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- Title
Period‐ and Pressure‐Dependent Hydraulic Properties of a Fractured Granite Inferred From Periodic Pumping Tests.
- Authors
Barbosa, Nicolás D.; Jiménez Martínez, Victoria A.; Gholizadeh Doonechaly, Nima; Müller, Tobias M.; Holliger, Klaus
- Abstract
Fractures 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. Plain Language Summary: Fractures govern the hydraulic and mechanical behavior of rocks, and their characterization is crucial for understanding natural and anthropogenically induced processes involving stress and fluid pressure changes in fractured environments. Periodic pumping tests are specialized experiments used to quantify the ability of fractures to transport fluids and to affect pressure distribution in the subsurface. These experiments consist of recording the pressure response of a borehole interval to an injected oscillatory flow. We performed periodic pumping tests on fractures intersecting a borehole and analyzed their response to different testing parameters, such as, the period and amplitude of the oscillations and the type of oscillation. We found that the pressure‐flow rate relation can be fairly well explained by a simple model for porous rocks that ignores any complexity associated with the presence of fractures. Nevertheless, notable effects that cannot be explained by the model are found to be correlated with the period and amplitude of the oscillations. Capturing the period dependence is useful for characterizing heterogeneities away from the borehole, while the oscillation amplitude dependence shows the impact of the deformability of fractures exposed to pressure variations. Future modeling and experimental techniques should consider these findings to improve interpretation methods. Key Points: We conducted periodic pumping tests on fractures intersecting a borehole varying their period and the mean‐ and amplitude‐pressure valuesThe observed flow rate versus pressure behavior can largely be reproduced by an uncoupled diffusion solution for radial flowWe observed a dependence of the hydraulic response on the period and on the pressure amplitude likely related to fracture deformation
- Subjects
HARMONIC oscillators; FLUID injection; ROCK deformation; FLUID pressure; GRANITE; HYDRAULIC fracturing; RADIAL flow
- Publication
Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 9, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB027075