Investigations into the opening of fractures during hydraulic testing using a hybrid-dimensional flow formulation

TL;DR

This study uses a hybrid-dimensional flow model to analyze fracture opening during hydraulic testing, revealing how fracture length and stiffness influence pressure responses and demonstrating the importance of non-linear hydro-mechanical coupling.

Contribution

It introduces a hybrid-dimensional flow formulation to model fracture opening behavior, linking fracture parameters with pressure transient responses in hydraulic testing.

Findings

Fracture length correlates with increased fracture stiffness.

Two distinct pressure response regimes were identified and modeled.

Non-linear hydro-mechanical coupling explains diverse hydraulic responses.

Abstract

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.