Does in situ state of stress affect fracture flow in crystalline settings

Whether in situ state of stress affects fracture flow in approximately 1-km-deep crystalline rock setting is investigated by analyzing a unique fracture database, consisting of 193,698 discrete fractures out of which 6,223 were detected as flowing during steady state pumping. The database is based on fracture data acquired in deep surface-based holes drilled during the site investigations for underground high-level spent nuclear fuel repositories in Finland and Sweden. By computing effective normal and shear stress magnitudes on the planes of all identified fractures, we show that the probability for critically stressed fractures to be flowing is low and low normal stress values provide a slightly better indicator for fracture flow. Our results further show that at shallow settings, fracture criticality is driven by low normal stresses as the distributions of shear stress values for flowing and nonflowing fractures are very similar. The results also show that the effect of in situ stress on fracture flow is relatively low and in situ stress or its associated derivatives (shear and normal stresses) cannot be used directly in a deterministic manner for the prediction of fracture flow. As a consequence, if hydromechanical concepts are invoked for modeling fracture flow, a probabilistic approach is needed. In such an approach, we emphasize the role of low normal stresses over the concept of critically stressed fractures, as the use of normal stresses does not require the selection of a certain friction coefficient value, defining the boundary between critical and noncritical fractures.