Period‐ and Pressure‐Dependent Hydraulic Properties of a Fractured Granite Inferred From Periodic Pumping Tests

Abstract

AbstractFractures can greatly impact fluid flow and pore pressure distribution in the upper crust. By conducting borehole hydraulic experiments, we can determine the ability of fractures to transport fluids and affect pressure diffusion under in situ conditions. We performed a series of periodic hydraulic tests in a borehole at the Bedretto Underground Laboratory for Geosciences and Geoenergies (Switzerland). We investigated the period‐ and pressure‐dependent response of a fractured granite by controlling the flow rate of harmonic and non‐harmonic oscillations, covering a wide range of periods as well as mean‐ and amplitude‐pressure values. By analyzing the phase and amplitude relation between the flow rate and fluid pressure in the injection interval, we found that the intersected fracture zone exhibits a period‐dependent hydraulic response close to radial flow perturbed by fracture‐related hydromechanical coupling effects. The main features of the interval's hydraulic response were reproduced by an uncoupled diffusion solution for radial flow, which was used to derive apparent hydraulic properties. The largest deviations from this model occur for the phase shifts and correlate with the pressure amplitude. This non‐linear expression of hydromechanical coupling associated with deformable fractures was interpreted in terms of a pressure‐dependent effective compliance of the fractures that affects their effective storativity. The period dependence is likely related to the spatial heterogeneity expected in fractured rocks and the varying sensitivity of the hydraulic response for different periods. Accounting for these effects is necessary for a correct hydraulic characterization in fractured environments and is valuable for inferring fracture mechanical behavior.