Dynamic Stressing of Naturally Fractured Rocks: On the Relation Between Transient Changes in Permeability and Elastic Wave Velocity

Abstract

We report on laboratory experiments designed to explore the fundamental processes that impact fluid permeability during drilling and fluid injection. Coupled ultrasonic and permeability measurements are presented for Westerly granite samples fractured in situ and used to illuminate the relationship between nonlinear dynamic stiffness and fracture permeability indexed via fracture aperture. We perturb the effective stress field using normal stress and pore pressure oscillations. The velocity of ultrasonic waves transmitted across the fracture is used to infer the evolution of elastic properties and the fracture nonlinearity. Changes in permeability are measured concurrently. We observe that relative changes in wave velocity and permeability, due to both normal stress and pore pressure oscillations, are correlated, such that larger drops in wave velocity (higher nonlinearity) correspond to larger increases in permeability. Our observations suggest that dynamic stressing is more likely to enhance the permeability of fractures that exhibit greater nonlinearity.

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

“…Additionally, the elevated efficiency of the first stress perturbation in increasing permeability, as reported from the experiments by Candela et al 2015, is not observed in our experiments. A similar observation was recently noted also by Shokouhi et al (2019).…”

A review framework of how earthquakes trigger volcanic eruptions

“…Dynamic stresses may also enhance or reduce permeability by opening, closing or shearing cracks 15,132,141,[161][162][163] . Opening new cracks or widening already existing ones increases permeability whereas other fractures with less favourable orientations would be closed and hence decrease permeability.…”

Measuring SHmax with Stress-Induced Anisotropy in Nonlinear Anelastic Behavior

“…The relationship between stress, strain, and elasticity is complex in individual rock samples 32,33 with mechanical damage and weak grain contacts being primarily responsible for nonlinear elastic behavior 34 . Temperature, pressure, and the presence of fluids modulate the nonlinear behavior 34,35 . In the Earth, seismic velocities are commonly observed to be faster when rocks are compressed, usually interpreted as the closing of 48 cracks 36,37 , while they are typically slower after experiencing strong shaking, usually interpreted as the breaking or weakening of internal contacts 38,39 .…”

“…After the disturbance, the material relaxes back to its original or a new metastable state, the process of slow dynamics. Thus, rocks are 51 metastable in their elastic behavior and strongly influenced by relatively weak external forces 52 perturbing the material structure 35,38,39 .…”