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

Abstract

AbstractFracture 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.