  We applied a hybrid-dimensional flow model to pressure transients recorded
during pumping experiments conducted at the Reiche Zeche underground research
laboratory to study the normal opening behavior of fractures due to fluid
injection. Two distinct types of pressure responses to flow-rate steps were
identified and numerically modelled using a radial-symmetric flow formulation
for a fracture that comprises a non-linear constitutive relation for the
contact mechanics governing reversible fracture surface interaction. These two
groups represent radial-symmetric and plane-axisymmetric flow regimes from a
conventional pressure-diffusion perspective. A comprehensive parameter study
into the sensitivity of the applied hydro-mechanical model to changes in
characteristic fracture parameters revealed an interrelation between fracture
length and normal fracture stiffness that yield a match between field
observations and numerical results. Fracture stiffness values increase with
corresponding fracture length. Decomposition of the acting normal stresses into
a stresses associated with the deformation state of the global fracture
geometry and the contact stresses indicates that geometrically induced stresses
contribute the more the lower the total effective normal stress and the shorter
the fracture. Separating the contributions of the local contact mechanics and
the overall fracture geometry to fracture normal stiffness indicates that the
latter, the geometrical stiffness, constitutes a lower bound for total
stiffness; its relevance increases with decreasing fracture length, too. Our
study demonstrates that non-linear hydro-mechanical coupling can lead to vastly
different hydraulic responses and thus provides an alternative to conventional
pressure-diffusion analysis that requires changes in flow regime to cover the
full range of observations.
