposition holes can decrease during the post-closure period via processes such as self-sealing by clay-swelling at the EDZ's fracture surface or an increase in effective normal stress acting on the fractures owing to swelling of backfilling or buffer materials. Hydromechanical coupling models for the stress-dependence of fracture transmissivity are helpful to estimate the change in an EDZ's fracture transmissivity after closure. The applicability of the applied models should be confirmed by in situ tests at the given site; this appears to be facilitated by using constant-head step injection tests. However, injection testing is rarely applied to EDZ fractures. To investigate the applicability of injection tests, the present study performed them on single, tensile EDZ fractures in the Horonobe Underground Research Laboratory hosted by poorly swelling mudstone. Furthermore, the Barton-Bandis normal stress-dependent fracture-closure model quantified the stress-dependence of EDZ facture transmissivity. The fracture's hydraulic aperture increased gradually during injection , and its variation was well reproduced by fitting the model. Although the model requires the normal stress, this parameter was reasonably estimated by the fitting analyses. Constant-head step injection tests coupled with the Barton-Bandis model is believed to be a convenient method for preliminarily quantifying or verifying the stress dependence of EDZ fracture transmissivity, at least for poorly self-sealed, tensile EDZ-fractures.
... where ′ n is the effective normal stress (Pa) (with compression positive); K n is the fracture normal stiffness (Pa/m); ψ is the dilation angle (°); and d s is the shear-displacement increment across the fault (m), which can be calculated as Ishii 2020;Yin et al. 2020;Zeng et al. 2021) where K s is the fracture shear stiffness (Pa/m) and Δτ is the excess-shear-stress increment or static stress drop (Pa) from Cappa and Rutqvist (2011) where τ 0 and τ(t) are the shear stress at the initial time and t-th time (Pa), respectively. ...
Injecting large amounts of fluid into the ground is an inevitable step in many subsurface energy extraction operations and has the potential risk of inducing fault activation. The injection operation involves complex hydromechanical coupling problems that are difficult to solve directly by analytical methods. Therefore, numerical methods are needed to simulate the physical process, and the most commonly used method is the finite element method. General finite element software is widely popular due to its unique advantages in solving complex engineering problems. However, only a few general finite element simulators permit large-scale, complex analyses and prediction of injection-induced fault activation problems. Therefore, in this paper, a user-defined field was developed in the commercial finite element software ABAQUS to improve the hydromechanical coupling analysis of injection-induced fault activation. According to the effective stress law and dynamic friction law, the effective normal stress and shear displacement of the fault were defined as the field variables varying with time step in the user subroutine. Through the exchange and update of data between the mechanical module and the hydraulic module in each step of the interpolation calculation, hydromechanical coupling analysis of injection-induced fault activation was realized. Then, verification was performed to demonstrate the capabilities and reliabilities of the user-defined field. Four cross-scale examples ranging from laboratory mesoscale (centimeter level) to reservoir-scale (hundred-meter level) show that the subroutine can be an effective tool for analyzing injection-induced fault activation in geoengineering applications.
... In this respect, in situ test methods for verifying how the local permeability along the fault is affected by fault activation have been proposed, where fault slips are artificially induced by constant-head step injection into the faults. 14,15 However, few in situ investigations have focused on how the hydraulic connectivity of the fault is affected by fault activation. 16 Although the hydraulic connectivity of faults can be investigated by pressure/flow responses in multiple boreholes, [17][18][19][20] it can also be assessed by analyzing the pressure derivative during hydraulic packer tests in a single borehole. ...
... The packer pressure and test section pressure were monitored with pressure measurement devices, and fault movement was monitored by measuring the shear displacement and hydraulic aperture of the fault. 15 Figure 2b shows a schematic view of the test device used for the packer tests. A high-performance liquid chromatography pump was used for the packer tests because the water injection flow rate was less than the injection test. ...
