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Oil yield characterization by anisotropy in optical parameters of the oil shale Xinyang Miao, Honglei Zhan, Kun Zhao, Yizhang Li, Qi Sun, and Rima Bao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02443 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 6, 2016
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Oil yield characterization by anisotropy in optical
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parameters of the oil shale
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Xin Y. Miao, Hong L. Zhan*, Kun Zhao*, Yi Z. Li, Qi Sun and Ri M. Bao
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Beijing Key Laboratory of Optical Detection Technology for Oil and Gas, China University of
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Petroleum, Beijing 102249, China
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KEYWORDS
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Oil yield, Oil shale, Anisotropy, Refractive index
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ABSTRACT
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Oil yield is an important indicator for oil shale to optimize the comprehensive utilization.
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Generally, oil shale is highly anisotropic owing to the combined effects similar to shale. In this
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paper, THz-TDS was employed to investigate the anisotropic response of oil shale samples from
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Longkou, Yaojie and Barkol with different oil yield. All the samples had significant anisotropy
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of the refractive index (n) and absorption coefficient (α) with symmetries at the location of 180°,
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which were corresponded with the bedding plane and the partial alignment of particles. Besides,
15
the D-values of experiment n in the vertical and parallel direction of the bedding plane were
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calculated as ∆n’= n⊥-n‖, and samples from Beipiao and Huadian were also tested in the
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horizontal and vertical directions for a sufficient number of THz parameters. Linear regression
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was built between the ∆n’ of the samples from five regions and the oil yield, described as
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y=60.86x+3.72 for oil yield (y) and ∆n’ (x), with the correlation coefficient R equaled 0.9866
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and the residual sum of squares was 1.182, indicating THz technology could be an effective
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selection for evaluating the oil yield of oil shale.
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22 23
INTRODUCTION
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Shale formations have been found in most of the global sedimentary basins, acting as the cap
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rock of conventional reservoirs. Moreover, energy demands have motivated the development of
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shale formations as significant unconventional reservoirs. Oil shale, a finely grained sedimentary
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rock with kerogen contained, has been gradually developed in China since the 1920s 1.
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Numerous oil and gas products as fuels and raw materials in petrochemical industries can be
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yielded by pyrogenation of kerogen 2-3. In order to optimize the comprehensive utilization of oil
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shale, a series of methods have been carried out for oil content and oil yield evaluation, including
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pyrolysis 4, thermogravimetry analysis (TGA) 5, well logging 6, X-ray diffraction as well as
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diffuse reflectance infrared fourier transforms spectroscopy (DRIFTS) 7.
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Generally, shales as well as oil shales are often highly anisotropic owing to the combined
34
effect of partial alignment of platy clay particles, layering, microcracks, low-aspect-ratio pores
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and kerogen inclusions 8-10. Ultrasonic measurements have demonstrated that the elastic
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properties are isotropic in the directions parallel to the bedding, while anisotropic in other
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directions 11-13. Besides, solid organic matters are usually more compliant than other minerals in
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shales. Organics’ shapes and distribution exhibit some elongation parallel with bedding direction
39
14
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content has impact on the anisotropy degree in shales; thus, anisotropy can be employed to assess
, which makes organic materials a strong source of anisotropy in shales. The organic matter
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the oil content theoretically; actually, precisely quantitative relations between oil content and
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anisotropy were seldom built in relative reports 15-18.
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In the past decades, optics has been proved to be the ultimate means of sending information
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to and from the interior structure of materials 19. Birefringence, defined as the division of a light
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ray into two rays when it passes through an optically anisotropic material, is dependent on the
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polarization of the light. It derives from the electrical anisotropy of a material, and is applied for
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characterizing the internal material properties 20. Terahertz (THz) radiation, which is located
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between far-IR (infra-red) and millimeter-wave bands of the spectrum and spans the transition
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range from radio-electronics to photonics 21. At present, analysis with THz radiation has attracted
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much attention owing to the unique advantages 22-27. Various crystals (e.g. quartz 28, sapphire 29,
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LiNbO3 30 and ZnO 31) have been studied to exhibit THz birefringence. Alternating structures
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with two different dielectric materials in sub-millimeter scale were also investigated to show
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form birefringence, such as multilayered polymers 32 and stacks of silicon wafers with air gaps 33.
