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

Abstract

Fracture transmissivity in a fault zone is a significant parameter when solving certain geoscientific and geotechnical problems. However, the transmissivities are difficult to predict quantitatively owing to the complexity of in situ conditions such as the apertures of fractures. This study analyzes extensive data sets on flow anomalies (transmissive zones) detected from fluid/flow logs of boreholes in fault zones in the light of rock rheology, fracture mineralization/dissolution, and fracture orientation at six sites, namely Horonobe (Japan; siliceous mudstone), Wellenberg (Switzerland; argillaceous marl), Forsmark (Sweden; granite/granodiorite), Olkiluoto (Finland; gneiss), Northern Switzerland (granite/gneiss), and Sellafield (UK; volcaniclastic rocks and sandstone). The flow anomalies are correlated to fractures in fault zones. The data sets show that the transmissivities of the flow anomalies are strongly controlled by the ductility index, defined as the effective mean stress normalized to the tensile strength of the intact rock. An empirically derived power law relationship exists between the transmissivity and the ductility index, allowing predictions of the highest potential transmissivities of fractures in possible fault zones with maximum errors of about 2 orders of magnitude, due to the inevitable heterogeneity of a fault zone. The actual transmissivities may be further reduced by mineral precipitation in the fractures, or increased by mineral dissolution. Fracture orientation has no discernable influence on the transmissivity. The results may prove helpful for understanding and predicting the long-term transport properties of fault zones in the upper crust.

Far‐field stress dependency of the failure mode of damage‐zone fractures in fault zones: Results from laboratory tests and field observations of siliceous mudstone

“…Although the OHP films themselves are impermeable, the boundaries between the OHP films and the precut slot planes are permeable, and the behavior of pore pressure shows no significant difference between samples with and without the precut slot(s) (Figures b, f, and b). Therefore, reductions in pore pressure were probably the result of dilatancy hardening [ Brace and Martin , ], as open spaces are locally generated in fractures by shear‐induced dilation [e.g., Dehandschutter et al ., ; Ingram et al ., ; Ishii , ; Laurich et al ., ] during undrained triaxial tests.…”

“…Ishii [] introduced a ductility index (DI) to describe the rheological properties of faulted/deformed rock; the DI can be expressed in terms of σ ′ m and T as follows: …”

“…The fracture frequencies show numbers of fractures per 10 m interval of drillcore (i.e., …, 100–110 m, 110–120 m, …) and the DI values represent values at the centers of the 10 m intervals. Because previous studies have indicated that the width of the damage zone surrounding a fault core in the siliceous mudstone is generally 10 m or less [ Ishii , ], Figure only shows the fracture frequencies in the 10 m intervals including a fault core(s) and the adjacent intervals, so as to focus on damage‐zone fractures. For the calculation of DI values, the tensile strengths and mean stresses used in Ishii [] were applied, and the pore pressure and the effective stress coefficient were assumed to be hydrostatic and unity, respectively.…”

“…Because previous studies have indicated that the width of the damage zone surrounding a fault core in the siliceous mudstone is generally 10 m or less [ Ishii , ], Figure only shows the fracture frequencies in the 10 m intervals including a fault core(s) and the adjacent intervals, so as to focus on damage‐zone fractures. For the calculation of DI values, the tensile strengths and mean stresses used in Ishii [] were applied, and the pore pressure and the effective stress coefficient were assumed to be hydrostatic and unity, respectively. As shown in Figure a, the resulting frequency of shear fractures was high at DI > 2, that of hybrid fractures was high at DI ≤ ~2 and that of tensile fractures was high at DI ≤ ~1.…”

“…Ishii [] suggested that the highest potential transmissivity of damage‐zone fractures in fault zones may be controlled by shear‐induced dilation generated in the fault zone. Moreover, the shear‐induced dilation is closely related to the ductility index (DI), which is defined as the far‐field effective mean stress ( σ ′ m ) normalized to the tensile strength (T) of intact rock.…”