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

Abstract

The macroscopic failure mode (tensile/hybrid/shear) of damage‐zone fractures in fault zones may influence the hydrogeological properties of the fault zone. Application of the Griffith‐Coulomb failure criterion, combined with the simple assumption that failures are predominantly induced by an increase in differential stresses and/or a decrease in effective normal stresses resulting from stress concentrations generated at the asperities/tips of faults, suggests that (1) only tensile fractures propagate from faults when the effective mean stress is less than the rock tensile strength, (2) tensile/hybrid fractures form when the effective mean stress is less than twice the rock tensile strength, and (3) shear fractures can develop when the effective mean stress is more than twice the rock tensile strength, which suppresses the formation of tensile/hybrid fractures. In this study, thin slots were precut in siliceous mudstone samples and mechanical experiments were conducted under a range of effective confining pressures using core samples with and without precut slots. A comparison of fractures formed in the samples at different applied effective mean stresses gave results consistent with the proposed model. The correspondence of model predictions and results was also corroborated by observations of natural damage‐zone fractures observed in the field, in boreholes penetrating the same siliceous mudstone as used in the experiments. The results indicate that fault zones containing numerous tensile/hybrid fractures are limited to domains that have experienced effective mean stresses of less than twice the rock tensile strength.

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…This is accompanied by dilatancy and by microfracturing of the original fabric but also by progressive decrease in porosity and pore size in the gouge with the nonclay particles embedded in reworked clay. The structure of macro-scale fracture in the samples compares well with Ishii et al (2011Ishii et al ( , 2016.…”

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

“…In addition, the hydraulic connectivity of fractures is not necessarily uniform across a formation. Ishii et al () and Ishii () indicated that fracture‐connectivity varies within the formation, depending on depth and the ductility index (DI). The DI is defined as follows (Ishii, ): where σ ′ m is the effective mean stress (MPa), σ t is the tensile strength (MPa) of intact rock, σ 1 is the maximum principal stress (MPa), σ 3 is the minimum principal stress (MPa), and p is the pore pressure (MPa) of the rock mass.…”

“…The DI is defined as follows (Ishii, ): where σ ′ m is the effective mean stress (MPa), σ t is the tensile strength (MPa) of intact rock, σ 1 is the maximum principal stress (MPa), σ 3 is the minimum principal stress (MPa), and p is the pore pressure (MPa) of the rock mass. Based on the Griffith‐Coulomb failure criterion, laboratory experiments, and core logging, Ishii () reported that tensile/hybrid (dilational) fractures tend to develop along faults during fault movement when DI < 2, while shear failure suppresses the formation of tensile/hybrid fractures when DI > 2 (Figure ). Such tensile/hybrid fractures, or brecciated zones that originate from numerous tensile/hybrid fractures, may act as highly conductive features (Aydin, ; Dholakia et al, ; Holland et al, ; Mazurek et al, ; Sibson, ; Tokiwa et al, ).…”

“…Such tensile/hybrid fractures, or brecciated zones that originate from numerous tensile/hybrid fractures, may act as highly conductive features (Aydin, ; Dholakia et al, ; Holland et al, ; Mazurek et al, ; Sibson, ; Tokiwa et al, ). Conductive features generated at asperities (Bahaaddini et al, ; Ishii, ) may link fault zones at their extensional steps (Ishii, ; Ishii et al, ; Kim et al, ; Martel & Pollard, ) or bridge open channels isolated by contact areas (asperities; Chen et al, ; Crandall et al, ; Gutierrez et al, ; Kang et al, ; Vogler et al, ) within shear fractures and fault zones. Thus, the likelihood of the formation of tensile/hybrid fractures may yield a contrast in the hydraulic connectivity between the domains of DI < 2 and DI > 2 within the same formation.…”

“… (a, b) Example of the growth history of a sinistral strike‐slip fault (modified from Ishii ()). (a) At shallower levels (DI < 2), numerous tensile/hybrid fractures develop in the fault damage zone.…”