Modelling frequency‐dependent seismic anisotropy in fluid‐saturated rock with aligned fractures: implication of fracture size estimation from anisotropic measurements

Abstract

A B S T R A C TMeasurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data.

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

“…One way to distinguish crack size is by measuring and interpreting frequency-dependent seismic attenuation, where squirt flow is (probably) the dominant mechanism. The basic equations have been developed [53] and modelling data has been attempted [54]. However, correct interpretation requires very well-recorded observations which are difficult to obtain, and there are no definitive results at the time of going to press.…”

Reservoir stress path and induced seismic anisotropy: results from linking coupled fluid-flow/geomechanical simulation with seismic modelling

“…In this paper, we focus on the development of stress-induced anisotropy, assuming that the rock has no intrinsic anisotropy, and that large-scale fracture sets are not present. Although squirt flow has been shown to generate observable seismic anisotropy (e.g., Maultzsch et al 2003;Baird et al 2013), in this paper, we focus on the influence of stress on seismic anisotropy and so ignore fluid substitution and squirt-flow effects.…”

The One-Way Wave Equation: A Full-Waveform Tool for Modeling Seismic Body Wave Phenomena

“…There are several techniques to measure seismic anisotropy that can estimate fracture orientation and strength, such as Amplitude Versus Offset and Azimuth (AVOA). However, distinguishing between the various sources of seismic anisotropy as well as seismic heterogeneity is often not a simple task [e.g., 83; 84], and interpretation can be complicated further by frequency-dependent anisotropy [85]. Measurement of fracture compliance from static and dynamic measurement can provide valuable information on fracture strength and potential fracture infill [86; 87; 88].…”