Seismic Dispersion and Attenuation in Fractured Fluid-Saturated Porous Rocks: An Experimental Study with an Analytic and Computational Comparison

Although different fluid pressure diffusion mechanisms caused by fractures have been extensively studied using analytical and numerical methods, there is little to no experimental work completed on them in laboratory conditions. In this paper, hydrostatic-stress oscillations (frequency - 0.04 to 1 Hz) are used on an intact and saw cut sample, in dry and water saturated conditions, in a triaxial cell at different effective pressures in undrained conditions. The objective is to study the fracture's effect on the elastic properties of the sample and validate some computational fracture models, that have been explored in the literature. Experimental results highlight dispersion and attenuation in saturated conditions due to the fracture, which diminishes in amplitude as the effective pressure is increased, i.e. as the fracture is closed. From local strain gauge measurements, it is found that there is a local negative phase shift between stress and strain in water saturated conditions for the fractured sample, due to the location of the strain measurements. No attenuation observed in dry conditions. A simple 1D model using mass balance and mechanical equilibrium equations for a linear isotropic poroelastic homogeneous medium give prediction in very good agreement with the experimental results. A 3D model was also developed to allow a comparison between analytic, numerical and experimental results.