Understanding the response of faults to the injection of high-pressure fluids is important for several subsurface applications, for example, geologic carbon sequestration or energy storage. Lab-based experiments suggest that fluid injection can activate fault slip and that this slip can lead to increased fluid transmission along low-permeability faults. Here we present in situ observations from a cross-borehole fluid-injection experiment in a low-permeability shale-bearing fault, which show fault displacement occurring before fluid-pressure build-up. Comparing these observations with numerical models with differing permeability evolution histories, we find that the observed variation in fluid pressure is best explained by a change in permeability only after the fault fails and slips beyond the pressurized area. Once fluid migration occurs along the fault as a result of slip-induced permeability increase, the fault experiences further opening due to a decrease in the effective normal stress. We suggest that decoupling of fault slip and opening, leading to a rapid increase in fluid pressurization following the initial fault slip, could be an efficient driver for fluid migration in low-permeability faults.
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This work has been supported by the Federal Office of Topography, Swisstopo, and by the US Department of Energy (Spent Fuel and Waste Science and Technology Research Group). F.C. acknowledges support from the Institut Universitaire de France. We thank E. Dunham and R.M. Pollyea for constructive comments.
The authors declare no competing interests.
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Nature Geoscience thanks Ryan Pollyea, Eric Dunham and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Louise Hawkins and Rebecca Neely, in collaboration with the Nature Geoscience team.
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Geology and location of the Mont Terri underground laboratory in Switzerland.
a, Schematic plan of the SIMFIP probe. b, Details of the three-dimensional displacement sensor. c, Fault geology from Nussbaum et al.32. Green lines show the bedding planes. Blue lines show the main fault shear planes.
Extended Data Fig. 3 Conceptual illustration of the fault activation sequence during fluid injection.
The blue color represents the fluid pressure evolution, while the red color illustrates the mixed-mode deformation (slip and opening) along the fault.
Evolution of the fluid pressure on the fault at selected times (red dots in panel a represent the hydraulic loading at injection): b, 560 s, c, 580 s, d, 600 s, e, 620 s and f, 640 s.
Evolution of the effective normal stress on the fault at selected times: a, 560 s, b, 580 s, c, 600 s, d, 620 s and e, 640 s. Arrows indicate the spatial extent of fault opening.
Evolution of the fault-parallel displacement at selected times: a, 560 s, b, 580 s, c, 600 s, d, 620 s and e, 640 s. For comparison between panels, the displacement is normalized to the maximum value obtained at 640 s.
Time evolution of the modeled stress state in a diagram of shear stress as a function of effective normal stress. The stress paths calculated in the numerical model at the injection (square) and monitoring (circle) points are presented. The starting point indicates the initial stress before injection, and the end point show the stress at the end of injection. The black dashed lines correspond to the Mohr-Coulomb failure estimated for static and residual friction coefficients of 0.6 and 0.1, respectively. Here, we show a most “unfavorable” case where the effective normal stress falls to zero when fluid pressure reaches 5.4 MPa at injection. This graph shows that before the effective normal stress falls to zero, the Coulomb failure envelope is reached at about 3.5 MPa in good accordance with Guglielmi et al.15. Thus, the fault may start shearing before eventually opening.
Sensitivity of the fluid pressure response calculated at the monitoring point to the a, initial friction coefficient (μs), b, friction drop (μs-μd), c, critical slip distance (dc), d, initial hydraulic aperture (bho) at failure initiation and e, dilation angle (ψ) of the fault.
Supplementary Text S1
Model hydromechanical parameters for the fault and rock
injection_data.mat contains the values of the time, fluid pressure, flowrate, fault aperture and slip measured at the injection point. (Matlab file)
monitoring_data.mat contains the values of the time, fluid pressure, fault aperture and slip measured at the monitoring point. (Matlab file)
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Cappa, F., Guglielmi, Y., Nussbaum, C. et al. Fluid migration in low-permeability faults driven by decoupling of fault slip and opening. Nat. Geosci. 15, 747–751 (2022). https://doi.org/10.1038/s41561-022-00993-4