Cyclic Permeability Evolution During Repose Then Reactivation of Fractures and Faults

Abstract

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.

The influence of fault reactivation on injection-induced dynamic triggering of permeability evolution

“…This is consistent with stresses on the in situ field scale fault-reactivation studies conducted by Guglielmi et al (2015), as well as with a series of experiments equally exploring processes in brittle failure and friction (e.g. Im et al 2018Im et al , 2019. We initiate each experiment by applying a confining stress of 3 MPa that is retained constant throughout the experiment.…”

“…Previous studies have revealed complex interactions impacting permeability evolution during fracture reactivation; in that, shear slip deforms fracture walls through destruction and rearrangement of asperity contacts (Elsworth & Goodman 1986;Im et al 2019), which may result in permeability increases (e.g. Barton et al 1985;Wang et al 2017) or decreases (e.g.…”

“…Teufel 1987;Faoro et al 2009). Among the controls over the sense of shear permeability evolution, more recently, compaction-induced fracture surface mating during pre-slip interseismic repose has been demonstrated (Im et al 2018(Im et al , 2019. Such recent advances in our understanding of shear permeability behavior has proven even more that laboratory testing of injection-induced fracture permeability evolution is needed to more accurately account for the physical interplay of processes in application to stimulation engineering (e.g.…”

Fault valving and pore pressure evolution in simulations of earthquake sequences and aseismic slip

“…Typically k min is set to zero in fitting experimental data, but keeping k min finite is useful for numerical purposes (the very low k min we use plays little role in the system behavior). Permeability also evolves due to a range of mechanical and chemical processes [35][36][37]39,48,49 . Here we introduce an idealization that captures two fundamental processes: permeability increase with slip and permeability reduction from healing and sealing processes over longer time scales.…”

“…Geologists document mineral-filled veins that provide evidence for episodic fluid pressurization events in which pore pressure locally exceeds the least principal compressive total stress 26,30,31 . The intermittency of fluid pressurization and release, a concept known as fault valving 30,31 , is a consequence of feedback between fault slip and deformation, which typically elevate permeability [32][33][34] , and healing and sealing processes, like pressure solution transfer, that reduce permeability [35][36][37][38][39] . These feedback effects are amplified by nonlinear dependence of permeability on effective normal stress due to mechanical compression of pores and microfractures 13,19,40 .…”