Relations between hydraulic properties and ultrasonic velocities during brittle failure of a low-porosity sandstone in laboratory experiments. Issue 1 (27th September 2017)

• Record Type: Journal Article
• Authors: Ahrens, B; Duda, M; Renner, J
• Is Part Of: Geophysical journal international. Volume 212:Issue 1(2018:Jan.)
• Page Start: 627
• Page End: 645
• Publication Date: 2017-09-27
• Subjects: Fracture and flow -- Microstructure -- Permeability and porosity -- Creep and deformation -- Elasticity and anelasticity -- Acoustic properties; Geophysics -- Periodicals; 550
• DOI: 10.1093/gji/ggx419
• Languages: English
• ISSNs: 0956-540X

Abstract

SUMMARY: We continuously monitored hydraulic properties and ultrasonic P -wave velocity during triaxial compression of Wilkeson sandstone samples with an initial total porosity of about 9.5 per cent to elucidate the microstructural evolution and its consequences for the relation between these properties during elastic and inelastic deformation. Samples were deformed at drained conditions at a range of effective pressures from below to above the determined crack-closure pressure of about 50 MPa and hydraulic properties were determined using the oscillatory pore-pressure method. As a result of the employed period the evolution of hydraulic properties was resolved for strain increments of 10 −4 and thus constrained during imminent localized failure. Except during the development of a localized fault at the lowest imposed effective confining pressures, we found permeability and hydraulic diffusivity to generally increase during progressing brittle deformation associated with dilation. Thus, in situ faulting of fluid-bearing rocks should in general exhibit self-stabilization. Axial P -wave velocity shows a rather uniform correlation with radial strain during inelastic deformation suggesting a prominent role of cracks aligned with the direction of wave propagation, an inference that needs further analyses regarding the varying sensitivity of longitudinal waves for (micro)structural changes in their direction of travel. Hydraulic properties exhibit systematic correlations with radial strain during inelastic deformation, too, a rather expected result for permeability. During nominally elastic deformation, however, stress ratio and mean stress seem to control hydraulic properties. Differences between elastic properties derived from mechanical and hydraulic excitations probably indicate heterogeneity of the microstructures. An additional contribution of anisotropy in hydraulic properties cannot be ruled out that up to now evades experimental quantification. The observed fairly uniform correlation between hydraulic diffusivity and P -wave velocity in the direction of fluid flow suggests that monitoring changes in elastic wave velocities bears some potential to constrain changes in conditions for transient fluid flow.