Estimation of fracture normal stiffness using wellbore pressure recovery

SUMMARY Fractures are ubiquitous in Earth’s upper crust and strongly affect its physical properties. Their visibility for elastic waves hinges on their stiffness, a measure of the relation between changes fracture aperture applied stress, which itself significantly increases with increasing stress according to laboratory data. We constrained normal stiffness (and dependence) two fractures intersecting borehole research mine ‘Reiche Zeche’ Freiberg, Saxony, Germany, from pressure-recovery behaviour observed when pressure double-packer interval enclosing them was briefly released during shut-in sequences. For evaluation recovery, we developed generic model that accounts inherent convolution mechanical closing fracture, fluid flow exchange surrounding rock, here addressed as leakage, effects associated finite storage. address hydraulic system by considering three different ‘storage containers’, described non-dimensionless storage parameters: wellbore fixed capacity, capacities related compressibility rock constituting container unlimited capacity. To examine model’s characteristics, investigated transients predicted specific cases, such negligible subsequently numerically determined sensitivity predictions involved parameters. The application field data yield estimates properties (length, stiffness) mass (product permeability capacity). parameters derived modelling compare well independent constraints. An analysis currently available fracture-stiffness shows lack knowledge dependence severely hampers firm conclusions scale bias methods.