Macroscopic non‐Biot's material properties of sandstone with pore‐coupled wave‐induced fluid flows

Abstract

ABSTRACTWave‐induced pore‐fluid flows are recognized as an important cause of seismic‐wave attenuation at frequencies below 1 kHz within heterogeneous rocks. By using Lagrangian continuum mechanics, wave‐induced pore‐fluid flow mechanisms can be classified strictly on the basis of macroscopic non‐Biot's material properties. In this classification, type I and II wave‐induced pore‐fluid flows are identified by local internal deformations being elastically coupled with either the whole Biot's rock or only with its pore fluid, respectively. Type III wave‐induced pore‐fluid flow is defined as a mixture of types I and II. For all types of wave‐induced pore‐fluid flows, the model predicts all rock properties observed in rock‐deformation experiments, such as the frequency‐dependent poroelastic moduli. The observed attenuation peaks and effective moduli can be used to invert for new, non‐Biot's material properties of porous rock. In data examples, we focus on the pore‐fluid coupled (type II) wave‐induced pore‐fluid flow mechanism and compare it to a previous analysis of type I. Non‐Biot's elastic and viscous rock properties are inverted for by fitting the effective drained bulk moduli measured in two previously published experiments: (1) with real sandstone including two saturating fluids and multiple confining pressures, and (2) a numerical experiment with heterogeneous sandstone containing mesoscopic‐ and microscopic‐scale wave‐induced pore‐fluid flows. Compared with a previous study of type I wave‐induced pore‐fluid flow, the data‐fitting method is improved by focusing on attenuation peaks and additional points in the observed spectra. For both of these experiments, both type I and II interpretations yield accurate fitting of the observed attenuation and dispersion spectra. Combinations of type I and II models (type III) yield a broad variety of acceptable mechanical models. This ambiguity of inversion shows that the different types of wave‐induced pore‐fluid flows cannot be differentiated in conventional attenuation/dispersion experiments. However, these WIFF types are physically meaningful and lead to rigorous equations of rock deformation that can be used in many applications.