On the application of the Eshelby-Cheng effective model in a porous cracked medium with background anisotropy: An experimental approach
| Autor: | José J. S. de Figueiredo, Emilia Hartmann, Leo K. Santos, Murillo J. S. do Nascimento, Carolina B. da Silva, Bruce F. F. Chiba, Crislene Silva, Matias C. de Sousa |
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| Rok vydání: | 2018 |
| Předmět: | |
| Zdroj: | GEOPHYSICS. 83:C209-C220 |
| ISSN: | 1942-2156 0016-8033 |
| Popis: | Understanding the effect of cracks in elastic media is important for hydrocarbon recovery, especially in nonconventional reservoirs. Consequently, due to the presence of oriented cracks on these types of reservoirs, an anisotropic behavior can be induced. In terms of seismic or ultrasonic velocities, this means that elastic waves propagating on regions with oriented cracks have their velocities varying with the propagation and polarization directions. Thus, the analysis of the seismic wave velocities in a cracked medium can be used as a tool for reservoir characterization. For this reason, there is a variety of mathematical models to describe transversely isotropic cracked medium as well as the design of several experiments to test these models. We have experimentally analyzed the theoretical predictions of Eshelby-Cheng’s first-order model. For this proposal, we measured P- and S-wave ultrasonic velocities in 17 anisotropic samples. All samples indicate weak background anisotropy due to layering deposition; i.e., they are vertical transversely isotropic (VTI) media. Sixteen synthetic anisotropic samples with different crack densities and aspect ratios were simulated by penny-shaped void inclusions in a homogeneous porous matrix made with cement and sand. An uncracked sample, with weak VTI anisotropy, was constructed for reference. The crack densities and aspect ratios ranged from 0 to 0.102 and from 0 to 0.52, respectively. All measurements were performed in a dry condition. From the experimental and theoretical velocities, we calculated the Thomsen’s parameters and correlated them with the crack density. An efficient flowchart was developed to make feasible and clear the inversion of the output Eshelby-Cheng’s effective elastic coefficients in effective velocities. Our results suggest that the anisotropy increases with crack density. In general, we noted that the best fit between the Eshelby-Cheng’s model and the experimental results occurs when the crack density and aspect ratio were lower than 0.1 and 0.32, respectively, and it is largely dependent on the type of crack porosity’s equation used in the inversion of effective stiffness coefficients in the elastic effective velocities. |
| Databáze: | OpenAIRE |
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