... Before Event A, the hydraulic aperture increased asymptotically at ~6.1 MPa, whereas after Event A, the equivalent test pressure was ~5.6 MPa. The total shear displacement also exhibited the same trend as the hydraulic aperture, and resulted in residual shear displacement after Event A. These results suggest that normal and shear stiffness of the fault decreased after Event A. This decrease in fault stiffness indicates the occurrence of a dilational shear failure associated with loss of cohesion on the fault through Event A. 15 Figure 4 shows log-log plots of pressure derivatives and test section pressure changes obtained by the packer tests before and after the injection test. The derivative plots before injection show an upward trend (i.e., low hydraulic connectivity; Fig. 4a, b), whereas the derivatives one week after the injection exhibit a horizontal/downward trend (i.e., high hydraulic connectivity; Fig. 4c). ...
An injection test and repeated packer tests were conducted for a fault in siliceous mudstone in order to activate the fault and investigate changes in hydraulic connectivity of the fault before and after fault activation. The injection test successfully induced a significant dilational shear failure within the fault. Pressure changes measured by the repeated packer tests were analyzed before and after the failure, where log–log plots of the pressure derivatives changed after the failure from an upward trend indicating a limited extent of fluid flow paths to a horizontal trend suggesting well-connected fluid flow paths. After the borehole had been open for six weeks, the pressure derivatives were restored to an upward trend. This reversible change in pressure derivatives means that the hydraulic connectivity of the fault increased temporarily during and just after the injection test, but fault activation did not irreversibly affect the initially low hydraulic connectivity of the fault. This transition in the hydraulic connectivity of the fault is also consistent with the variation in fluid pressure monitored at a neighboring observation hole. We propose that analyzing the pressure derivatives obtained by repeated packer tests before and after an injection test in a single borehole is effective for assessing the sensitivity of the hydraulic disconnectivity of faults to fault activation, which is key information for risk assessment of radioactive waste disposal.
... is approximate (Barton, 2020), but this formula has been widely utilized to convert measured e into E or simulated E into e on the field scale in a variety of geoengineering problems (Blum et al., 2009;Ishii, 2020;Lei et al., 2014;Saeidi et al., 2013;Sun et al., 2019). ...
... At the Horonobe URL, a remarkable dilational-shear-failure associated with a loss of cohesion occurred in the tested single fault plane (a shear fracture). After failure, the transmissivity of the shear fracture decreased to one-third its pre-failing value (Ishii, 2020), and then increased by 1 order of magnitude or more with increasing injection pressure (Fig. 9c). During this injection after the failure, the shear displacement was up to a few centimeters (Ishii, 2020), and the observed change in transmissivity matched well the trend of the DI dependency of the highest transmissivities defined by Eq. (15) (Fig. 9c). ...
... After failure, the transmissivity of the shear fracture decreased to one-third its pre-failing value (Ishii, 2020), and then increased by 1 order of magnitude or more with increasing injection pressure (Fig. 9c). During this injection after the failure, the shear displacement was up to a few centimeters (Ishii, 2020), and the observed change in transmissivity matched well the trend of the DI dependency of the highest transmissivities defined by Eq. (15) (Fig. 9c). ...
The transmissivity of a fracture can be related to fracture roughness (JRC0), initial aperture (E0), effective normal stress (σ'n), and tensile strength (σt) of the intact rock, based on the Barton–Bandis model and their data, and the transmissivity (or E0) can increase by shear-induced dilation. Previous studies revealed that the transmissivities of fractures in fault zones, detected as flow anomalies (highly transmissive zones) during borehole investigations at six sites, decrease uniformly with an increasing effective mean stress normalized to σt. If this uniform change in transmissivity is explained by σ'n-dependent fracture-normal displacement following the Barton–Bandis model, those transmissivities represent the upper limit of transmissivities of fractures in fault zones that can increase by shear-induced dilation. To verify this possibility, the E0 of fractures was estimated using those transmissivities, σt, and possible JRC0 and σ'n. Then, using this estimated E0, the changes in transmissivity were simulated, varying σ'n. The results reproduced very well the observed uniform change in transmissivity. The estimated values of E0 are tens of micrometers to a few millimeters, which can occur by slight shear displacements (e.g., 0.05–2.00 mm) during shear-induced dilation, easily achievable in fault zones. Thus, the requirements for the highest transmissivities are slight shear displacements and no/limited fracture-sealing rather than large shear displacements. Transmissivities in fault zone fractures that have already reached the highest transmissivities do not change significantly by shear displacement, while the transmissivities of fractures sealed by mineral filling can increase by orders of magnitude, as confirmed by recent fault-stimulation-experiments.
