Abstract
Rocks in earth's crust usually contain both pores and cracks. Typical examples include tight sandstone and shale rocks that have low porosity but contain abundant microcracks. By extending the classic Biot's poroelastic wave theory to include the effects of cracks, we obtain a unified elastic wave theory for porous rocks containing cracks, adding crack density and aspect ratio as two important parameters to the original theory. The new theory is applied to interpret acoustic velocity log data from tight sand and shale gas formation, whereas the classic Biot theory has difficulty in explaining such data. Because the flat- or narrow-shaped cracks can easily deform under acoustic wave excitation, the acoustic property of a cracked porous rock is quite different for different saturation conditions. This allows the new theory to correctly predict the trend of velocity variation with gas saturation in low-porosity rocks, providing a useful interpretation tool for acoustic logging in tight formations.
Published Version
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