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In addition, THz birefringence was observed for fibrous materials (e.g. textiles 34, leaf and wood
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35
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), which exhibits an orientation arrangement of the fiber during the production process. Recently, THz time-domain spectroscopy (TDS) has been utilized for the investigation of
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dielectric properties of reservoir rocks. Optical parameters in THz range were sensitive to the
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mineral and structures in rocks, e.g. marble, limestone, sandstone, clay as well as mudstone 36-38.
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Spectral features of organics were also studied, and THz-TDS was proved efficiently in probing
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the evolution of kerogen with different maturity and disaggregation of crude oil under magnetic
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fields 23, 25. Besides, combination of THz technology and stoichiometry had provided a practical
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means to analyze crude oils 26 as well as fuel oils blended with various additives 44. In our
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previous study, oil yields of oil shales were studied with laser-induced voltage (LIV), showing
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correlations between oil yields and the LIV parameters 39. In this paper, the anisotropic responses
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of oil shales were initially studied by THz-TDS. The anisotropy of experimental n were then
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calculated in order to investigate organic matters content based on the anisotropy in THz
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parameters.
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EXPERIMENTAL METHODS
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The oil shales used in the experiments were obtained from three districts in China, including
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Longkou, Yaojie and Barkol. The oil yields of the samples from these regions were measured as
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∼14.16, ∼9.05, and ∼5.66%, respectively 39. From each shale block, we cut slices at different
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angles (θ0) relative to the bedding plane (shown in Table 1). The size of each slice equaled ~8×8
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mm2 and the thickness were measured by micrometer caliper one by one. Therefore, totals of 17
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slices were prepared. In order to avoid the influence of water, all the slices were dried in vacuum
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at 90 °C for 10 h before testing.
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Table 1 Cutting and testing angles of the slices from three kinds of blocks Region
Longkou
Yaojie
Barkol
Cutting angles of
0°, 15°, 30°, 45°,
0°, 30°, 45°, 60°,
0°, 30°, 45°, 60°,
the slices (θ0)
Testing angles of the slices (θ)
60°, 75°, 90°
90°
90°
0°, 15°, 30°, 45°,
0°, 30°, 45°, 60°,
0°, 30°, 45°, 60°,
60°, 75°, 90°,
90°, 120°, 135°,
90°, 120°, 135°,
105°, 120°, 135°,
150, 180°, 210°,
150, 180°, 210°,
150°, 165°, 180°,
225°, 240°, 270°,
225°, 240°, 270°,
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195°, 210°, 225°,
300°, 315°, 330°,
300°, 315°, 330°,
240°, 255°, 270°,
360°
360°
285°, 300°, 315°, 330°, 345°, 360°
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The experiments were performed by a transmission THz-TDS system (shown in Figure 1(a))
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under air atmosphere with the temperature 294.1 K and the humidity 30%. The signal-to-noise
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ratio (SNR) of the setup was ~1500. A detailed description and schematic drawing of the THz
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system based on photo conductive antenna and electro-optical sampling has been explained in
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our previous study 26. All the slices were tested and then the refractive index (n) and absorption
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coefficient (α) anisotropy were calculated. As shown in Figure 1 (b), the measurement process
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was divided into four steps: (i) each slice was placed in the THz-TDS equipment at the focus
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position, with the angles between the input orientation of terahertz waves and the normal
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direction of the bedding plane (θ); (ii) revolved 180° of the slice around the THz-propagation
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direction (define as X-axis) with θ’=180°-θ; then the slice was turned 180° around (iii) Z-axis
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and (iv) X-axis, with θ’’=360°-θ and θ’’’=180°+θ, respectively.
88 89 90
Figure 1. Top view of (a) experimental setup for the detection of slices with transmission THz-TDS. (b) Schematic diagram of the four steps during the test.
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RESULTS AND DISCUSSION
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THz-TDS of the reference and the slices were initially measured. Fast Fourier transform (FFT)
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was used for deriving the THz frequency domain spectra (THz-FDS), and the n as well as α
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spectra were then calculated 25. Owing to the intense absorption of the slices, the spectra were
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not only composed of the samples’ characteristic features, but also many kinds of noises.
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Therefore, weaker signals at some frequencies were covered by the noises, and the effective
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frequency range was reduced to 0.4-1.0 THz for acceptable signal-to-noise ratio. Frequency
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dependent α and n spectra of slices with the testing angle 0° and 90° were exhibited in Figure 2.
