Cyclic Permeability Evolution During Repose Then Reactivation of Fractures and Faults

摘要

Cyclic growth and decay of permeability in fractures is explored during repeated reactivation and repose of saw-cut fractures of Green River shale. These slide-hold-slide experiments are supported by measurements of fracture normal deformation and optical surface profilometry. Overall, we observe continuous permeability decay during repose (holds) and significant permeability enhancement during slow reactivation (slide). The permeability decay is accompanied by fault compaction. Both hydraulic aperture change (Δb_h) and measured compaction (Δb_s) are consistent with time-dependent power law closure with a power exponent of ~0.2-0.4. These dual compaction magnitudes are positively correlated but Δb_h > Δb_s in late stage holds. Permeability enhancement during reactivation is typically also accompanied by fault dilation. However, we also observe some cases where hydraulic aperture change decouples from the measured deformation, conceivably driven by mobilization of wear products and influenced by the development of flow bottlenecks. Pretest and posttest surface profiles show that the surface topography of the fractures is planed down by shear removal. The shear removal is significant with initial laboratory prepared surface (~10 μm of aperture height) but less significant following consecutive reactivations (~2 μm). The flattened surfaces retain small-scale, ~10-20 μm wavelength, roughness. Flow simulations, constrained by the surface topography and measured deformation, indicate that small-scale roughness may control permeability at flow bottlenecks within a dominant flow channel. These results suggest cycles of permeability creation and destruction are an intrinsic component of the natural hydraulic system present in faults and fractures and provide an improved mechanistic understanding of the evolution of permeability during fault repose and reactivation.