The highest potential transmissivities of fractures in fault zones: Reference values based on laboratory and in situ hydro-mechanical experimental data

The transmissivity of a fracture can be related to fracture roughness (JRC(0)), initial aperture (E-0), effective normal stress (sigma'(n)), and tensile strength (sigma(t)) of the intact rock, based on the Barton-Bandis model and their data, and the transmissivity (or E-0) 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 sigma(t). If this uniform change in transmissivity is explained by sigma'(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 E-0 of fractures was estimated using those transmissivities, sigma(t), and possible JRC(0) and sigma'(n). Then, using this estimated E-0, the changes in transmissivity were simulated, varying s'n. The results reproduced very well the observed uniform change in transmissivity. The estimated values of E-0 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.

Automated summary

The transmissivity of a fracture in fault zones can be influenced by various factors such as fracture roughness, initial aperture, effective normal stress, and tensile strength of the intact rock. Studies have shown that transmissivities decrease uniformly with increasing effective mean stress, although they can increase by shear-induced dilation. Understanding these interactions is vital for predicting flow behavior in fault zone fractures.