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Figure 2. Frequency dependent spectra of the absorption coefficient (α) and refractive index (n)
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measured with the propagate direction parallel (dotted line, θ=90°) and vertical (solid line, θ=0°)
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to the bedding plane at 0.4-1.0 THz.
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As shown in Figure 2, the values of n were almost unchanged in the range of 0.4-1THz,
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while those of α augment with the increasing of the frequency. Owing to the divergence in
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mineral compositions and oil yields, the values of n and α in the three regions differed from each
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other. Generally, oil shales collected from Barkol have the maximum n, followed by Yaojie and
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Longkou. On the contrary, the α spectra of oil shales from Barkol were the minimum and those
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samples from Longkou led across most of the frequency range. Divergence was also found in the
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spectra of n and α for the samples from the same place with 0° and 90° , respectively.
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Then we investigated the θ dependences of n and α at various angles range from 0° to 360°.
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Expressions of n and α variation with the angle were obtained firstly. Oil shale was assumed as a
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uniaxial birefringent media with transverse isotropy, in which the optical axis was perpendicular
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to the bedding plane. The dielectric constant of the oil shale was denoted by ε0 in direction of the
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optical axis, as well as ε90 along the layer. The dielectric tensor was expressed as a tensor in the
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principal axes. i.e.
ε 90 0 0 ε = 0 ε 90 0 0 0 ε 0 .
117 118
(1)
Then, we defined the propagation velocities of electromagnetic wave in x, y and z directions,
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with 0 and 90 represent the transmission direction parallel and vertical to the optical axis,
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respectively.
121 122 123
vx = vy = v90 vz = v0 ,
.
(2)
According to the theory of light propagation in an anisotropy media 40, the relationship between the propagation velocity (v) and the angle (θ) in a uniaxial crystal was given by:
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vθ =
124 125 126
2 v 02 cos 2 θ + v 90 sin 2 θ
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(3)
the refractive index (n) could be calculated by:
127
129
.
Where θ represented the angle between the propagation direction and the optical axis. Thus,
nθ =
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1 cos2 θ sin2 θ + 2 n02 n90
.
(4)
As n0 and n90 were constants associated with the properties of samples, nθ varied periodically as a function of θ with the cycle 180° in the case of n0 ≠ n90. Besides, the angle dependent attenuation of THz field were also calculated. The output field
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was expressed as a function of θ, and then α was obtained based on that. Considering the
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diagram shown in Figure 3, the output fields of the shale parallel and perpendicular to the layer
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were expressed as:
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E⊥out = Ein cos(θ )exp(iΓ⊥ )exp(−α⊥d )
,
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Eout = Ein sin(θ )exp(iΓ )exp(−α d )
.
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In which Ein was the incident electric field amplitude, Γ⊥and Γ‖were the phase retardances
(5)
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parallel and vertical to the layer, respectively. d was the thickness of the oil shales. α⊥ and α‖
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were defined as the absorption coefficients along the two directions.
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142 143
Figure 3. Schematic diagram of the shale sample for the input THz fields. Directions α⊥ and α‖ were parallel and perpendicular to the layer, respectively.
In Figure 3, the output fields in directions perpendicular and parallel to the layer recombined and the output THz field was given by:
Eout = E⊥out cos(θ ) + Eout sin(θ )
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146 147 148
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(6)
Then, angular dependent Eout was obtained as:
E out = Ein exp(−α d ) cos 2 (θ ) exp(−∆α d ) exp(i∆Γ) + sin 2 (θ )
.
(7)
Where ∆α was the difference value of α‖and α⊥, and ∆Γ represented the phase retardance which could be expressed by:
∆Γ =
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.
2π d
λ
∆n
.
(8)
With the birefringence (∆n) and the wavelength (λ) 27.
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Then, the relation of α could be expressed as –ln (Eout /Ein) /d.
αθ = α −
1 ln cos 2 (θ ) exp( −∆ α d ) exp(i ∆Γ ) + sin 2 (θ ) d
(9)
Using Eq. (8) in Eq. (9), we obtained the variation of α with θ, d as well as λ:
αθ = α −
1 2π l ln(cos 4 θ e −2 ∆α d + 2 cos 2 θ e − ∆α d cos ∆ n + sin 4 θ ) 2d λ
(10)
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Figure 4. (a) θ dependent measured n values of the samples (dots) and calculated n (dotted
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curve) from Longkou, Yaojie, and Barkol, respectively. (b) Experimental value of α at 0.8 THz
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(dots) as a function of angle along with the expected angular dependence of α (dotted curve)
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calculated by Eq. (10). Error bars represent 0.5-fluctuation of standard deviation of n and α.
