Flow-bearing structures of fractured rocks: Insights from hydraulic property scalings revealed by a pumping test
Résumé
A long duration pumping test conducted over 151 days in a fractured sandstone and shale formation displays a nonstandard drawdown response and anomalous pressure diffusion, which cannot be properly interpreted using existing frameworks (e.g., homogeneous, double porosity, boundary conditions, and fractal models). An alternative framework with simple geometry and more complex hydraulic properties is thus proposed to interpret such kind of drawdown responses. The analytical development allows first to demonstrate all scaling relations in this interpretation framework. Then, and most importantly, the multi-scale hydraulic test provides consistent scalings of transmissivity, T, to storativity, S, over distances ranging from 83 to 383 m in a faulted area. Drawdown analysis in several monitoring wells shows persistent decrease of transmissivity in highly channelized fracture flow structures. In one structure, the cubic dependency of transmissivity to storativity identifies a well-defined fault and also demonstrates the validity of Poiseuille flow at a scale rarely investigated. In the other structure, the linear dependency of transmissivity to storativity indicates that the flow-bearing structure is the surrounding fracture network. Well-designed pumping tests combined with scaling analysis driven by geological evidence thus provide essential information on flow-bearing structures for site characterization and modeling tasks. At least for moderate to low permeable fractured rocks, the scaling of transmissivity to storativity appears to be more informative than any separate interpretation of hydraulic property scaling exponents.
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