| China is rich in shale gas resources,the development and utilization of shale gas is an important means of rational of energy structure adjustment in China.The physical property of shale is the key factor affecting the development of shale gas.Due to preferential orientation of mineral particles and finely laminated bedding planes,several of the macroscopic physical properties of shales are anisotropic.Compared with conventional sandstone and carbonate reservoirs,the properties of shale are special.Therefore,the development and application of new technologies and methods will help eliminate the influence of uncertain factors encountered in the interpretation and evaluation process,provide new indicators and parameters for shale physical property evaluation,and thus establish a more complete and reliable rock physical property evaluation system.Based on two kinds of optical methods which have been rapidly developed in recent years,including terahertz(THz)spectoscopy and laser-induced voltage(LIV)method,we characterizate the dielectric properties,electrical properties as well as anisotropic characteristics of shale.1.Alternating layers presented in shales affect the propagation of waves,causing the anisotropy at various frequencies.Simplifying the complicated interior structures of shales is conducive to characterize the anisotropic properties.Therefore,simulative shale core samples are designed and fabricated by 3D printing method,and the layer-caused dielectric anisotropy is investigated by THz time-domain spectroscopy.On the basis of the effective medium theory,the change of optical length resulted by refraction of the ray is discussed and modeled.It is believed that the refraction of the ray 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.2.THz spectroscopy is employed to investigate the anisotropic response of oil shale samples from different places in China with different oil yield.All the samples have significant anisotropy of the refractive index and absorption coefficient with symmetries at the location of 180°,which are corresponded with the bedding plane and the partial alignment of particles.Linear regression is built between the anisotropic degree of the samples from five regions and the oil yield,indicating THz technology can be an effective selection for evaluating the oil yield of oil shale.3.A 248 nm ultraviolet laser is employed to assess the anisotropic electrical response of a dielectric shale.Angular dependence of LIV are obtained,with a data symmetry at the location of 180°.The double-exponential functions have provided an explanation for the electrical field controlled carrier transportation process in horizontal and vertical directions.The accumulation of interface charges is considered resulting in the transient voltage response.The results demonstrate that the combination of optics and electrical logging analysis is a promising technology for the investigation of unconventional reservoirs.4.A 248 nm ultraviolet laser is used to study lateral LIV response of shale.The voltage waveforms at the dielectric shale and sandstone are varying with the laser spot position in accordance with the arc structures of the cores under a bias of 210 V.Besides,dependences of the laser-induced voltages on the laser spot position were also observed in the low resistance black shale.Owing to the Dember effect and the layered structure of shale,peak amplitudes show an approximately linear dependence with the laser irradiated position for the 0° shale sample but nonlinearity for the 45° and 90° ones.The results demonstrate that the laser-induced voltage method is very sensitive to the structure of materials,and thus has great potential in oil and gas reservoir characterization. |
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