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Figure 4 showed the angular dependent n and α (at 0.8 THz) of the samples from Longkou,
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Yaojie, and Barkol, respectively. It was clear from Figure 4 (a) that all the oil shale samples from
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three regions had significant anisotropy of n, and there was data symmetry at the location of 180°.
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Moreover, the data also showed a significant attenuation of n in the THz propagation direction
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parallel to the layering compared with that perpendicular to the layering. Then, calculated n were
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obtained and plotted (the dotted lines in Figure 4(a) by Eq. (4) with the experimental n0 and n90.
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Slightly larger differences existed between the calculated and observed n for Longkou and
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Barkol samples compared to that of Yaojie, but followed the same overall trend. Likewise, the
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measured (solid curves) and calculated (dotted curves) αθ as a function of the angle were shown
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in Figure 4(b). For all the three samples, the peak values of measured α were obtained at ~45° in
170
the range from 0° to 90°, which remained coincide with the calculated values. The experimental
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data met the expectations in Eq. (10), and the results clearly demonstrated the anisotropy
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behavior of the measured oil shale samples. Various studies have shown that the observed THz
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anisotropy of materials with natural fibers are contributed by their intrinsic structure 34, 35.
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Similarly, for the oil shale samples, the anisotropy in THz parameters are caused by the
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preferential orientation of basic structural units. The data symmetries at the location of 180° in
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the experimental data as well as the calculated lines are corresponded with the bedding plane and
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the partial alignment of particles in oil shales.
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In addition, the presence of alternating layers with varying organic matter content has
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influence on the degree of anisotropy of elastic wave velocity and attenuation 14. The organic
180
matter appears to have little intrinsic birefringence, nevertheless, the effect of organic matter on
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the anisotropy was speculated to be strongly related to the texture of shales 15-19. Organic matter
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such as kerogen has always exhibited some elongation parallel with the bedding, which is caused
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by lithostatic overburden and deformation, and by the original orientation when kerogen first
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deposits 42. The organic matter content resulted in the variation of ∆n with the oil yields in this
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research. Herein, we calculated the D-value of n⊥ and n‖as ∆n’, in which n⊥ and n‖represented
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the average value of experiment n in the vertical and parallel direction of the bedding plane.
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Organic matter has a very low refractive index in THz range, 36,43 in addition, owing to the effect
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of oriented organic matters, n‖decreased more than n⊥ with the increasing in organic matter
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content. The possibility of using ∆n’ to infer oil yield promoted us to test two more samples with
190
different oil yields. Oil shale samples from Beipiao and Huadian were tested in the horizontal
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and vertical directions, with the oil yields ~5.00 and ~9.96, respectively. ∆n’ of the samples from
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five places were calculated and plotted with oil yield in Figure 5. Significant positive correlation
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was shown between the anisotropy parameters and the oil yield, indicating that the increase in
194
organic matter has promoted the anisotropy in THz range. Therefore, THz technology was
195
expected to be valid in evaluating the oil yields of oil shales.
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Figure 5. Linear fit of oil yield as a function of ∆n’, with the correlation coefficient R equal to
198
0.9866 and the residual sum of squares equal to 1.182. The inset is the n spectra of Beipiao and
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Huadian samples with the propagate direction parallel and vertical to the bedding plane. Error
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bars represent 0.5-fluctuation of standard deviation of the oil yields.
