Seismic Wave Scattering and Dissipation in Fractured Shales

TL;DR

This study investigates seismic wave scattering and absorption in fractured shales, revealing how micro-cracks and clay swelling affect wave attenuation and how pressure and temperature cycles influence fracture development, aiding subsurface exploration.

Contribution

It introduces a method to infer intrinsic attenuation from multiply scattered waves in fractured shales, combining experimental observations with numerical simulations.

Findings

Increased confining pressure reduces scattering attenuation but raises absorption.

Cyclic heating causes microcrack growth and macro-fracture nucleation.

Numerical simulations confirm multiple scattering and fracture evolution.

Abstract

Seismic attenuation in granular porous media is of paramount importance in rock physics and seismology. Unlike sandstones, shales are mixtures of sand grains and clays with extremely low porosity and permeability. Swelling of clays upon wetting induce micro-cracks at grain-clay interfaces and results in the strong elastic wave scattering. Such scattering prevents adequate measurements of the absorption from ballistic wave attenuations. Here we infer this intrinsic attenuation from multiply scattered waves as in seismology and ultrasonics. We find that increasing confining pressure reduces the scattering attenuation by micro-crack closure but increases surprisingly the absorption, likely due to the viscous dissipation involved with more liquids adsorbed in clays and at grain surfaces. Also, we observe that cyclic heating and cooling causes the shrinkage of clays and the growth of microcracks as well as the nucleation of macro-fractures. This leads to a predominant chaotic reverberation in this fractured shale. Numerical simulations based on X-ray tomography of the fractured sample confirm the multiple scattering behavior and reveal the increase of a characteristic length from an initial intact to a finally fractured shale. This study helps to improve acoustic techniques for multiscale exploration of gas and oil in shales and other fractured rocks.