Simulation of Induced Acoustic Emission in Fractured Porous Media

Abstract

Acoustic/microseismic Emissions (AE) in naturally fractured porous media are the result of local instability along internal interfaces and the sudden release of strain energy stored in the rock matrix. This rapid release of energy, stimulates high-frequency components of the dynamic response of the rock mass, inducing mechanical wave propagation. In this article an enriched finite element model is employed to concurrently simulate the interface instability and the induced wave propagation processes in a fractured porous media. Harmonic enrichment functions are used in the context of the Generalized Finite Element Method (GFEM) to suppress the spurious oscillations that can appear in wave propagation/dynamic modelings using regular finite elements. To model the fractures, the Phantom Node Method (PNM) is employed with the GFEM. The frictional contact condition at material interfaces is modeled using a stable augmented Lagrange multiplier approach. Through various parametric studies it’s shown that (i) decreasing the permeability leads to an increase in the frequency and a decrease in the amplitude of the acoustic signal; (ii) increasing viscous damping leads to narrower frequency spectrum and decreased magnitude of the emitted acoustic signal; (iii) increasing damping leads to a transition from transient wave propagation to diffusion-dominated AE response; (iv) increasing interface friction leads to more pronounced stick-slip behavior and higher amplitude AE-without interface friction there is no AE. Lastly, the numerical illustrations demonstrate the superior capability of the enriched model (in comparison with regular finite element models) in suppressing the spurious oscillations in AE solutions.