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Anisotropic degree of n in horizontal and vertical directions have provided alternative
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parameters for oil yield characterization, with small amount of samples required. According to
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previous researches, a variety of methods can be implemented to evaluate the organic matter
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content in oil shales, mostly assessed by heating and combusting 41. Fisher assay has been
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extensively applied in oil yield characterization, which is time consuming, destructive and
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expensive with large amount of samples required 7. TGA measurements have been utilized to
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determine the temperature affected pyrolyzation and kinetic parameters of oil shales with the
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heating rates 5-40 °C/min and final temperature 950 °C 5,7. Other methods such as well-logging
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provides the total organic carbon content (TOC) by relations between the TOC and ∆logR,
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however, the log curve is always influenced by the well environment and cannot reflect the
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information on undisturbed formation accurately 6. According to our previous studies, positively
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related relationships were observed between the photo induced voltage (∆v) and the oil yield, in
213
which ∆v directly affected by organic matter content 39. In this paper, we determined the content
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of organic matter by its promotion of the anisotropic degree. In terms of the anisotropy in THz
215
parameter, by measuring n of oil shale slices with two bedding plane direction, n0 and n90 could
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be obtained based on relations formulated in Eq. 4, and ∆n’ can be quantitatively related to the
217
oil yield. Linear correlation has been shown with the results of the samples from three places,
218
described as y=60.86x+3.72 for oil yield (y) and ∆n’ (x), with the correlation coefficient R
219
equaled 0.9866 and the residual sum of squares (RSS) was 1.182. Therefore, THz technology has
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provided a promising means in non-destructive detection of oil yields in oil shales from different
221
places.
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CONLUSION
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In summary, THz-TDS was utilized to study the oil shale slices with different angles between the
224
surface and the bedding plane. The samples from three regions had significant anisotropy of n
225
and α with the variation of θ, and the data symmetries corresponded with the bedding plane and
226
the preferential alignment of particles in oil shales. In addition, n anisotropy of the oil shale
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samples from five districts were compared with each other and plotted as a function of the oil
228
yield, showing that the increase of organic matter has promoted the anisotropy in THz range.
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Therefore, THz technology is supposed to be a valuable tool for the oil yields evaluation of oil
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shales.
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AUTHOR INFORMATION
232
Corresponding Author
233
*Telephone: +86-10-89732270. E-mail:
[email protected];
[email protected].
234
Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval
236
to the final version of the manuscript.
237
Notes
238
The authors declare no competing financial interest.
239
ACKNOWLEDGMENTS
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This work was supported by the National Nature Science Foundation of China (Grant No.
241
61405259 and 11574401), National Basic Research Program of China (Grant No.
242
2014CB744302), the Specially Founded Program on National Key Scientific Instruments and
243
Equipment Development, China (Grant No. 2012YQ140005) and the China Petroleum and
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Chemical Industry Association Science and Technology Guidance Program (Grant No. 2016-01-
245
07).
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(b) 1
(a) THz Emitter
2
X
θ 180
THz Detector
180-θ
Z Samples
180
4
180+θ
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180
3 360-θ
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Barkol
2.6
2.0 1.6
n
2.4
Yaojie
2.2
Longkou
(mm )
2.0
-1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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1.2
1.8 0.4
0.5
0.6
0.7
0.8
0.9
1.0
Frequency (THz)
0.8 0.4 0.4
90
0 0.5
0.6
0.7
0.8
Frequency (THz) ACS Paragon Plus Environment
0.9
1.0
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αǁ Ein
Eout θ
α
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2.05
Longkou
2.00 1.95
n
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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1.90 1.85 1.80
0
40
80 120 160 200 240 280 320 360 o
Angle( )
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2.22 2.20
Yaojie
2.18 2.16
n
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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2.14 2.12 2.10 2.08 0
40
80 120 160 200 240 280 320 360 o
Angle( )
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Barkol
2.53 2.52 2.51
n
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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2.50 2.49 2.48 0
40
80 120 160 200 240 280 320 360 o
Angle ( )
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1.6 1.5 1.4
mm )
1.3 -1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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1.2 1.1 1.0 0.9 0.8
Longkou 0
40
@0.8THz
80 120 160 200 240 280 320 360 o
Angle ( )
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1.00 0.95
mm )
0.90 -1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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0.85 0.80 0.75 0.70 0.65
Yaojie 0
40
@0.8THz 80 120 160 200 240 280 320 360 o
Angle ( )
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0.75
Barkol
mm
0.70 0.65
-1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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0.60 0.55 0.50 0.45
@0.8THz 0
40
80 120 160 200 240 280 320 360 o
Angle ( )
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18
2.15
Longkou ~14.16
2.05
n
14
Beipiao
2.10
16
Oil yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
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2.00
Huadian
1.95
12 10
1.90 0.4
90°
0° 0.5
0.6
0.7
0.8
0.9
1.0
Frequency (THz)
Huadian ~9.96
Yaojie ~9.05
8 Barkol ~5.66
y=60.86x+3.72
6 4
Beipiao ~5.00
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18
n'
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