Layer Caused an Anisotropic Terahertz Response of a 3D-printed Simulative Shale Core

Abstract

Energy demands have motivated the development of shale formations as significant unconventional reservoirs. The anisotropy of shales plays a significant role in both the mechanical behavior and engineering activities. Alternating layers presented in shales affect the propagation of waves, causing anisotropy at various frequencies. Simplifying the complicated interior structures of shales is conducive to characterize the anisotropic properties. Therefore, simulative shale core samples were designed and fabricated using additive manufacturing processes, and layer-caused dielectric anisotropy was investigated by terahertz (THz) time-domain spectroscopy. On the basis of effective medium theory, the change of the optical length caused by refraction of rays was discussed and modeled. It is believed that the refraction of rays at the interfaces is the source of THz propagation anisotropy in the multilayered structure, and the anisotropy degree is mainly influenced by the layer thickness as well as the refractive index.

Terahertz Dielectric Permittivity Anisotropy and Fabric of Oil Shales: Comparison to Acoustic, Magnetic Susceptibility, and X-ray Microtomography Results

“…As presented in Figure , THz-TDS is sensitive to many anisotropy sources, but the results up to day were mainly linked to the organic matter content and strata. , Therefore, many questions remain about anisotropy dependency related to magnetic minerals, pores, fluids, etc. In particular, magnetic mineral influence is considered negligible at high electromagnetic frequencies, but to our knowledge, it was never studied deeper to be certain about this.…”

“…In the same figure, we also present the four applied experimental methods (AMS, X-ray microtomography, acoustics, and THz-TDS), indicating to which physical properties they mainly show sensitivity. ,,,,,− The methods have different spatial resolutions and volume interactions. Thus, it is highly expected to obtain different degrees of anisotropy in each method.…”

“…Although the results obtained more than 30 years ago indicated encouraging results, − it has not been widely developed mainly due to experimental difficulties, such as the use of electrodes. However, recently, thanks to advanced technologies, a better way to measure dielectric permittivity using high-frequency electromagnetic (EM) waves (>0.8 THz) has been proposed. ,− In particular, in refs and − , the authors link the measured terahertz (THz) signal to the organic matter of the shales, and in refs − , to its change as a result of temperature-maturation. Specially, in three papers, ,, the authors have worked on the anisotropy of the dielectric permittivity in the THz band.…”

“…However, recently, thanks to advanced technologies, a better way to measure dielectric permittivity using high-frequency electromagnetic (EM) waves (>0.8 THz) has been proposed. ,− In particular, in refs and − , the authors link the measured terahertz (THz) signal to the organic matter of the shales, and in refs − , to its change as a result of temperature-maturation. Specially, in three papers, ,, the authors have worked on the anisotropy of the dielectric permittivity in the THz band. In refs and , they related it to the organic matter content, and in ref , they related it to the shale layering.…”

“…Specially, in three papers, ,, the authors have worked on the anisotropy of the dielectric permittivity in the THz band. In refs and , they related it to the organic matter content, and in ref , they related it to the shale layering. Hence, to our knowledge, THz dielectric anisotropy information has not been used to obtain the dielectric fabric and no comparison to widely applied methods was made to show whether good results are achieved.…”