Seismic Dispersion and Attenuation in Shale Oil Reservoir: Laboratory
Experiments and Theoretical Analysis
Abstract
Different lithofacies of shale oil reservoir invole different
lithology(mineral composition), structure(laminated and interbedded
crack), pore type and permeability. One of our objectives is to explore
the frequency dependent elastic properties and attenuation of the rocks
with different lithofacies characteristics for shale oil reservoir. For
another significant purpose, we attempt to explain the possible
dispersion and attenuation mechanisms in shale using existing
theoretical models. We first executed two sets of stress–strain
oscillation experiments on partially white-oil saturated samples with
different lithofacies, which comes from Inter-Salt shale oil in
QianJiang Sag, to investigate the dispersion of elastic moduli, elastic
and anelastic parameters, anisotropy and attenuation from seismic to
ultrasonic frequencies. Assuming that the formation conforms to the
characteristics of VTI medium, the experiments were carried out at a
confining pressure range between 5 and 30MPa in the frequency range 1 to
1000Hz using two samples drilled in vertical and parallel directions to
the formation bedding. And then we not only evaluate the applicability
of the anisotropic Gassmann theory to the Inter-Salt shale, but also
discuss the experimental phenomenon at mesoscopic and microscopic
scales. The results of our broad-frequency experiment study illustrated
that the dispersion and attenuation for compression/extension vertical
to bedding is larger than that parallel to bedding in partial fluid
saturation, and exhibiting different attenuation characteristic peaks.
The increase of shear stiffness tensor with frequency seems to indicate
the inapplicability of anisotropic Gassmann theory. The interpretation
of the attenuation measurements in terms of well-established theoretical
models which depict the wave-induced flow of pore fluid at the
mesoscopic scale was discussed in terms of the Lithofacies
characteristics, include intergranular pores and horizontal interlayer
fractures,et al.