Transmissivity and connectivity changes sustained by stimulated fault damage zones – New observations
Résumé
Understanding how the transmissivity and connectivity of damage zones around brittle faults respond to hydraulic stimulation is critical as we continue to pump fluids and extract energy underground worldwide. Changes in hydraulic properties, however, are generally only quantified along the borehole section stimulated, yielding no information on how such properties evolve in the near field. We report here results from hydraulic tests performed at the Grimsel Rock Laboratory (Switzerland) on stimulated faults that dissect meta-granitoids from the crystalline basement of the Aar Massif. These tests, including single-hole pressure pulse tests (to determine local transmissivity) and cross-hole constant rate injections (to characterize the network-scale connectivity – here up to 20 m), were performed before and after a two-week stimulation campaign involving high-pressure fluid injections. We use analytical flow models to show that the hydraulic properties of damage zones sustain significant, non-reversible changes. As expected, all stimulated intervals experienced an increase in transmissivity; however, in the near field, we observed that transmissivity can decrease by more than one order of magnitude, revealing that fluid-induced deformation near stimulated wells can also permanently decrease permeability. Cross-hole tests show that changes in fracture connectivity follow a negative relation with the initial pre-stimulation connectivity state. The final (post-stimulation) connectivity state of fracture systems around faults may therefore depend on their initial connectivity. Comparing our results with the maximum earthquake magnitudes and rupture areas observed during stimulation also shows that a link exists between the redistribution of flow in damage zones and their seismicity. Such a link suggests that the hydraulic performance of fault stimulations (i.e. the ability to enhance permeability) could be predicted from knowledge of their hydrogeological properties.